Page images
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]
[ocr errors]


[ocr errors]

is computations, will soon commence to decrease in bright with warships and smash the statues to pieces in bombarding

the city." Mr. Lick was so struck by this, that he asked, Berlin, Midnight,

“What shall I do with the money, then?" How this question R.A. Decl. Log r. Log A. Br. was answered is now well known, and astronomical science was h. m. S.

presented with the finest object-glass that was ever made. 30 ... 15 57 15 ... 58 31.0

Mr. Lick's deed prescribed that the Observatory should be 31 ... 16 16 30 ... 60 21'3 ... O'0820 ... 9.8589 .. 7:66

"made useful in promoting science," and up to the present ... 16 38 18 ... 62 19

these words have been carried out to the letter. The big 2 ... 17 2 46 ... 63 29-7 ... 0'0812 ... 9.8530 ... 7.89 telescope has not been preserved for one side of astronomical 49 ... 64 417

science, but has dived into all branches, as every astronomer is 4 ... 17 59 0 ... 65 34 5 ... O'0807 ... 9:8521 ... 7.95 aware Not only have minute double stars been observed and 5 ... 18 29 40 ... 66 54

measured, but the spectroscope has been employed, from which LET HOLMES (NOVEMBER 6, 1892). — The following is a

excellent resulis have been published, while lunar photographs,

equalling, is not excelling, those that had been previously uation of the ephemeris of this comet for the present

obiained have brought 1o light much to set us thinking about. Berlin, Midnight.

Jupiter's fifth moun is perhaps the latest arrival of which we

have heard, and this, following just 300 years alter Galileo's R.A. (app.) Decl. (app.). Logr. Log A.

discovery, would alone render ihe Observatory famous. That h. m. s. I 2 22 ... +33 59.5

The Lick Astronomical Department, during the few years of iis 572

existence, has done an immense amount of good work, especially 4 27 55'1 ... 0.4096 ... 0*3284

when one takes into account the comparatively small staff on

hand, cannot be denied, and we hope the day will come when 513

the number of such telescopes will be increased, for the ever7.42

opening fields of research point out how necessary they are. 8 50 ... 33 47.9 ... 0'4119 ... O'3400 WASHINGTON Magnetic OBSERVATIONS. -- The United

States Naval Observatory has quite recently published their E MARKINGS ON Mars.-In No. 25 of the Publications

magnetic observations that were made during the past year, Astronomical Society of the Pacific, Mr. Schaeberle has a

prepared on the same plan as that sor 1889-90. The observasinary note on the question as to whether the darker and

tions for 1891, as Mr. Hoogewerff (who was in charge for the righter areas on Mars are water and land or vice versa.

greaier part of the year) informs us, are better than those of g observed the planet from June II up to the present time

former years, owing to the fact that the reductions took place s been led to the conclusion opposite to that of Schiapa

at no very distant dates from the observations, the experience Flammarion, and other observers, and considers that after thus gained helping to correct and guard against conditions

dark portions should be considered as land and the bright which might have tended to give rise to errors. The introducter. In raising such a question as !his Mr. Schaeberle | tion contains a description of the buildings, methods of observ. en very reserved, for should his opinion receive due atten- ing, together with the personnel during the year, concluding s of course it should do, and be corroborated, the planet's with a description of the jabular results. The tabular results, as will be looked upon in quite a different light than for: usual, show ihe mean hourly readings for the elements for each In this note he sets forth a few of his reasons for coming

month, Table I. containing the mean values for the four years h a conclusion, and it may interest many of our readers if

1888-91. te some of them briefly. If the dark marking. be taken

Simultaneous with this volume was also issued the meteorod, would not the irregular gradations of shade be more logical observations and results for the year 1888. lly ex,ected than if we consider them as fixed surlace !s? “Light reflected from a spherical surface of water ght state of agitation would vary uniformly in intensity. vosition, the centre of the planet would, for a water sur.

GEOGRAPHICAL NOTES. ppear brightest. Observations show that within a certain e from the edge of Mars there is a gradual increase in

A SPECIAL number of the Mouvement Géographique is ady lustre of the brighter areas towards the centre of the

devoted to a series of important despatches from M. Alexandre " Assuming these dark areas to be water, then they

Delcommune, chief of the Lomami expedition of the Katanga thus be least dark near the centre, which is somewhat

Company. Entering the Lomami from the Congo, the pariy y to observation. With reference to the “canals," he

lest ihe river on May 13, 1891, and explored the entirely it they on this hypothesis “correspond to the ridges of

untraversed territory between its upper valley river and that of ins wbich are almost wholly immersed in water," while

the Sankuru as far as 80 S. Thence they turned eastward and ard to their observed doubling he remarks that they can

reached Lake Kassali on the Lualaba, and struck south through lined as “representing parallel ridges of which our own

Garenganze's country to Bunkeia. Making a circuit through nishes examples." As a concluding argument he takes | Katanga and westward, they found the Lualaba near ils source, ved terrestrial observation, the view of ihe lower end of and following it for 200 kilometres, discovered a grand gorge at ncisco Bay from Mount Hamilton, San Francisco being

Nzole, where the river Howed in a succession of wild cataracts es away. At all hours of the day, he says, “The

between cliffs nearly a thousand feet high, and not more than of San Francisco Bay (as seen from ihe top of Mount

forty yards apart. From the rapids they returned to Bunkeia, ) is much brighter than the neighbouring valley and

travelled north-eastward over the plateau, cro sing the is at the same distance.” He further adds that the line Luapula at its outflow from Lake Moero, and ultimately reached nakes an angle of more than 87° with the normal to the

Lake Tanganyika. The difficulties overcome were very great, the bay, while the observer's po-ition “ varies all the

and the sufferings of the caravan have rarely been surpassed being nearly in a direct line between the bay and the even in the grimmest records of African u avel. e position in which the sun is nearly in the direction of AMONGST the English travellers who have recently arrived in

London are Mr. Selous, the famous South African hunter, and

Mr. Conway, who has probably climbed higher than any other ICK OBSERVATORY.--Miss Milicent W. Shinn is the

European in the Karakoram range. Buth gentlemen will read a very interesting pamphlet on the history of the Lick ical Department of the University of California. In

papers to the Royal Geographical Society early next year pages she brings together much with regard to the The arrangements for the Royal Geographical Society's evenis connected with the sounding of the giant refrac:or ing meetinis alter Christmas are unusually varied. Mr. Huse of generally known. For instance, it is curious to will describe his journey up the Burram river in Sarawak to

Mr. Lick wished to be immortalized by leaving Mount Dulit, at ihe first meeting in January. The second or costly statues of himself and his family, and when meeting will be devoted to the Islind of Yezo, when I'rof. Milne t such statues would be preserved for all time, was and Mr. Savage Landor will read pipers. Papers by Captain by Mr. Staples that “more likely we shall get into a Bower and the American traveller, Mr. Rockhill, on Tibel, will

occia or somebody, and they will come round here be given la:cr ; and Lieutenant Peary will personally describe his experiences in the north of Greenland. In March Prof. frost on their upper sides, and had a thin film or coat of tre Bonny will lecture on the action of ice in producing geogra. parent ice on their lower sides. Leaves and sticks as phical forms, and there will be other papers dealing with the ground were less frosted on the sides facing the ground thus scientific basis of geography,

the top. Thick planks between a few inches and one foot to The death of Cardinal Lavigerie on November 24 removed

the ground were about a third as much frosted on the lower one of the most powerful personages who have recently influ.

on the upper sides. Considering that the upper side of a plan enced the geography of Alrica. It is very largely on account

I inch thick would fall to a considerably lower temperatur: 2 of his labours that the French Roman Catholic missions have

radiation than the lower side, it may be supposed that the played so conspicuous a part in coinbating the slave trade, and

position would have been largest on the lower side if they to him also is due the formation of a much-needed Belgian

bien at the same temperature. That much frost came from a Anti slavery Society.

air independently of the ground, was shown by the white sau

12 feet abuve the surface of the earth. On the other halt. The British Government having decided to relieve the

grass was much more heavily frosted. Moreover, tumbleri East African Company from the responsibility of occupying

verted and pressed down on dry, hard, bare earth, on san., a Uganda, an Imperial commission, under the charge of Sir

erial commission, under the charge of Sir on hard turf, were moderately frosted inside, besides being that Gerald Portal, will set out from Mombasa as soon as it frosted outside. The indications, on the whole, seems can be got ready to take over the administration of the country. resemble those of the previous June, but the vapour code Another fact of some interest is the revival by Mr. Cecil Rhodes ation attributable to exhalation from the earth bore a ne of the idea of exploring Africa by telegraph. He proposes to smaller proportion to the total deposit than in the case de lay down a line from the Cape to Uganda, and uliimately to

on interior surfaces observed in summer extend it to Egypt. In a few months the South African Com. Boards, tiles, and stones (sandstone) in heaps were 12 pany's wires will have reached the mission station of Blantyre

on the top, and especially in cracks and indentations of te north of the Zambesi, and there are no serious physical diffi şurface, but not in the interstices between the separate pe culties in continuing the line to the head waters of the Nile. Stones on the ground were sometimes not frosted at all a The effect on the exploration of Africa will be enormous, not top, but much on the parts against the sandy earth, and 13 the least important result being the possibility of arriving at the bedded in the ground. true longitudes of places in the interior of the continent.

Further experiments in May and in the summer sur

1892 gave strong confirmation of the evidence that mic DEW AND FROST.

and frost are caused by exhalation of vapour from the eartas

in dry weather. A PAMPHLET recording some interesting “Observations The facts that

on Dew and Frost," by the Hon. R. Russell, has just been (1) A large quantity of dew was invariably found 15 published by Mr. Edward Stanford. We reprint Mr. Russell's nights in the interior of closed vessels over grass and sas ** Summary of Results" :

(2) Very little or no dew was found in the interior of The observations were begun with the object of verifying the inverted over plates on the ground. commonly rıceived theory of dew, and with a strong feeling (3) More dew was found on the lower side of a square, that the results obtained hy Col. Balgeley, described in the | raised, china plate over grass or sand than on the lower set Proceedings of the Royal Meieorological Society for April, 1891, similar plate placed upon the first. opposed as they were in some measure to the accepted teaching (4) The lower sides of stones, slates, and paper on on the subject, would not be corroborated. When, alter ex. sand, were much more dewed than the upper sides I posing inverted glass tumblers and pans on grass and bare earth wooden back of the minimum thermometer on cleared in the summer of 1891, dew was often found in surprising when lying on earth, sand, or grass was almost iovarmaan amount in the interior, I attributed the deposit to vaporous air before the upper surface. which might have entered under the rim and parted with its (5) The lower side of plates of glass, I or 2 in. abore moisture in the calm of the inclosed space. But when it was were as much or more bedewed than the upper sides found that a tumbler pressed down into dry earth, and other (6) Leaves of bushes, leaves lying on the ground, 2003 vessels admitting little air from outside, were considerably be. of grass were about equally bedewed on both sides. dewed in the interior ; and when, further, similar vessels inverted ! (7) The interior of closed vessels inverted on the on earthenware or metal plates were found to be very slightly or covered with two other inverted vessels of badly not at all bedewed inside, it became more probable that the substance was thickly bedewed, and the grass in the vapour condensed in the interior of vessels over grass and garden circular inclosures also thickly bedewed. earth had proceeded from the earth beneath. Next, it was found (8) The deposit of dew on the interior of closed vesses that china plates, admitting a flow of air between their lower over dry garden earth was much less than over surfaces and the ground, were more heavily bedeu ed on their although the powdery condition of the earth in the set lower than on their upper surfaces, and that a cylinder of glass showed that no deposit from the air had taken place was most bedewed on the lower outer and upper inner surfaces. | face during the night, These observations confirmed the suspicion that the dew on the (9) Usually a greater amount of dew was depaste inside of the hollow ves els was derived from the ground. It was interior of vessels when the earth was moist at 1 1* for a long time a matter of doubt and difficulty that vessels below the surface than when the earth was at its drie inveri ed over dry, dusty earth and dry turf were found copiously (10) The temperature of the space under a giis bedewed within on the morning following exposure. On many other object suspended near the surface of the ground mornings the amount of dew in the interior increased in some than that of the upper surface of the object, and, best proportion to the precautions iaken to exclude free air, and it a cloudy film was produced first on the lower surface. seemed highly improbable that moist air penetrated, without de to a proof that a large part of the dew formed is dere positing on its way much of its moisture, either through the vapour from the earth. dusty earth banked round the edges of the vessel, and exposed Moreover, the large difference often observed bere to the sky, or else through the dusty covering of earth below quantity of dew deposited in the interior of a vessel 10 the vessel from lower layers.

plant, and the quantity of an empty vessel, proved 35 In December, 1891, during hard frost and very fine weather, dew may be derived from the earth through plants with calm or very light airs, the ground being frozen hard, leaves Drinking glasses inverted over grassy turf, and one of bushes, ferns, &c., were seen to be frosted both on their upper by, from which the grass was removed, showed as and lower sides, though much less on the lower sides facing ine of deposit on the glasses inclosing grass. Moen bare ground than on the upper sides facing the open sky. Where condensed on plates suspended over grass than ove thick fern grew between ihe observed leaves and the ground, In these cases the conditions are somewhat artifak there was no rime on the lower sides of the overhanging ferns or grass, which was covered by a suspended plate or to leaves. This seemed to show that the rime on the lower sides glass, would be warmer than if the exposure to the same of ferns was due to exhalation from the ground, for the interruption but the disturbance thus caused would tell as bids of radiant heat from the earth by dry litter would rather favour in favour of deposition on the interior surface..! than reduce the frosting of the under sides. Live leaves on objected that the air in and above the grass work bushes, and dead leaves on the ground, were whitened with owing to the radiating grass, th an over the baredo

efore more dew would be deposited from the air ; but this of the stratum of air near the ground, which is continually cool. ction would scarcely be valid where a small plant was in- | ing by contact and radiation. Thus dew goes on forming while ed on bare earth and the deposition on the interior of the the air falls lower and lower beyond its original dew point, and s compared with that on a glass not inclosing a plant. while by a very genıle movement an interchange is kept up be. ecent investigations have proved the evaporation from plants tween the warmer air touching ihe ground beneath the grass, and e very large, and since evaporation proceeds by night as the cold air on the surface of the grass, and between differently

as by day, there can be no reason why a moderate pro. cooled layers and portions of air above it. If the air is very ion of the dew deposited on the surface of blades of grass humid, a very slight air or breeze is favourable to heavy de. on leaves of plants generally should not be derived from the position. On ordinary clear nighis, calm and light airs allow pur which they exhale. The fact that an equal quantity of ihe reduction of the lowest stra'um of air to the dew-point, and is deposited on glass, china, painted wood, &c., exposed to there is no liability to evaporation of the minute deposited parsky to that depo-ited on grass, may seem to minimize the ticles by portions of air above the dew-point being driven against ence of plant exhalation, but we must remember that the them. When the air is rather dry, as often happens at night in e of the stratum of air near the ground is rendered inore dry summer weather, and in winter frosts, calm is frequently a rous by these exhalations, and that therefore the dew-point necessary condition for the deposit and appearance of dew and oder reached on the surface of any body exposed to the sky white frost. The deposit may be observed to take place on the e midst of vege'a ion than on bare open ground. Moreover, cessation of wind, and again, the change from calm to wind soon hickness of the substance prevents earth heat from much dries off the dew which has already formed. On other occasions, ting the upper surface. The effect of grass in promoting | when there is a gentle air or breeze, dew and frost are deposited formation is owing-(1) To its radiative power cooliny its only in sheltered places, as on the most sheltered slopes of fields, ce below the dew-point. (2) To the consequent cooling of on banks sloping to leeward, on leaves on the lee side of bushes tratum of air in and over the grass to a point much below and trees, on the lee side of mole-hills, posts, railings, and other of the air a few feet higher. (3) To the obstruction offered objects. Hollows, depressions, and cracks, in paper, glass, le grass to any light air or breeze on a nearly calm night, stones, tiles, wood, and leaves, are more bedewed than Alat the consequent settling down, without much disturbance, of surfaces from the same reason, the reduction below the dewd heavy stratum. (4) To the prevention by the grassy point of air less diluted than thit which is more free by currents ring of the drying up process by sun and wind which takes of higher temperature and greater dryness. With a fresh west

on bare ground, and to the moist earth which therefore wind in a clear night, the raised and ribbed parts of leaves, &c., s under grass near the surface even in dry weather. (5) To may be thickly bedewed and frosted, but the hollows and folds xhalation of vapour from the grass. .

scarcely if at all less, and the sides of buds, thorns, &c., are e realization of these causes explained what was always, more frosted than the points. The wind is, in fact, often ous to these observations, a difficulty to me, the almost sufficiently removed from the dew point to prevent deposition . absence of dew on heather and dry fern in the summer. or continuance of moisture on all parts which are fully exposed after heavy dews, heather was in variahly found perfectly to it. Not even free radiation to a clear sky then avails

In fine, calm winter weather, with white frost, heather to plant frost.growths upon the object whose temperature is be a good deal whitened, and the frost is then derived being perpetually supplied by the forcible impact of warmer air. y from the open air. Wood, being a good radiator and Free radiation or exposed situation is, on the whole, perhaps onductor, is heavily bedewed and frosted.

the most effectual cause of dew on very many nights in the year. nes, whether of sandy composition and appearance, or of In a level country thoje parts of a field which are least sheltered grain like fint, pebbles, and slate, are not oíten visibly | by trees and hedges gather most dew and frost on calm nights. red or frosted on the top on clear nights. On their sur- | Similarly, those parts of any flat substance, such as a sheet of

touching or very close to the ground, they are heavily be- glass or paper, which have the most uninterrupted exposure to 1 and frosted. A moderate radiative power, their usual the sky are most bedewed. The tops of bushes, posts, railings, ion removed from grass an i vegetation, and in the case of inverted drinking glasses, pans, &c.; are on calm nights, and ose grained stones, a conductive power greater than that sometimes breezy nights, more bedewed than the sides. Greater ves, grass, and wood, though less than that of metals, pre- | cold by greater radiation in these cases produces greater deposition the deposition of much atmospheric moisture on their from the cooled air which comes in contact with the freely radiated sides. But when air highly charged with vapour im. ing surfaces. It must be remarked, however, that radiation from on them in a confined space, as on their lower sides, con- ! fine points, such as the tips of sharp thorns, is often not sufficient ion readily takes place, just as it will take place when any | to counteract in air which is not very humid the effect of the nce, even polished meral, is held above ihe spout of a continual impact of air above the dew-point and higher in temof boiling water. It is apparent that since stones act as perature. Close to the ground the case is different, for there the isors to the vapour constantly arising from the earth, and temperature of the low stratum of air is lower, and usually about he heat of the sun and temperature of the air by day only the dew-point, there is little movement, and vapour from the " raise the temperature of the earth immediately beneath ground increases humidity ; but even in this situation the points

stone, while the radiation of heat from the stone and low of grasses, &c., are often less bedewed than the sides. aperature of the night cause the lower side of the stone to | That free radiation is by no means necessary for the formation

cold at night, a rather large amount of moisture must be of heavy dew on grass is proved by the experiments detailed ed on its lower surface in every twenty-four hours, and the | above, made during the summer of 1892. The grass was found on which it rests must in our climate remain always very | heavily bedewed in dry weather wiihin three enclosures of

The space between the stone and the ground conse. earthenware by which radiation was arrested. becomes the abode of many insects which live well in Since grass covered by hollow vessels, and the interior of id darkness.

hollow vessels themselves, are thickly covered with dew, it would ional observation of the distribution of dew, without seem likely that the grass under overhanging trees would be as omparison with the state of the weather, gives an im thickly bedewed as the exposed grass in a field, and that the of capriciousness which only continuous records com under sides of the overhanging leaves would also be wetted. 'arious conditions can remove.

This is not the case. And there are differences in the two sition is generally favoured by a humid air, and therefore situations sufficient to account for the absence of dew under puntry by southwesterly and westerly winds, which bring lealy trees. In the first place, on a calm night, the air under a land the vapour derived from evaporation of the Atlantic tree is warmer than in the open owing to radiation from the

A smaller fall of temperature by radiation brings about ground being arrested. Secondly, whatever vapur escapes from tion, and there is less tendency in any deposit to the earth is unable to condense on the grass which covers it, the

than in a drier air. Radiation may produce a greater | grass being but little colder than the air and vapour. Thirdly, operature in dry air, but the distance froin the dew. and herein lies the chief difference, the air under the tree mnves

ommonly too wide to compensate greater humidity freely and is above the dew-point, since the earth and other ter cooling

objects which it touches are warmer than the grass and air outis also very favourable to dew-formation. It allows side. If the air were confined in a small space, the increments vapour in the air to remain sufficiently long in contact of vapour issuing from the earth, and the gradual cooling of the radiating substances to become greatly cooled, and so l grass under the tree and of the tree itsell, might cause deposition,

condensed upon them, and it prevents the dispersion but air which has parted with much of its moisture outside is constanly mixing with a considerable bɔdy of air already warmed though similarly exposed with bodies which are attracting in under the sheltering canopy. Thus all objects under the tree and ihat wool laid upon a metal acquires much less dewibar remain above or not much below the dew-point of the air which equal quantity laid upon grass in the immediate vicinity; 'ket touches them. Yet, on a calun night, long grass and other sub. metal plate on grass always became moist on the loser stances a little raised above the ground are sometimes heavily during the night, though the upper side was often very dry, bedewed, though largely hindered by overhanging branches from that is the plate was elevated several feet in the air, the condes losing their heat by radiation. They often remain nearly dry of both sides was always the same, whether dry or woist: 1 till the morning hours, and then reach a temperature below the wool on a raised board was commonly colder than on the dew-point. The absence of dew under trees and bushes is, within on very still nights, and that the leeward side of the board to limits, roughly proportional to the area of ground covered. A colder than the windward ; that bare gravel and garden t'u large surface of dry ground slowly parting with its heat during were very much warmer after sunset than neighbouring as the night has a powerful effect in preventing condensation. that on dewy nights the temperature of the earth balf an inde Small bushes on a humid clear night are often much bedewed an inch beneath its surface was much warmer than the gres? even on their lower leaves. On the night of October 5, 1892, it, and than the air ; that metal covering grass was only both sides of the leaves of bushes in all sheltered situations were colder than the grass covered, and this again colder there found thickly bedewed, but where leaves were either exposed earth; that melal thus exposed was warmer than air 4 let akce to the slight breeze which was blowing, or near the wall of the it, and much warmer than neighbouring grass ; that the ter house on which the sun had shone, they were dry. The warm, dry in the quantitie, of dew, formed upon bodies of the same wall of a house acts a part similar to ihat of the earth under a in different situations, was occasioned by the diversity of te tree in radiating warmth to neighbouring objects, and in warming | perature existing among them; and that on nights savarze the air by contact. The vapour emerging from earth sheltered ihe production of dew, only a very small part of what comes by foliage several feet above it has time to mix well with air | owing to vapour rising from the earth. before coming in contact with solid objects. In the hollow The last of these conclusions Wells supported by the stern vessels, and even in the space between a raised plate of glass and vation that the dew on the grass increased con-iderabij the earth, the vapour which rises from the earth has no time to sunset, the same time at wh ch dew began to show it ea become equally distributed in the air besore meeting with sub raised board, and by the reflection that, "though boiliessen stances coider than itsell ; in the closed vessels the initial amount on the ground after they have been made sufficiently care of vapour is augmented so as to produce constant saturation. radiation to condense ihe vapour of the atmosphere wul ben Objects, such as drinking.glasses, raised several feet above the to retain the moisture which they acquire by condex grass, were seldom much bedewed, and often quite dry.

vapour of the earth ; yet, before this happens, the rising The increase of pasture-land in England must have a con. must have been greatly diminished by the surface of the siderable effect in increasing cold by radiation, and in diminish having become much colder.” He adduced the fact ing the amount of vapour in the air at night by deposition on substances on the raised board attracted rather more grass. The sensible moisture at night must be increased near throughout the night than substances lying on the gras the ground, the dew-point being quickly reached on a clear admitted that all the dew on calm, cloudy nights might pight over grass.

tributed to condensation of the earth's vapour, since ca The large quantity of dew found on plates and other objects nights the raised board was dry. over sandy ground, dry to a depth of several inches, proves the But if the grass was moist on these calm, cloudy night possibility of a large émanation of noxious vapours from soil the moisture were owing to earth-vapour, it is only reset containing decaying organic matter below a covering of sand. | 10 inser that a very much larger quantity was owing the The ague of parts of East Anglia and of sandy malarious districts va pour on clear nights when radiation was comparativeir may be thus accounted for

Moreover, the fact that substances on the raised board be Houses built on sandy ground over a damp subsoil may be wetter than substances on the grass may be attributed considered as scarcely more wholesome than is built on the damp non-conducting wood intercepting the warmth radiated IF soil itself.

ground, and thus allowing a substance on the upper suns In late summer and early autumn the high temperature of the ihe board to become colder than a substance on the gras: soil in comparison with the temperature of the surface and of the with regard to the “rising vapour" being greatly dimas air near the ground at night, must have a powerlul effect in the the surface of the ground having become colder, it cx production of vaporous exhalations. The heavy rains which so appear that such diminution actually occurs, owing po often occur in October, the wellest month of the year, must co the influence of the high temperature of the preceding day = operate with a falling air-temperature in driving out air from ing the moist earth at a little depth below the surface 132 the pores of the earth.

same time. I have found the dep ısition of earth-vapes In nearly all the conclusions of Wells, as stated in his ad. ceed at a rapid rate after sunrise over grass. mirable “Essay on Dew," my observations lead me to concur. Wells explains with much ingenuity the reason why las He found that calm is favourable to the precipitation of dew ; trees often remain dry throughout the night, while those that is, in the course of the night, the weather, from being calm are covered with dew. But he does not, I think. | and serene, became windy and cloudy, not only did dew cease to sufficient weight to the fact which he mentions amon form, but that which had formed either di-appeared or diminished that the air near the ground is near one of its sources of 1 considerably; that if the clouds were high and the weather calm, while the tops of trees are removed from that source ! dew sometimes formed io no very inconsiderable extent ; that is both damper and colder near the ground; a stratus dew often forms on shaded grass even several hours before sun- | air rests upon warm earth emitting vapour. The up set, and continues to form after sunrise ; that, if the weather be are pervaded by air which is drier and warmer, and favourable, more dew forms a little before, and, in shaded do not allow air to rest long enough on their cooled places, a little after sunrise, than at any other time ; that on part with sufficient heat in order that condensation in substances elevated a sew feet above the ground it forms much I have found that when the air is clear and not hue later in the evening, but continues to form as long after the tion into space is often not sufficient to cause visiblecer rising of the sun as upon the ground ; that dew is more abun- except in sheltered calın places, and in the same cow dant shortly after rain than during a long tract of dry weather ; / air deposition takes place more on broad surfaces than a that dew is always very copious on those clear and calm nights shoots, threads, and points, and more on the face thos which are followed by misty or foggy mornings, and also on edges of leaves. It appears necessary that a certain clear mornings after clou ly nights, and generally after hot days ; temperature below that of the air, and a certain protec that more dew was formed between midnight and sunrise than re-absorption by the drier portions of air which p-sS CR between sunset and midnight, owing doubiless "to the cold of be attained in order that dew and frost may accumolas the atmosphere being greater in the latter than in the prior part | on the other hand, the air is very moist, with a C of the night ;” that whatever diminishes the view of the sky mist or fog, a very large condensation takes place diminishes the quantity of dew; that a substance placed on a l objects, and especially on those which are at some be raised board of some extent acquired more dew on a very stili the ground, such as the branches and twigs of the night than a similar substance lying on grass ; that bright metals thorns, spiders' webs, and other thin filaments are ** attract dew much less powerfully than other bodies, that a metal | bedewed. Mist or fog osten follows. which has been purposely moistened will often become dry When some mist has formed on such a night, thos

cipitation on trees, &c., which is increased by wind, and large mud on the Aagstone, the rest of the flagstones being perfectly ps of rain on to the ground beneath them. This condition | dry already by the early morning sunshine.” ms best explained by Aitken's discoveries showing the possi Prof. Meldola sends us the following interesting letter which ity of a super-saturation of air when the number of dust

he has received from Corbridge on-Tyne :rticles is unusually small in a mass of air which is humid and pled to saluration. The dust-particles from their minuteness,

"I was much interested by your note in NATURE the other d from their inability to fall below the temperature of the air

day, anent the frost markings of a vegetable pattern. I have ing to the cloud canopy above, do not condense much of

seen just the same forms several times in the north, but it is I e vapour, and consequently any solid object of the same or

think the least common of the patterns usually met with. I ghtly lower temperalure brings about precipitation from the

write, however, to call your attention to Figs. I and 7 of . Plate ssiog air, which may possibly be super saturated. A slight Tor temperature in ihe air, or sometimes an increase of dusi. rticles, then pro tuces fog. A dry fog may thus result from

causing con lensation on a very large number of dust-parles which are radiating heat rather freely, and a damp mist om partial con lensation fro'n super-satura: ed air on a comAratively small number of dust particles not radiating freely wing to a clouded sky.

These considerations explain why a dry fog is densest in ondon and a wet mist den-est in the country. A dry fog is ve work of cold radiating particles, a wet mist is the work of old air mixing with warm. “In a fog,” says Angus Rankin,

the watery vapour in condensing has more particles to condense In, and consequently the particles of sog are smaller, and on neeting with an ohject with a higher temperature, instead of Netting it, the object dries them up by parting with some of its neat. On the other hand, in a mist, the particles of dusi, being ew, have more water condensed on each, and so are larger and do not readily evaporate with small increinents of heat." Yet in a damp mist the addition of a large number of dust-particles, as in a town by day, scarcely increases the density of the mist. In fact, the wet mist is less dense in London than in the country, owing to the higher temperalure and lower humidity of the air. Dry or radiation fogs, which cling to the ground, are the most dense in smoky places.

Io fogs with Trost in winter, such as have occurred several times in the last few years, I have always found the windward side of objects to be much more heavily frosted than the lee. Ward, a'i the rime to attach usel! most to points and edges. ! Fig, 1.-Ice crystallite. " Nature printed” by A Anderson, January 1887. Trees have i hus become laden with rime, even so as to break

Facsimile tracing by J. Maclear, January 1887. Size of crystal 141 down branches ; iron points of railings, splinters of wood, wires, inches X 13 inches. and blades ol grass have borne spikes and fern-like growihs an inch or more long, and heather and sern in hollows have been

vii, illustrating the article on Meteorology in the "Encyclopædia whitened as if with a lall or snow. In weatber of this kind it is

Metropolitana" (1845, vol. i. of plates, vol. v, of text). These difficult to say what is dew or frost proper, and what is deposited

figures are very like yours and some of the others given with moisture from super-saturated air and from fog. On the same

them are also very interesting. I have often shown my students night a white frost may present the characteristics of foy.

when out in the fields in cold weather how exactly the muddeposition in a valley and of clear condensation on a neighbouring hill.

Dew and frost are in fine the result of many causes which inler.operate in a complex manner. The importance of the laws of gases of the multitude of fine adaptations in the relations ni vapour, air, water, earth, and plants; the importance, too, of the thermal receptivity of boundless space, gives an inerest to Inis branch of meteorology which is second only to its beauty.

[merged small][ocr errors][merged small][merged small]

DROF. MELDOLA'S account of Arborescent Frost Patterns

has excited a good deal of interest, and we have received many letters on the subjeci, some of which we have already published. To-day we give retroductions of photographs we Fig 2.--Photograph from the original “Nature print," made have received froin Mr. J. Maclear, Cranleigh. Fig. I re Anderson, of an ice crystallite, January 1887. Size of crystal, 10 inches presents a phtograph of a facsimile tracing of a “ Nature x 7 inches. print" or an ice crystallite taken by Mr. A. Anderson on a still and sunny early morning in lanuary 1887, asier a not very | cum-frost markings of the common feathery volute type imitate severe frost. The sunshine had just dried the rest of the frost the so-called Cauda-galli fossil fucoid (?) which is one of the off the fagsone, and left this mud and ice-crystallization, whii h| most abundant objects on the surface of the carboniserous limele promptly secured on solt piper by means of a sost pad stone courses about here. As far as form goes they are identical, pressure, and thus got a perlec Nature printed impression. and there is no structure to be discovered in the fossil markings. The original (now unfortunaiely lost) showed an appearance “ Corhridge-on-Tyne, December 16. G. A. LEBOUR.” of vegetable (mos) growth, even more strikingly than in this

PROF. Sollas writes to us : The correspondence on this Tracing from it. With regard to Fig. 2 Mr. Maclear writes :-" The melting

uhject that has lately appeared in your columns (particularly ice under the dabbing pad formed a natural pigment with the

| Prof. Bonney's reference), leads me to anticipate a communica.

'ion I hope shortly to present to the Roval Dublin Society on Journal of the Scottish Meteorological Society. Third Series. No. viii lare merely a special case of a very general problem-that on the

he growth of crystals. The arborescent forn.s assumed hy ice

« PreviousContinue »