466 



NATURE 



[March 20, 1902 



the phenomena of meteorological optics had, I believe, been 

 worked out by Babinet about ten years earlier. 



As to the vertical pillar of light frequently observed in high 

 latitudes after sunset and before sunrise, and occasionally seen 

 in latitudes as low as ours, il may be attributed to spiculiv of ice 

 which, whether isolated or radiating from a centre as in crystals 

 of snow, will assume a horizontal position if they subside through 

 a portion of the atmosphere which is quite free from convection 

 currents. Those of the horizontal spicuUi; which are vertically 

 over the cone connecting the spectator's eye and the sun will 

 include some that can directly reflect solar light to his eye, and 

 it is these that produce the plienomenon which was seen by Mr. 

 Knight. The pillar may be ex]iecled to be white till the sun 

 gets some distance below the horizon, when it will in succession 

 assume the colours due to the absorption and dispersion of light 

 by the atmosphere. 



It can easily be shown experimentally that if the air be 

 free from the minute convection currents which so trouble 

 astronomers (which it seldom is), then subsiding spicule of ice will 

 be horizontal. To show this, cut from a sheet of stiff paper a 

 straight, long and narrow strip, and let it fall through the air. 

 The experiment is a pretty one when the strip of paper is thrown 

 out of an upper window on a calm day. The strip falls not 

 lengthwise, but sideways, and spins round its long horizontal 

 axis. The dynamics of this phenomenon have not, I think, as 

 yet been worked out. The explanation would require an in- 

 vestigation of the stream lines surrounding a body rotating as 

 well as progressing through a fluid. It seems to be a problem 

 which might with advantage be proposed to the mathematical 

 research scholars of our Universities. 



30 Ledbury Road, W., March i6. G. Johnstone Stoney. 



Proofs of Euclid I. 5. 

 Besides the proofs cited by I'rof. Bryan (p. 438), another is 

 ■equally worthy of notice, and requires no construction. The 

 sides of the triangle ABC may be regarded as taken in two 

 ■orders — 



AB, AC and l A 

 = AC, AB and / A 

 . •. L opp. AB = z opp. AC. 

 This is a variation of the proof by duplication, but avoids this 

 process. As in the case of the proof cited by Prof. Bryan and 

 involving limiting values, the proof given above is not altogether 

 satisfactory for the use of beginners, and is, of course, of no 

 value to the advancing student except as an interesting illustra- 

 tion of method. H. W. Croome Smith. 

 Bristol, March 15. 



As Prof. Bryan is discussing proofs of Euclid's I. 5, may I 

 call attention to the way I proved it in my " Foundations of 

 Geometry," namely as a corollary to the equivalent of I. 4 ? 

 Thus— 



" For if AB in the above proof had been equal to AC, the 

 triangle ABC might also have been moved so that AB fell on 

 DF, and AC on DE, and the triangles would have been con- 

 gruent so. Hence both the angles ABC and ACB would be 

 shown to be equal to DEF, and therefore to each other." 



This seems to me far and away simpler than any other proof 

 I know of, and it has the advantage of directing attention to 

 the fact that the proof of I. 4 as often as not involves turning 

 the triangle over in the air, while moving it ; so that, for 

 example, the proof would not apply as it stands to spherical 

 triangles. Edward T. Dixon. 



Racketts, Hythe, Hants, March 16. 



THE NATIONAL PHYSICAL LABORATORY. 

 COME further account of the National Physical 

 ^ Laboratory, which is being opened by H.R.I 1. the 

 Prince of Wales, accompanied by H.R.H. the Princess, 

 as these lines go to press, may be of interest to readers 

 of N.'VTURE. A description of Bushy House, with plans, 

 has already appeared ; the alterations required to fit it 

 for a laboratory are now complete, and the new Iniild- 

 inv;s erected for the engineering depaitment aie ready 

 for use. The following extracts from the report of the 

 ■executive committee will indicate what has been done : — 



NO. 1690, VOL. 65] 



The basement and ground floor of Bushy House have 

 been transformed into a physical laboratory, while the 

 upper floors forjn offices and a residence for the director. 

 The basement is covered with a brick groining, on which 

 the main building rests, but the more important labora- 

 tories are in four large wings, one at each corner, and 

 these have no basement below, thus steady supports are 

 everywhere possible. 



One wing, containing the original dining-room and 

 library, has been fitted as an electrical and magnetic 

 laboratory. .\11 iron has been, as far as possible, removed 

 from the structure, and, with a view of preventing a stray 

 magnetic field from any currents which may be used, 

 concentric wiring has been employed for all large 

 currents, while the wires for smaller currents have been 

 twisted. 



In this room will be placed the Lorenz apparatus 

 which the Drapers' Company has recently with great 

 generosity given to the Laboratory in memory of the 

 distinguished services to science and to education of the 

 late Pnncipal J. \'. Jones, F.R.S., of Cardiff. Along 

 with this there will be other apparatus for the absolute 

 measurement of current and of electromotive force. 



Another wing has been fitted for thermometric work. 

 A special study will be made of high-temperature ther- 

 mometers, and the laboratory owes to the generosity of 

 .Sir \. Noble the means for installing a number of electric 

 ovens for testing thermopiles and other instruments for 

 the measurement of temperature up to 1000' or 1200° 

 Centigrade. 



In a third wing a metallurgical laboratory has been 

 fitted in which to continue the work begun at the Mint 

 by Sir William Roberts-.'\usten and the Alloys Research 

 Committee. For this purpose apparatus for cutting and 

 polishing sections and further photomicrographical 

 examination has been obtained. The committee has 

 to thank Mr. Stead for his assistance in arranging this. 

 The fourth wing is fitted as a chemical laboratory. In 

 the basement are a number of constant temperature 

 rooms. 



Sir Andrew Noble's fund, referred to in the last report, 

 has provided a measuring machine, a dividing engine 

 and a comparator, which will be placed in some of the 

 basement rooms. In an adjoining rooin the resistance 

 measurements of the British Association Committee will 

 be continued, while in another, apparatus for the pro- 

 duction of liquid air is being set up. The testing of 

 pressure gauges will form an important branch of the 

 work, and for this a mercury column some fifty feet in 

 height has been erected in one corner of the house. 



Gas and water have been laid on freely throughout 

 the building — also electricity. A loo-volt circuit is con- 

 nected to the main dynamo and battery in the power- 

 house, and supplies light. Numerous plug points enable 

 a supply to be taken off for lights for experimental 

 purposes or for small motors. For experimental work a 

 special battery of fifty-five cells has been installed. This 

 is divided into groups of five. Wires run from the switch- 

 board to the various rooms in such a way that one or 

 more of these groups can be switched on to any circuit. 

 Thus voltages between 2 and 1 10 volts can be obtained 

 as required. 



The house is heated on the Webster low-pressure 

 system by steam from a Lancashire boiler in the boiler- 

 house at a distance of about 100 yards. The boiler also 

 supplies steam to one of Parson's 60-kilowatt turbo- 

 generators, which is the main source of power. The 

 power-house also contains an iS-h.p. Crossley gas- 

 engine, driving a 1 2-kilowatt dynamo by T. Parker and 

 Co. This serves as a stand-by and for charging the 

 main battery of fifty-eight chloride cells. 



The engineering laboratory, a building eighty feet by 

 fifty feet, adjoins the power-house. This is divided into 

 two bays ; a shaft, driven by a motor supplied by Mather 



