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problems and exercises are likewise provided. The style of the author is attractive, and the course as a whole has great educational value; in fact, we know of no text-book which presents the subject in a way more suited to the natural capacities of the youthful reader, or which is better adapted to impart a thorough knowledge of concrete geometry, and at the same time to develop the reasoning faculties in a legitimate manner.

There is a chapter describing the vernier, spherometer, callipers, and the micrometer screw gauge, and also treating briefly of the mensuration of the simpler geometrical solids. There are selections of recent examination papers, four-figure logarithms and trigonometrical ratios, answers to numerical problems, and a very useful general index.

If a draughtsman were to criticise the book he would probably say that in measuring and setting uff lengths the scale should be directly applied without the intervention of dividers; that a line to be accurately measured should have its ends clearly defined by short cross-lines; and that diagonal scales, being of little or no practical use, are made rather too much of in the chapter devoted to them. But these are very minor matters, and do not detract from the general rxcellence of the work. We know of no text-book of elementary geometry which can be more fidently recommended to teachers, and none from which students are likely to derive more profit. Les Procédés de Commande à Distance au Moyen de l'Electricité. By Captain Régis Frilley. Pp. vii+ 190. (Paris: Gauthier-Villars, 1906.) Price 3.50 francs.

con

THE problem considered in this volume is that of communicating to a distant mechanism a movement the magnitude, direction, and sense of which are definite functions of those of a transmitting mechanism. The character of the movements which it is desired to transmit varies very much in degree from the simplest of all (traction), in which the three "commands "forwards, backwards, stop—are alone the orders to be obeyed. The author classifies the different mechanisms employed, not according to their complication, but according to the methods that are characteristic of them. These form seven groups-(1) direct action apparatus, (2) apparatus using relays, (3) apparatus employing rotating fields, (4) Wheatstone's bridge devices, (5) apparatus based on the use of induction sparks, (6) escapements, (7) Hertzian waves. The various devices that have been used from time to time are very clearly described under these headings with the aid of diagrams. In chapter viii. an account is given of the commutating device of Lieutenant-Colonel Rivals, by which the sending and receiving instruments can be used as either in turn. Altogether the book forms a very useful and suggestive summary of this very important branch of modern military practice.

Das Radium und die radioactiven Stoffe. By Karl Frhr. von Papius. Pp. viii+90. (Berlin: Gustav Schmidt.) Price 2 marks.

THIS book contains a semi-popular account of radioactive phenomena. The leading experimental facts and the conclusions of their discoverers are described clearly enough, but with little in the way of suggestive comment. The printing and illustrations are good, but we notice a serious error in Fig. 10, which suggests that the B-rays of radium, when deflected by magnetic force, lie in the same plane as the poles of the deflecting magnet. The contrary is, of course, the fact, and such a mistake cannot but suggest serious doubts as to the competence of the author's general scientific knowledge. R. J. S.

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.] Ionisation and Temperature.

THE discourse by Prof. J. J. Thomson, published in NATURE of March 22 (vol. Ixxiii., p. 495), was of importance from several points of view. The explanation of the method of ionisation which he suggests was of especial interest to myself, and I should be pleased if I might be allowed to raise one query concerning it.

Prof. Thomson does not regard the temperature of the gas as having any effect upon the ionisation. It has, indeed, never been shown that high temperature alone would produce ionisation. On the other hand, is there any reason for supposing that ionisation by impact may not take place much more easily at high temperature than at low, and that this is the explanation of the discharge observed by Prof. Thomson? That the gas in this case must have a very high temperature would seem exceedingly probable, for the amount of electrical energy lost in the discharge is very great when compared with the thermal capacity of the gas through which the discharge occurs. Thus in one case when the discharge became luminous the current was 0.045 ampere, the potential difference 50 volts, the distance between the electrodes 5 mm., and the pressure of the gas o-01 mm. The dimensions of the tube are not given, but if we assume the volume of the gas to be 2 c.c., the residual gas to be atmospheric gas, and that the whole electrical energy is used in heating the gas, we should conclude that it would raise it 7.4 X 10 degrees. It is, of course, not to be supposed that the temperature does reach any such value, but we have reason to believe that it reaches a very high temperature, and may it not be that this has a very great effect upon the production of the ions? C. D. CHILD.

Colgate University, Hamilton, N.Y., May 11.

THE average temperature of the gas when the discharge first became luminous was comparatively low; for example, a fine platinum wire immersed in it did not become hot enough to be visible. The figures quoted by Prof. Child refer to the current after the luminous discharge had been well established; the current when the transition from dark to luminous discharge took place was very much smaller, generally less than 10-5 ampere.

A Horizontal Rainbow.

J. J. THOMSON.

J'AI étudié récemment un arc-en-ciel horizontal qui se montrait à la surface d'un petit étang dans les premières heures de la matinée. On l'observait, comme celui dont Mr. W. R. M. Church a envoyé la description à NATURE (April 26, p. 608), en tournant le dos au soleil; et il disparaissait quand la hauteur du soleil était de 44° environ. Il avait la forme d'un arc d'ellipse dont un foyer se serait trouvé à peu près dans l'ombre de la tête de l'observateur. Ses caractéristiques étaient les mêmes que celles de l'arcen-ciel ordinaire ouverture angulaire de 42° sur le bord rouge, largeur de 2°, apparition à 53° (plus rare) d'un second arc plus faible et plus large avec les couleurs disposées dans l'ordre inverse, obscurité de l'espace compris

entre les deux arcs.

Tout invitait donc à chercher l'origine du phénomène dans des sphérules d'eau, qui ne pouvaient être que répandues sur la surface calme. C'est effectivement ce qu'une étude attentive m'a fait découvrir. Les sphérules en question ont généralement quelques dixièmes de millimètre de diamètre. Elles sont très nettement visibles quand on se penche sur l'étang, mais la moindre agitation les fait disparaître. Je les attribue à la rosée déposée à la surface de la nappe tranquille, laquelle est un peu grasse par suite végétaux dans ses eaux stagnantes. L'arc-en-ciel observé de l'existence de nombreuses colonies d'animalcules et de par Mr. Church me semble dû à la même cause: dépôt du brouillard à l'état sphéroïdal sur la surface calme du lac. V. SCHAFFERS.

Louvain (Belgium), rue des Récollets, 11.

NOTES ON SOME CORNISH CIRCLES.'

III.

Boscawen-un, N. lat. 50° 5' 20''.

My wife and I visited Boscawen-un on a pouring

day, when it was impossible to make any observations. Mr. Horton Bolitho, who was with us, introduced us to the tenant of Boscawen-noon-Mr. Hannibal Rowe-who very kindly, in spite of the bad weather, took us to the circle and the stone cross to the N.E. of it.

Lukis thus described this monument 2:"The enclosed ground on which this circle stands is uncultivated and heathy, and slopes gently to the south. Twenty years ago a hedge ran across it and bisected the circle.

monolith enclosed within it was inclined, it is possible that it was upright at that time.

"Dr. Stukeley's supposition was that it originally stood upright, and that somebody digging by it to find treasure disturbed it.'

64

On the north-east side there are two fallen stones which Dr. Borlase, in 1749, imagined to have formed part of a cromlech. It is more probable that they are the fragments of a second pillar which was placed to the north-east of the centre, and as far from it as the existing one is. There are instances, I believe, of two pillars occupying similar positions within a circle. One of the stones, that marked C in my plan, on the eastern side of the ring was prostrate in the doctor's time.

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"This monument is composed of nineteen standing | stones, and is of an oval form, the longer diameter being 80 feet and the shorter 71 feet 6 inches. One of the stones is a block of quartz 4 feet high, and the rest, which are of granite, vary from 2 feet 9 inches to 4 feet 7 inches in height. On the west side there is a gap, whence it is probable that a stone has been removed. Within the area, 9 feet to the south-west from the centre, is a tall monolith, 8 feet out of the ground, which inclines to the north-east, and is 3 feet 3 inches out of the perpendicular.

"In 1594 Camden describes this monument as consisting of nineteen stones, 12 feet from each other, with one much larger than the rest in the centre. It must have been much in the same condition then as now. As he does not say that the

1 Continued from v l. lxxiii., p. 563. 2" Prehistoric Stone Monuments of the British Isles: Cornwall," W. C. Lukis, p. 1.

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and farming operations have changed the conditions of the sight-lines, so that 1 and 3 are just invisible from the circle. This is by no means the only case in which the sighting stone has just been hidden over the brow of a hill and in which signals from an observer on the brow itself have been suggested, or a via sacra to the brow from the circle; there are many monoliths in this direction which certainly never belonged to the circle. From menhir P (No. 2) a fine view is obtained from N. to S. through E., so that the Blind Fiddler and the two large menhirs, and almost the circle, are visible. The curious shapes of 1 and 2 are noted, the east face vertical and the west boundary curved, like several sighting stones on Dartmoor.

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The circle itself has several peculiarities. In the first place, as shown by Lukis, it is not circular, the diameters being about 85 and 65 feet; the minor axis runs through the pillar stone in the centre and the fallen stones" of Dr. Borlase towards the "stone cross (which is no cross but a fine menhir) in Az. N. 43° 15' E. This would suggest that this was the original alignment in 2250 B.C., but against this is the fact that the two stones of the circle between which the "fallen stones 99 lie are more carefully squared than the rest. It is true, however, that this might have been done afterwards, and this seems probable, for they are closer together than the other circle stones.

The one quartz stone occupies an azimuth S. 66° W. It was obviously placed in a post of honour. As a matter of fact, from it the May sun was seen to rise over the centre of the circle.

As there are both at Tregaseal and Boscawen-un alignments suggesting the observation of the summer solstice sunrise, it is desirable here to refer to the azimuths as calculated. For this

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feet in diameter, and the whole monument is, in Lukis's opinion, the most interesting and remarkable in the country. Surrounding the platform there is a ditch I feet wide, and beyond that a penannular vallum about 10 feet in width. The peculiarity of the vallum is that it has three bastions situate on the north-east, north-west, and east sides. It is to the north-east bastion that I wish to refer.

Sighting from the huge monolith, which is now prostrate but originally marked the centre of the circle along a line bisecting the arc of this bastion, we find that the azimuth of the sight-line is N. 25° E.; the angular elevation of the horizon from the 1-inch Ordnance map appears to be about o° 22'. Thus we get in the same form as before :-

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FIG. 9.-Showing the azimuths at the present time and in 1680 B.C. at which the sun rose in Cornwall at the solstice, with different elevations of the sky-line. These are shown at the side.

purpose Fig. 9 has been prepared, which shows these for lat. 50° both at the present day and at the date of the restoration at Stonehenge.

My readers should compare this with the table on p. 33, vol. Ixxii., which gives the solstice sunrise conditions of Stenness in lat. N. 59°. Such a comparison will show how useless it is to pursue these inquiries without taking the latitude and the height of the sky-line into account.

The "Stripple Stones" (lat. 50° 32′ 50′′ N.,
long. 4° 37' W.)

This is a very remarkable circle consisting of 5 erect and 11 prostrate stones situated on a circular level platform 175 feet in diameter on the boggy south slope of Hawk's Tor on Hawkstor Downs in the parish of Blisland. The circle itself is about 148

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THE

THE STABILITY OF SUBMARINES. HE construction of submarines for the Royal Navy began about five years ago. On March 31, twenty-five vessels of the class had been completed, fifteen were building, and twelve more were projected in the Navy Estimates for 1906-7. France at the same date had thirty-nine submarines completed, and fifty building or projected. Russia had thirteen vessels completed and fifteen building. The United States had eight vessels completed and four building, while Congress has recently sanctioned a special vote of 200,000l. for further work on submarines. Germany, Italy, and Japan as yet have done but little, but they are moving in the same direction. An American engineer, Mr. Holland, has exercised the greatest influence on recent submarine design, having worked at the problem for thirty years, and proved himself a worthy successor of his fellow-countrymen Bushnell and Fulton, who were pioneers in submarine construction in the closing years of the eighteenth century and the commencement of the nineteenth. The first five British submarines, ordered in 1900, were repetitions of a type designed by Mr. Holland, tried and approved by the United States Navy Department. Great developments have taken place in later British submarines. Those first built had displacements of 120 tons, surface speeds of eight to nine knots, and gasoline engines of 160 horse-power. Vessels now building have displacements exceeding 300 tons, a surface speed of thirteen knots, and gasoline engines of 850 horse-power. The cost of the earlier vessels was about 35,000l.; that of the later vessels must be twice as great. Other countries have taken similar action, and some are building still larger vessels.

British submarines are kept continuously at work, and this experience has yielded valuable information leading to successive improvements. The vessels chiefly used for experimental purposes up to date belong to the "A" class-200 tons in displacement and ten knots surface speed. Vessels of this class consequently have been most before the public. Their active employment has not been free from accidents, but, having regard to novelty of type and special risks which unavoidably accompany the power of submergence, it is a matter for congratulation that these accidents have not been more numerous and serious in their consequences. Official inquiries have been made into the causes of accidents, and reports have been published. In the opinion of the writer these proceedings showed a tendency to minimise risks necessarily encountered in working submarines. consequently undertook a lengthy series of calculations for typical submarines of different dimensions, in order to ascertain their conditions of stability in various conditions which occur on service. The results for one class are embodied in a paper presented to the Royal Society on May 3, which paper contains also the results of similar calculations made for a cruiser of ordinary form. The distinctive conditions of submarines were emphasised by comparing these results, and the editor of NATURE has suggested that an explanation in popular language of the principal conclusions, based on the investigations, may be of general interest.

He

Submarines are generally "cigar-shaped," with circular or nearly circular cross-sections. This form is adopted in order to provide, with a minimum expenditure of weight, structural strength sufficient to meet severe external fluid pressures which may come upon the hulls if submarines sink to considerable depths. Such depths are not reached intentionally, but experience shows that they may be attained accidentally, and that very quickly.

In ordinary vessels the freeboard is considerable, and the sides are approximately vertical between the lightest draught reached on service and the deepest (load) draught; consequently, within these limits of draught, horizontal sections of the vessels coincident with the water-surface-known as planes of flotation -remain practically constant in form, area, and moments of inertia. In cigar-shaped submarines, with circular cross-sections, the freeboard is small, and the lightest draught of water bears a large proportion to the diameter of the largest circular cross-section. For the typical submarine dealt with in the Royal Society paper, the extreme breadth (diameter of largest crosssection) is a little more than twelve feet, and the lightest draught of water is about ten feet. The circular form of cross-section involves rapid diminution in lengths, breadths, areas, and moments of inertia of successive planes of flotation as the draught of water is increased from light to load. These changes are accompanied by rapid and considerable losses in the stability, and the conditions differ radically from those of ordinary ships. For the typical submarine the extreme length is 150 feet, and breadth extreme 12.2 feet; but the length of water-line at the lightest draught is only 94 feet, and breadth 8.2 feet. When the draught of water is increased eighteen inches (by admitting water-ballast) and the vessel is prepared for diving, the length at the water-line falls to 41 feet, and the breadth to 3.6 feet. In the cruiser of ordinary form an equal change of draught produces small change in length, breadth, and area of the planes of flotation, and these dimensions are practically equal to the extreme length and breadth of the vessel. For the cruiser the moments of inertia of successive planes of flotation about their principal axes remain nearly constant within these limits of variation in draught; whereas for the submarine moments of inertia diminish rapidly as the draught of water is increased. In the cruiser the extreme length is 260 feet, and the metacentre for longitudinal inclinations is 352 feet above the centre of buoyancy at light draught, and 328 feet when the draught is increased by eighteen inches. In the submarine the extreme length is 150 feet, but the corresponding height of longitudinal metacentre above centre of buoyancy is only 37 feet at lightest draught, and falls to feet when the vessel is prepared for diving. At the lightest draught the power of the submarine to resist longitudinal inclinations (changes of trim) is relatively small; in the diving condition it is diminished almost to vanishing point. It will be understood, therefore, that when a submarine is prepared for diving every man has to remain at his station, and no weights must be moved; every opening into the interior must be closed hermetically. The reserve of buoyancy is extremely small in the diving condition. A submarine of more than 200 tons weight may have only 400 to 800 pounds reserve-representing 40 to 80 gallons of water.

Even at their lightest draughts the reserve of buoyancy of submarines is very small as compared with that in other vessels. In good examples it is 6 per cent. of the corresponding displacement-little more than half the lowest percentage accepted for low-freeboard monitors when fully laden, and about one-fourth the corresponding percentage for the deepest laden cargo steamers. Openings into the interior are placed at the tops of conning towers at a considerable height above water, and Admiralty regulations provide that all openings shall be closed before water-ballast is admitted to bring a vessel into the diving condition. Further, it is now provided that before proceeding at full speed at the surface, the maximum reserve of buoyancy shall be secured by emptying ballast tanks. One of the most serious acci

dents that have occurred to British submarines--that to A 8-was unquestionably due in great measure to proceeding at full speed with about half the maximum reserve of buoyancy, certain tanks containing waterballast. The vessel was driven under water as she gathered speed, dipped her bow suddenly, brought the upen top of the conning tower to the waterlevel, was partly filled, and foundered.

Maintenance of the full reserve of buoyancy and lightest draught of water when proceeding at the surface increases safety in two directions. It secures much greater longitudinal stability, and diminishes the tendency to plunge produced by the relative motions of the water surrounding the vessel, especially at the bow. These motions are largely discontinuous, broken water being piled upon the bow, and the phenomena being of such a character that only direct experiment on models or vessels can give accurate information. Such experiments have been made both in this country and abroad, and they indicate the occurrence of a tendency to plunge at certain critical speeds. The problems are still only partially solved, but it is certain that the maximum reserve of buoyancy should be maintained. It also appears desirable to keep the vessels on an even keel, since a cigar-shaped form has then its maximum longitudinal stability for a given mean-draught of water. In the Royal Society paper calculations are recorded showing the diminution of stability accompanying changes of trim in submarines.

especially when applied to large vessels, involves risks of reaching great depths in a short time before buoyancy can be restored. This is recognised in vessels which work on that system, and detachable external weights are fitted, so as to restore buoyancy in cases of emergency.

There has been a considerable increase in the speed of submarines, both at the surface and when submerged. Our latest types are said to have surface speeds of thirteen knots and a radius of action of 500 miles with their gasoline engines, while the underwater speed is ninc knots and radius of action go miles. These higher speeds are attainable, no doubt, but they necessarily involve greater risks, especially in the diving condition. Pressures on horizontal rudders increase as the squares of the speeds, and the extreme sensitiveness of submarines when submerged to the action of external forces tending to produce changes of trim must demand much greater watchfulness, skill, and promptness of action on the part of steersmen than are now required, if greater speeds are to be attained under water. The risks of attaining rapidly excessive depths of submergence must increase as speeds are raised, and they are now far from negligible. At the lightest draughts increase of speed would also involve greater risks of accidental plunging. Exhaustive experiments are necessary, therefore, before designers of submarines commit themselves to the production of vessels having much greater surface speeds, and still more of vessels having much greater under-water speeds. Submarine design is not a task to be lightly undertaken by amateurs; it requires thorough experimental and scientific treatment by competent naval architects, who should be furnished by naval officers with the strategical and tactical conditions to be fulfilled in the completed vessels, and should ascertain what is involved in the fulfilment of these conditions. W. H. WHITE.

I1

THE RISE AND PROGRESS OF THE
ZOOLOGICAL SOCIETY.1

T was a happy thought on Mr. Scherren's part to tell the story of the Zoological Society of London, and he is to be congratulated on the success with which he has accomplished his evidently congenial task. The history of a development is always interesting, especially when it is still progressing, and there is, moreover, a strong personal interest in the book, since many eminent workers, whose names and deeds are familiar, have cooperated in various ways in furthering the welfare of the society since its inception in 1826. Mr. Scherren's book is not only a careful contribution to the history of zoology in Britain during the last eighty years, but is at the same time good reading for its revelation of what goes on behind the scenes in a scientific society, and for its record of many interesting events in what is familiarly called the "Zoo.”

In modern submarines of large size the operation of diving is performed when the vessels have headway. Horizontal rudders, controlled by skilled men, are employed as the active means of depressing the bow. The pressures on the upper surface of the vessel resulting from the relative movement of the surrounding water develop a vertical component acting downwards which overcomes the small reserve of buoyancy and the vertical component of the pressures on the rudder. The submarine then moves obliquely downwards. When the desired depth below the surface has been reached the steersman operates the horizontal rudders in such a manner that the vessel shall advance on a practically horizontal course, although it really is an undulating one. Watchfulness and skill are necessary to achieve this result, and there must be no movements of men or weights which would vary the position of the centre of gravity. If such movements become necessary-as, for example, when torpedoes are discharged-compensation must be arranged to take effect at once. Failures to comply with these conditions may involve serious consequences, and have caused submarines to dive to great depths. With trained and disciplined crews such accidents are rare. Plans for automatic maintenance of any desired depth-similar to those used in locomotive torpedoes have been brought forward and tried; but for large submarines manual control has been found preferable. In small submarines it has been found possible to dive without headway On November 29, 1822, John Ray's birthday, a by varving the volume of displacement, admitting bud from the Linnean Society formed itself into a water into suitable chambers from which it can be "Zoological Club," which four years afterwards took readily expelled when the desired depth has been shape as the Zoological Society. There were 342 reached, and a balance restored between weight and members at the close of the year, and there are now buoyancy. Such methods involve the necessity for ten times as many, In 1828, when the gardens were minute and rapid adjustments, which can be secured opened to the public, there were about 600 specimens, on a small scale much more readily and certainly and there is now a specimen for each F.Z.S. A than on a large scale. As a consequence, horizontal farm for breeding purposes and experimental work rudders and headway have been generally adopted for (from which nothing very noteworthy ever resulted) large submarines, and have answered well on the was established in 1829 at Kingston Hill, and scientific whole. One great advantage of the plan is that when meetings began to be held in 1830. Such were the headway ceases the horizontal rudders become in1 "The Zoological Society of London: a Sketch of its Foundation and operative the small reserve of buoyancy reasserts Development, and the Story of its Farm, Museum, Gardens, Menagerie and Library." By Henry Scherren, F.Z.S Pp. xii+252; 12 coloured itself, and the submarine comes to the surface. The other system—varying the volume of displacement-plates, so uncoloured plates, 9 plans. (London: Cassell and Co., Ltd., Price 30s. net.

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