industrial discoveries along these lines have been very far-reaching. The success attending the investigation of various chemical reactions occurring at high temperatures has caused a marked revival in the interest taken in inorganic chemical research. This has been especially noticeable on the Continent, where, to a much greater extent than with us, the brilliant and rapid development of organic chemistry had led to a marked neglect of this older branch of the science. The technical results are hardly less important. Several new and flourishing industries have been firmly established, some of them supplying hitherto unknown materials, which are proving themselves of great value in the arts. A still wider field of usefulness for the electric methods of heating seems now to be opening up. So far as the electrolytic and high temperature applications are concerned, there has been no direct competition with any existing technical processes. But now that the engineer and chemist have become familiar with the use of the electric furnace, there is a great tendency to extend its employment to work which requires temperatures already attainable by fuel heating if properly applied. The possibility of generating the heat just where it is required, the ease of regulation of temperature, and the accompanying economy of heat losses, are the chief factors which tell in favour of electric heating under these conditions. The production of carbon bisulphide and the rapid development of the electrical manufacture of steel form excellent examples of what is being achieved technically in this direction; whilst even in the laboratory electrically heated tube and muffle furnaces are being largely employed in place of those heated by gas. It is with the interesting details of such subjects as these that M. Minet is concerned. In view of the fact that this is but the first part of his complete work, it is impossible to do more than point out these main divisions of the subject. The author has drawn largely on the patent literature, and has copiously illustrated his descriptions with excellent diagrams and with the portraits of many of the leading investigators in this field of work. R. S. HUTTON. OUR BOOK SHELF. Elementary Microscopy. By F. Shillington Scales, F.R.M.S. Pp. xii + 179. (London: Baillière, Tindall and Cox, 1905.) Price 38. net. No instrument of research has such wide application in various branches of science and commerce as the microscope. It is, perhaps, scarcely too much to say that the principles underlying its construction and use are often disregarded by those who employ it, and sometimes totally ignored. Any treatise, therefore, on this subject, however unpretentious, is to be cordially welcomed, and the book now under notice is one that should meet a pressing need. It is written for beginners or for those who have used a microscope without troubling to understand it, and who consequently have never by any chance used it at its best. The book commences with a description of various simple magnifiers and a descriptive diagram showing the essential parts of a microscope. These parts and the various accessories are in turn described more fully, as well as such appliances as are usually only found in the best instruments. use. The most important points, such as substage condensers and fine adjustment construction, are treated somewhat fully. As to the choice of a microscope, reference is made to the fact that in medical schools and elementary science laboratories, where the cheaper form of instrument is usually provided, still no instruction is given as to its use, and that it is too often looked on as a mere magnifying glass. This is unquestionably true, and it is much to be deprecated that, in cases where the microscope performs such an important part in the work of instruction, no attention whatever is bestowed on its principles and The most interesting paragraphs in the book are, perhaps, those in which a comparison is made between the English and Continental stand. That the form of instrument now known as the English model is generally much superior in design and construction to the Continental stands is admitted and insisted on by the majority of those whose opinion is of value. At no period for many years past has the English microscope stand held such a high place, and it is greatly to be hoped that those who are in a position which gives them opportunities of recommending one form or another will recognise this. It is much to be regretted that, so far as objectives are concerned, the same cannot be said. Some English makers do undoubtedly produce lenses of good quality, but the average is not so high, and the finest objectives produced by Messrs. Zeiss are still unexcelled by those of any other makers. In the production of substage optical appliances, this country holds, as it has always done, a very high position, and it is difficult to understand why the same cannot be said of objectives. All the usual microscope accessories, as well as their method of use, are described as fully as the circumstances permit. Chapters vi. and vii. are devoted to the practical optics of the microscope and its manipulation. This be carefully studied. Perhaps more space might have is the most important section of the book, and should been devoted to this, although it is quite easy to understand the difficulties that might arise in attempting anything like an exhaustive treatise on microscopic optics, debatable as the subject still is. a Altogether, the book is to be commended as genuine attempt to treat the subject in a simpl straightforward manner, so that the reader for whom it is primarily intended may grasp its meaning without difficulty. J. E. B. THESE pages, as we are told in the preface, are intended to serve no other purpose than to aid the memory of the busy photographer, and if possible to anticipate his daily needs. An examination of the book shows that the editor has very successfully accomplished his task, and at the same time has not made the volume of such a bulky nature as to render its size inconvenient. It is true that more references might have been inserted, but such an addition would perhaps be questionable. The four sections into which the book is divided include a dictionary of practical hints, dodges, &c.; a collection of tables, weights, measures, everyday formulæ, &c.; a directory of the photographic societies of Great Britain and Ireland; and finally, a set of indices to the first twelve numbers of the present (library) series of the Practical Photographer. Each of these sections is arranged so far as possible alphabetically, so that ready reference is greatly facilitated. We thus have a concise and practical dictionary which should be found of very general utility. Nineteenth fractive index n conveying progressive and regressive waves Murray's Handbook of Travel-Talk. THAT this little pocket-book meets the requirements of travellers is shown by the fact that this is the nineteenth edition that has been issued. The success of such a companion depends mainly on the arrangement and scope of the material which it contains, and on these points it seems difficult to suggest any improvements. This edition is divided into fourteen distinct but comprehensive groups of subjects, each one containing exclusively those words and phrases which naturally belong to each section. Great pains seem to have been taken to bring the information up to date, motoring, for example, having quite a large part devoted to it. The Britisher is equally helped in either French, German, or Italian, and such a vade mecum as is here presented should be found of great service to everyone who crosses the Channel. LETTERS TO THE EDITOR. [The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.] a The Pressure of Radiation on a Clear Glass Vane. IN NATURE, June 29, a letter from Mr. G. F. Hull appeared under the above title. In it the writer claims to have verified experimentally that the pressure upon transparent vane is equal to the difference in the density of energy in front of and behind the vane, and reference is made to a difference of views regarding the theory of the pressure in a non-absorbing medium. In regard to the latter point, the same result is obtained for the particular case in question whether the beam of light is considered simply as a carrier of momentum or whether the pressure due to radiation is regarded as arising from a mechanical bodily force integrated throughout the material medium in which the radiation is being propagated. Consider the latter theory for steady radiation consisting of plane polarised waves of simple harmonic period 2/Kc propagated along Ox (see Larmor, Phil. Mag., vol. vii., p. 578, 1904). If all the interfaces are perpendicular to Ox, then y and Y are continuous throughout, whether the medium vary continuously or abruptly; consequently the mean value of the mechanical force upon any slice of the medium can be expressed as a pressure per unit area upon each surface equal in amount to the mean value of (y2+Y2)/8 at the surface. Thus for any vane suspended in free æther (or air) the resultant mechanical force is equivalent to a pressure per unit area equal to the difference in energydensity in front of and behind the vane. The apparent confusion arises from the usual statement that the mean value of 2+ Y can only vary along Ox in the case of an absorbing medium, but this is true only for progressive waves. For a transparent medium of re 1 consequently the resultant pressure is equal to St. John's College, Cambridge, July 14. An Omitted Safeguard. IN two schemes set out in a recent issue of NATURE, one dealing with the requirements of Oxford and one with the organisation of applied science in London, there appears a noteworthy omission. If the weather is proverbially the first topic of conversation of Englishmen, it is surely because of the influence it has on the well-being of the community. Yet in both the schedules referred to no provision is made for research in meteorology. It is singular how tardy is the recognition of so important a factor in the national welfare. It is to meteorology that we constantly appeal for help. By its daily survey of rainfall it safeguards our water supply (now a very anxious problem, being outpaced by the ever-increasing demands of population, sanitation, railways, or manufacturing machinery). We turn to it for the comparison of localities and to study the effects of climate or fog upon health and disease, or to ascertain the relations of temperature, sunshine, or rainfall to the prosperity of the crops and fruit gardens. We look to the readings of the barometer to protect the safety of those working underground. Meteorology takes cognisance of the force of the wind for the protection of structures, or of storms likely to imperil the mariner on his voyage, and by the extension of, and the improved modes of, forecasting the weather is becoming each year of greater service to all. Without encroaching further upon the limits of your space, sufficient has perhaps been said to show primâ facie grounds (while so much is proposed to be devoted to physics, geology, or botany) for the consideration of a possible chair in meteorology, or for in some other way repairing an omission of so serious a kind in the schemes lately propounded. The large amount devoted annually to meteorology in the United States shows the appreciation of its utility to all classes of the community by so practical a people as the Americans, and that the outlay is amply recouped by the value of the services rendered by it. RICHARD BENTLEY. The Hydrometer as a Seismometer. IN NATURE of June 29 Mr. Bennett discusses the motion of a floating hydrometer when vertical motion is imparted to the (rigid) vessel containing the (incompressible) fluid in which the hydrometer floats. The solution offered is that the whole system moves precisely as a rigid body would move, and this solution clearly satisfies the very simple equations of motion in the problem considered. But is such motion stable? In general it is not, and I believe that Faraday studied experimentally the crispations of a free surface of liquid when small vertical oscillations were imparted to the containing vessel. This hardly affects Mr. Bennett's conclusion that a floating hydrometer is an unsatisfactory form of seismometer, but perhaps it may explain the positive results which some observers have obtained; elastic yielding of vessel or hydrometer, although conceivably an adequate explanation, is not the only one open to us. NOTES ON STONEHENGE. VIII. ON THE DARTMOOR AVENUES (Continued). MY Y inquiries began at Merrivale because there is a circle associated with the avenues a little to the south of the west end of the longest; and again nearly, or quite, south of this there is a fine menhir, possibly used to give a north-south line. There is another menhir given on the Ordnance map, azimuth N. 70° 30′ E., which, with hills 3° high, points out roughly the place of sunrise from the circle in May (April 29). Although this stone has been squared and initialed, I think I am justified in claiming it as an ancient monument. There is still another, azimuth N. 83° E., giving a line from the circle almost parallel to the avenue. I hope some local achæologist will examine it, for if ancient it will tell us whether the N. avenue or the circle was built first, a point of which it is difficult to overrate the importance, as it will show the strict relationship between the astronomy of the avenues and that of the circle, and we can now, I think, deal with the astronomical use of circles after the results obtained at Stonehenge, Stenness and the Hurlers as an accepted fact. With the above approximate values the date comes out 1750 B.C., the declination of the Pleiades being N. 6° 35': I now pass on from Merrivale as an example of those avenues the direction of which lies somewhere in the E.-W. direction. Others which I have not seen, given by Rowe, are at Assacombe, Drizzlecombe and Trowlesworthy; these Mr. Worth adds Harter or Har Tor (or Black Tor). to The avenues which lie nearly N. and S. are more numerous. Rowe gives the following:-Fernworthy, Challacombe, Trowlesworthy, Stalldon Moor, Batterdon, Hook Lake, and Tristis Rock. Of these I have visited the first two, as well as one on Shovel Down not named by Rowe, and the next two I have studied on the 6-inch Ordnance map. either of the remaining avenues, but one large menhir terminates one row of stones. The others may have been removed. So it is probable that the alignment was to the north. If so, we are dealing with the setting of Arcturus, warning the summer solstice sunrise in 1860 B.C. To the S. the hills are 4° 48′, to the N. 4° 50′. To this result some importance must be attached, first, because it brings us into presence of the cult of the solstitial year, secondly, because it shows us that the system most in vogue in Brittany was introduced in relation to that year. In Brittany, as I have before shown, the complicated alignments, there are 11 parallel rows at Le Ménac (p. 99) (there were 8 To Barrow Solstice. noverver Samet To Cross FIG. 20.-The sight-lines at the Hurlers, showing high northern azimuths among others. From the Fernworthy (lat. 50° 38').-Here are two avenues, one with azimuth N. 15° 45' E., hills 1° 15'. There is a sighting stone at the N. end. We appear to be dealing with Arcturus 1610 B.C. This is about the date of the erection of the N. avenue at Merrivale. The second avenue has its sighting stone built into a wall at the south end. Looking south along the avenue, the conditions are azimuth S. 8° 42' W., hills 3° 30'. Both these avenues are aligned on points within, but not at the centre of, the circle. Challacombe (lat. 50° 36'). This is a case of a triple avenue, probably the remains of eight ows, in a depression between two hills, Challacombe Down and Warrington. There is no circle. The azimuth is 23° 37′ N.W. or S.E., according to direction. The northern end has been destroyed by an old stream work; there is no blocking stone to the south on 1 Continued from p. 248. Ordnance map. parallel rows at Challacombe), were set up to watch the May and August sunrises, and the solstitial alignments came afterwards. The Brittany May alignments, therefore, were probably used long before 1860 B.C., the date we have found for Challacombe, where not the sun rise, but the setting star which gave warning of it was observed. It is worth while to point out that at Challacombe, as elsewhere, the pries; astronomers so located their monuments that the nearly circumpolar stars which were so useful to them should rise over an horizon of some angular height. In this way the direction-lines would be useful for a longer period of time, for near the north point the change of azimuth with change in the declination of the star observed is very rapid. Shovel Down, near Batworthy (lat. 50° 39′ 20′′).— A group of five rows of stones, four double, one single, with two sets of azimuths. One set gives us 220, 250, and 28°. They seem to be associated. I will call them A, B, and C. A is directed to the circle on Godleigh Common. Its ends are free. B is a single line of stones to the E. of the triple circle, about which more presently. It is not marked on the Ordnance map; its ends are also free. C has its south end blocked, I think in later times, was towards the north; the height of the horizon I measured as 45'. It may have been an attempt to mark the N. point of the horizon. The triple circle to which I have referred is not an ordinary circle. I believe it to be a later added, much embellished, cairn. According to Ormerod, the diameters are 26, 20, and 3 feet, and there are three stones at the centre. 66 All the above avenues are on the slope of the hill to the north. On the south slope we find the longest of all, as shown on the Ordnance map survey of 1885. There is a long stone" in its centre, and at the southern end was formerly cromlech, the "three boys." " Part of this avenue, and two of the three "boys," have been taken to build a wall. The long stone remains, because it is a boundary stone! FIG. 21.-The sight-lines at Trowlesworthy, showing high northern azimuths among others. the Ordnance map. by a kistvaen. The astronomical direction may be, therefore, either N.W. or S.E. We find, however, a probable use in the N.W. quadrant, as at Challacombe, Arcturus setting at daybreak as a warner of the summer solstice. From a The azimuth is 2° 30′ W. of north or E. of south. Looking N. from the long stone, the heigh of the horizon is 2° 30'. I think this avenue was an attempt to mark the S. point. most Trowlesworthy (lat. 50° 27' 30"). --The remains here are interesting. This is the only monument on Dartmoor in which I have so far traced any attempt to locate the sun's place at rising either for the May or solstitial year. But I will deal with the N.-S. avenue first, as it is this feature which associates it with Fernworthy and Challacombe, and in order that a comparison may be made I append a map showing the sight-lines at the Hurlers (Fig. 20). As at Merrivale, the avenue has a decided "kink " or change of direction. The facts as gathered from the 6-inch map are as follows: S. part of Avenue N. 41 6 20 2130 B.C. 2080 B.C. FIG. 22. The remains of the eight rows of the Challacombe Avenue looking North of East, terminal menhir to the extreme right. This date is very nearly that of the use of the S. circle at the Hurlers, and it is early for Dartmoor; but it is quite possible that local observations on an associated avenue a little to the west of the circle which terminates the N.-S. avenue will justify it. This is not far from parallel to that at Merrivale, but its northern azimuth is greater, so that if it turns out to have been aligned on the Pleiades its date will be some time before that of Merrivale, that is, before 1680 B.C. I can say nothing more about it until I have visited it. The new features to which I have referred are two tumuli which in all probability represent more recent additions to the original scheme of observation, as we have found at Stenness; and show that Trowlesworthy was for long one of the chief centres of worship on Dartmoor. Their azimuths are S. 64° E. and S. 49° W., dealing, therefore, with the May year sunrises in November and February and the solstitial sunset in December. It is probable that, as at the Hurlers, tumuli were used instead of stones not earlier than 1900 B.C. Stalldon Moor (lat. 50° 27′ 45′′) I have already THE incidentally referred to. The azimuth of the stone row as it leaves the circle, not from its centre as I read the 6-inch map, is N. 3° E.; as the azimuth gradually increases for a time, we may be dealing with Arcturus, but local observation is necessary. The differences between the Cornish and Dartmoor monuments give much food for thought, and it is to be hoped that they will be carefully studied by future students of orientation, as so many questions are suggested. I will refer to some of them. (1) Are the avenues, chiefly consisting of two rows of stones, a reflection of the sphinx avenues of Egypt? and, if so, how can the intensification of them on Dartmoor be explained? (2) Was there a double worship going on in the avenues and the circles at the same time? if not, why were the former not aligned on the circles? On a dead level, of course, if the avenues were aligned on the centre of the circle towards the rising or setting of the sun or a star, the procession in the via sacra would block the view of those in the circle. We have the avenue at Stonehenge undoubtedly aligned on the centre of the circle, but there the naos was on an eminence, so that the procession in the avenue was always below the level of the horizon, and so did not block the view. (3) Do all the cairns and cists in the avenues represent later additions, so late, indeed, that they may have been added after the avenues had ceased to be used for ceremonial purposes? The cairn at nearly the central point of the S. avenue at Merrivale was certainly not there as a part of the structure when the avenue was first used as a via sacra for observing the rising of the Pleiades. I have always held that these ancient temples, and even their attendant long and chambered barrows, were for the living and not for the dead, and this view has been strengthened by what I have observed on Dartmoor. There was good reason for burials after the sacred nature of the spot had been established, and they may have taken place at any time since; the most probable time being after 1000 B.C. up to a date as recent as archæologists may consider probable. Mr. Worth, whose long labours on the Dartmoor avenues give such importance to his opinions, objects to the astronomical use of those avenues because there are so many of them; he informs me that he knows of 50; I think this objection may be considered less valid if the avenues show that they were dedicated to different sacred uses at different times of the year. For instance, Challacombe is not a duplicate of Merrivale; one is solstitial, the other deals with the May year, and a complete examination of them-I have only worked on the fringe--may show other differences having the same bearing. In favour of the astronomical view it must be borne in mind that the results obtained in Devon and Cornwall are remarkably similar, and the dates are roughly the same. Among the whole host of heaven from which objectors urge it is free for me to select any star I choose, at present only six stars have been considered, two of which were certainly used afterwards at Athens; and these six stars are shown by THE BOTANICAL CONGRESS AT VIENNA. HE International Botanical Congress, held at Vienna on June 11-18, was an impressive demonstration of the activity of botany as a science, and of the enthusiasm of its adherents. Vienna is not the most central town for a meeting-place, but, nevertheless, more than six hundred botanists, men and women, representing nearly all the important, and many of the less important, botanical institutions of the world, met together there. As might have been expected, the central European element predominated, but there were a goodly number of Americans representing the southern and far western as well as the eastern States, while from the Far East came a deputation of two Chinese. On the first day of the Congress, members were invited to be present at the opening of the Botanical Exhibition, which was held in the orangery of the historic Palace of Schönbrunn, just outside the town. The exhibition was an interesting one, and gave a good idea of the present position of botany from a teaching as well as from a more general point of view. I here were fine series of diagrams, and coloured photographic lantern-slides of microscopic preparations, flowers, plant associations, and other objects; living cultures of Algæ; apparatus of all kinds; and some beautiful photographs of tropical vegetation in Brazil, Malaya, and elsewhere. A remarkable feature was the unique specimen of Fockea capensis, a member of the family Asclepiadaceæ, which, originally brought from the Cape, still remains the only known specimen. The plant has a hard, woody rhizome, as big as a child's head, from which in the rainy season numerous shoots are developed. It was figured and described by Jacquin in his Fragmenta" at the beginning of the last century. The Botanic Garden of Schönbrunn brings to mind. at any rate for the systematic botanist, the name of Jacquin, and some of his manuscript and original drawings were an important feature of the exhibition, a subject of envious admiration of certain American botanists; we in London are proud to possess some of Jacquin's work, in the form of botanical letters to Sir Joseph Banks's librarian, Dryander, copiously illustrated with exquisitely delicate drawings. His herbarium, consisting largely of plants cultivated in the Vienna and Schönbrunn gardens, was bought by Banks, and is now in the general collection at the Natural History Museum Nicolas Joseph Jacquin was professor of chemistry and botany at Vienna from 1768-96; later in the week of the congress a bust was unveiled in his honour in the Fest-Saale of the university. To quote from Prof. Wiesner's appreciation at the ceremony:-"His broad horizon and great powers of organisation were shown in the fact that, in the second half of the eighteenth century, no scientific, and especially no natural scientific, undertaking was started in which Jacquin did not take an important part. He embodied the ideal of the academic teacher." On the same occasion was also unveiled the bust of Jan Ingenhousz (1730-99), a Netherlander by birth, who spent the greater part of his working life in Vienna. Physician to the Empress Maria Theresa and the Emperor Joseph II., botanists know him best as one of the earliest workers in the sphere of plant and |