626 



NA TURE 



[October 24, 1901 



LETTERS TO THE EDITOR. 



[ T/u Editor does not hold himself responsilile for opinions ex- 

 pressed by his correspondents. A'eit/ur ean he undertake 

 to return, or to cot respond with the writers of reja.ei 

 :nanns(ripls intended for this or any other part of Natuke. 

 No notice is taken of anonvmous communications.'^ 



A Simple Model for Demonstrating Beat. 



The phenomenon of beat produced by the interference of two 

 series of waves having nearly the same wave-length can be 

 objectively represented by a model of simple construction. 



A spiral, whose diameter and pitch are respectively 2 cm. 

 and 25 cm., is made of a steel wire about i mm. thick and 

 hung vertically before a white screen. At a distance of a few 

 metres we observe a very regular series of transverse waves. 

 Another spring of exaclly the same dimensions is suspended in 

 front of the first spring so as to coincide with each other when 

 they are seen at a distance. If one of the springs is then 

 slightly stretched, there results a small difference in wave-length 

 of the two sets of waves, thus causing them to strengthen in one 

 place and destroy in the other. The distance between these 



A. G. Nathorst, himself renowned as an investigator of the 

 Polar regions : — 



" It may perhaps be of interest at the present time to recall 

 the fact that we in Sweden have always taken for granted that 

 the leader of a scientific expedition must be a naturalist, to 

 whom the comm,ander of the vessel has to be subordinate." — 

 Gcofp-aph. Anzeiger, ii. p. 129, September, igot. 



The marked success of the Swedish expeditions, not merely 

 in pure science, but also in geographical discovery and the safe 

 return of their members, gives this opinion weight as well as 

 interest. Of course it is not what the president of the Geogra- 

 phical Section of the British Association calls " the good old 

 British plan." Civilian. 



Fig 2 





Fig h 





Fi c 4 



two places becomes less as the difterence of wave-length in- 

 creases. Figs. I, 2, 3, 4 are the photographs of the springs 

 suspended in the manner just mentioned, and show successive 

 stages of interference produced by stretching the length of the 

 second spring. The result of interference of two such waves 

 evidently corresponds to the phenomenon of beat. 



For practical purposes, two springs are suspended from a 

 veitical board, one in front of the other, as shown in Fig. 5. 

 Both ends of the first spring ate fixed, while the upper end of 

 the second is likewise fixed and the lower end pulled downwards 

 by means of a string passing through a hook attached to the 

 stand. Standing at a distance in front of the springs we can 

 gradually stretch the second spring by pulling the string and 

 easily observe the corresponding stages of interference in its 

 difl^erent phases. ■-'> K. Honda. 



Physical Laboratory, Imp. Univ., Tokio, Japan, May 26. 



Polar Exploration. 



The following sentence is extracted from an admirable notice 

 of the Arctic explorer, the late Baron Nordenskiold, by Prof. 



OlV THE CLUSTERING OF GRAVITATIONAL 

 MATTER IN ANY PART OF THE L'NIVERSE. 

 T N the Mathematical and Physical Science Section of 

 ■'■ the British .Association, Lord Kelvin delivered a 

 discourse on "The Absolute Amount of Gravitational 

 Matter in any large ^'olume of Interstellar Space." Gravi- 

 tational matter, according to our ideas of universal gravi- 

 tation, would be all matter. Now was there any matter 

 which was not subject to the law of gravitation? He 

 thought he might say with absolute decision that there 

 was. They were all convinced, with their President, that 

 ether was matter, but they were forced to say that the 

 properties of molar matter were not to be looked for in 

 ether as generally known to them by action resulting from 

 force between atoms and matter, ether and ether, and 

 atoms of matter and ether. Here he was illogical when 

 lie said between matter and ether, as if ether were not 

 matter. It was to avoid an illogical phraseology that he 

 used the title " gravitational matter.'' Many years ago 

 he had given strong reason to feel certain that ether 

 was outside the law of gravitation. They need not 

 absolutely exclude, as an idea, the possibility of there 

 being a portion of space occupied by ether beyond 

 which there was absolute vacuum — no ether and no 

 matter. They admitted that that was something that 

 one could think of: but he did not belie\e any living 

 scientific man considered it in the slightest degree pro- 

 bable that there was space surrounding our universe 

 beyond which there was no ether and no matter. Well, 

 if ether went through all space, then it was certain that 

 ether could not be subject to the law of mutual gravita- 

 tion between its parts, because if it were subject to 

 mutual attraction between its parts its equilibrium would 

 be unstable, unless it were infinitely incompressible. 

 Hut here again he was reminded of the critical 

 character of the ground on which they stood in 

 speaking of or giving very definite propositions beyond 

 what they saw or felt by experiment- He was afraid 

 he must here express a view different from that which 

 Prof- Riicker announced in his address, when he said 

 that continuity of matter implied absolute resistance to 

 condensation. They had no right to bar condensation as 

 a property of ether. While admitting ether not to have 

 any atomic structure, it was postulated as a material 

 which performed functions of which they knew some- 

 thing, and which might have properties allowing it to 

 perform other functions of which they were not yet 

 cognisant. If they considered ether to be inatter, they 

 postulated that it had rigidity enough for the vibration of 

 light, but they had no right to say that it was absolutely 

 incompressible. They must admit that sufficiently great 

 pressure all round could condense the ether in a 

 given space, allowing the ether in surrounding space 

 to come in towards the ideal shrinking surface. When 

 he said that ether might be outside the law of gravitation 

 he assumed that it was not infinitely incompressible. He' 

 admitted that if it were infinitely incompressible, then it 

 might be subject to the law of mutual gravitation between 

 its parts ; but to his mind it seemed infinitely improbable 

 that ether was infinitely incompressible, and it appeared 



NO. 1669, VOL. 64] 



