592 



KNOWLEDGE & SCIENTIFIC NEWS. 



[November, 1906. 



Late Stay of Swifts in Somerset. 



In tho Fiihl (Oclober 6), Mr. V. A. Knii^Iil rci.inl- llie 

 fact that he saw, on September 30, at 8 a.m., a large num- 

 ber of swallow.s and martins, and amonsj them were two 

 swifts. The birds were flying east, along the north side 

 of the Men<lip Hills, at ^^"ln<;combe. 



PHYSICAL. 



Bv Alfred W. Porter, B.Sc. 



The Efficiency of Brakes. 



The recent accidents in connection with the running away 

 of an electric tram at Highgate and the train near (irantham 

 have both called renewed attention to the non-effectiveness 

 of brakes when applied so as to lock the wheels. The pro- 

 blems raised are very difficult, and it does not seem very 

 clear as to what the correct solution is. At one time I 

 thought that the explanation was easy, in terms of energy 

 considerations. The argument ran somewhat as follows : 

 If the wheels arc locked there is dissipation of energy only 

 at the rails; but if the wheels arc only partially locked there 

 is dissipation also at the brake blocks ; therefore the energy 

 of the train will be more quickly removed in the latter case. 

 This argument is, however, wholly fallacious, and may 

 serve as an illustration of the danger attending arguments 

 based on the doctrine of energy. It is easy to see that it 

 must be fallacious by applying the method of forces. The 

 motion of the train as a whole can only be altered by means 

 of e:ctcrnal forces. Now, the forces between the brake 

 blocks and the wheels are internal forces, and, therefore, 

 cannot directly affect the motion of the train. The only 

 external forces on the train are between the wheels and the 

 rails, and the brakes act by changing the rolling motion 

 of the wheels to sliding motion. Unless, therefore, it can 

 be shown that the energy dissipated at blocks and rails 

 is greater when the wheels are partiallv locked than when 

 they are wholly so, the energy argument proves nothing. 



The important question to be answered is as to whether 

 sliding friction, even when the rails are slippery with rain 

 (or otherwise) or a smooth flat is formed on the wheels, is 

 likely to be less than the mixed rolling and sliding friction 

 which comes into play when the locking is imperfect. If 

 we take the supposed superiority of partial locking as being 

 an experimental fact, the answer to this question must be 

 that the sliding friction is less. It must be admitted that 

 this answer is not in agreement with preconceived expecta- 

 tions. It is certain that pure rolling is much less than 

 sliding friction ; and it is not clear how a mixture of the 

 two can give rise to a resistance which is greater than 

 either of them. The greater smoothness brought about 

 by the formation of a flat on the wheel commonly gets the 

 credit for this unexpected result ; and it is possible that this 

 is a sufficient reason. Again, the grip-and-let-go action 

 which accompanies the mixed friction may not be without 

 some influence, since static friction is always somewhat 

 greater than kinetic. This paragraph is written rather with 

 the idea of getting clear notions on the subject than of 

 dogmatising upon it. 



The Rate of Decay of Phosphorescence. 



This subject has been recently attacked bv the Rev. B. J. 

 Whiteside, S.J., who has experimented on Balmain's paint. 

 The method of experimenting consisted in placing a sur- 

 face coated with the paint alongside a piece of glass of such 

 a tint that when illuminated from behind it appeared of the 

 same tint as the paint when excited by light. The source 

 cf illumination of the glass was a small hole in front of a 

 lamp, and its strength could be adjusted by moving the 

 lamp and aperture from the glass. This movement was 

 effected in such a w'ay as to maintain the brightness of the 

 glass always of the same strength as that of the surface of 

 the paint, as the phosphorescence died away. Bv means 

 of a chronographic apparatus, the mechanism bv which the 



movement of the lamp was made (and which was con- 

 trolled by hand), automatically recorde<l the times at which 

 successive measured distances were reached. Hence, by 

 applying the inverse-square law, the intensitv at anv instant 

 could be calculated. As a result it was found that the in- 

 tensity could be represented very well by the formula 

 I (a+bt) const., where I ^^ intensity, a and b are constants. 

 The great interest in this result is that a similar formula 

 has been found by Professor Trouton to apply to the release 

 of strain of solids when stressed beyond their elastic limits, 

 and the plausible inference is that phosphorescence is 

 intimately connected with the effects following an actual 

 deformation protluced by the action of light. 



Models of Atoms. 



Professor Mayer's experiment with floating magnets has 

 been utilised considerably by Professor J. J. Thomson, in 

 illustrating the possible stable positions of electrons in an 

 .itom. A simple modification of this experiment was 

 described by me in Nature, Oct. 4. Small floating magnets 

 (i.e., bits of magnetised needle, j-inch long, each pierced 

 through a small piece of cork, so that it will float vertically 

 on water) are placed on water with their north poles (sav) 

 all pointing upwards. These, of course, will repel one 

 another and in the usual arrangement this repulsion is 

 balanced by placing below the dish a bar magnet with its 

 north pole upwards ; the magnets then take up equilibrium 

 positions, which are of a regular kind. But instead of 

 using this additional magnet, a force tending towards the 

 centre of the dish can be obtained by filling the vessel almost 

 to overflowing with water. A single magnet placed on the 

 water will then float to the centre ; two such magnets will 

 float into stable positions, depending upon their strength of 

 magnetisation. Three come to rest at the corners of an 

 equilateral triangle. The chief interest in this modification 

 is that the grouping is sometimes very different from that 

 obtained in the usual form of the experiment. For ex- 

 ample, in the latter, when six magnets are used, they w-ill 

 form an equilibrium figure, with one acting as a. central 

 nucleus, and the rest distributed round it at the corners of 

 a regular pentagon. In the modified arrangement, a large 

 number may be arranged in a single ring without anv 

 central nucleus. The maximum number depends upon the 

 size of the dish and the particular magnets employed. Thus 

 in a particular dish, it is possible to arrange ten in a single 

 ring. These also form a stable group as a ring of nine with 

 one in the middle, or of eight with two in the middle. 

 But a ring of seven with three inside is not possible ; if 

 temporarily so placed, one of the three moves out and joins 

 the seven. Not until eighteen magnets are introduced is it 

 possible to form a group with a nucleus and two rings. 

 These are stable when placed twelve in an outermost ring, 

 five in an intermediate ring and a single one in the centre. 

 The moral to be drawn is that too much reliance must not 

 be placed on applications to atomic structure of the be- 

 haviour in either of these forms of experiment, because the 

 lav.' of the forces between the constituents of an atom may 

 be, and probably are, different from those existing in either 

 of them. 



ZOOLOGICAL. 



By R. Lydekker. 

 The Fauna of Lake Tanganyika. 



.A. CONSIDERABLE portion of the August issue of the Zoologi- 

 cal Society's Proceedings is occupied by an account of the 

 organisms obtained during a recent expedition to Lake Tan- 

 ganyika. Several naturalists have taken a share in this 

 work, and while some of them have not expressed a definite 

 opinion on the subject, the general result of the investiga- 

 tions appears to show that the theory of the Tanganyika 

 fauna being originally a marine one, which became cut off 

 from the ocean and eventually adjusted itself to fresh- 

 water conditions, will not hold good. Support to this view 

 is afforded by a note in the same issue regarding a jelly-fish 



