472 



NATURE 



^August 29, 1878 



A limited number of experiments were made with wrought 

 iron blocks upon the steel tyres, a mean of which gave the fol- 

 lowing result : — 



The following table shows the result obtained by the sliding 

 of the wheel on the rail — that is, a steel tyre on steel rails : — 



The general results of these tables show that the coefficient of 

 friction between movmg surfaces varies inversely in a ratio de- 

 pendent upon the velocity at which the surfaces are moving past 

 each other ; probably the expression would be of the form of 



b -^v 



The coefficient of friction, moreover, at these velocities be- 

 comes smaller also after the bodies have been in contact for a 

 short time. That is to say, the longer the time the surfaces are 

 in contact, \he smaller apparently does the coefficient of friction 

 become. This result appears more marked in the case of cast- 



iron blocks than of the wheel sliding on the rail. This effect, 

 however, does not appear to be unnatural, as the friction de- 

 velopes heat, and the consequent expansion tends to close up 

 the pores and to make the heated surface a more united surface 

 than the colder surface ; besides which it is probable that, in the 

 act of rubbing, small particles may be detached which may act 

 as rollers between the surfaces. 



It will also be observed that the coefficient of friction between 

 the cast-iron block and the steel tyre is much larger than that 

 between the steel tyre of the wheel and the rails, which were also 

 generally of steel. As has been above-mentioned, the sliding of 

 the wheel on the rail takes place when the friction of the brake 

 blocks is greater than the adhesion between the wheel and the 

 rail, whidi is due to the weight upon the wheel. This was 

 found to amount generally to about 24 to 28 per cent, of the 

 weight. The influence which these results have upon brakes 

 for railway trains may be briefly summed up as follows : — 



In order to produce a given result at different velocities the pres- 

 sure applied to the brake blocks must increase in the proportion 

 shown by the coefficient of friction. Thus at fifty miles an hour 

 the pressure required to make one pair of wheels slide on the 

 rail was nearly 27,000 lbs., whilst at twenty miles an hour a 

 pressure of about 10,300 lbs. was found sufficient to obtain the 

 same result. The strain on the draw-bar showed that the 

 retarding force or the tangential strain between the brake blocks 

 and, the wheels followed very nearly the same law of variation ; 

 that is to say, in order to produce a degree of friction on the 

 wheel at fifty miles an hour which shall exert a retarding force 

 on the train equal to that at twenty miles an hour the pressure 

 applied to the brake blocks at fifty miles an hour must be nearly 

 2.\ times as great as that required at twenty miles an hour, and a 

 still greater pressure is required for higher velocities. Therefore 

 whilst a comparatively low pressure would make the wheel slide 

 at low velocities, it was difficult to obtain any sufficient pressure 

 to make the wheel slide at velocities over sixty miles an hour. 



A satisfactory brake, therefore, should be capable of bringing 

 on a very high pressure almost instantaneously, and then the 

 pressure should be gradually reduced as the train comes to rest. 



The figures given in the above tables must at present be 

 accepted as only provisional until an accurate mean has been 

 obtained from the diagrams, which are not yet all worked out. But 

 it may be assumed as an axiom that for high velocities a brake is 

 of comparatively small value unless it can bring to bear a high 

 pressure upon the surface of the tyre almost instantaneously, and 

 it should be so constructed that the pressure can be reduced in 

 proportion as the speed of the train is reduced so as to avoid the 

 sliding of the wheels on the rails. 



I must add that these experiments were made upon the London, 

 Brighton, and South Coast Railway, who, through their able 

 general manager, Mr. Knight, and locomotive engineer, Mr. 

 Stroudley, gave every assistance in the construction of the van and 

 the running of the train. The apparatus was mainly devised by 

 Mr. Westinghouse and the experiments were carried on under 

 his immediate supervision. The earlier experiments were also 

 made with the assistance of Mr. Horace Darwin. 



SECTION B.— Chemical Science. 



On a Simplification of Graphic Formula, by Oliver J. Lodge, 

 D.Sc. — In the graphic formulae of a compound the elements 

 were ordinarily represented by their chemical symbols (capital 

 letters), and the connection between the atoms was represented 

 by straight lines joining the letters. Graphic formulae were of 

 most use in organic chemistry, where the principal compounds 

 consisted only of the elements C, H, O, N, whose atomicities 

 were 4, i, 2, and 3 or 5 respectively. In any formulae, there- 

 fore, four bonds always radiated from the letter C : N was the 

 meeting place of three or five bonds, according to circumstances ; 

 two bonds met at each O, and a single bond terminated at every 

 H. Supposing that the letters were omitted and the bonds 

 joined together, the position of the atoms would still be apparent 

 as the meeting-place of a definite number of bonds, and therefore 

 the letters were unnecessary. The simplification he proposed, 

 then, was the omission of the usual symbols used to denote the 

 atoms, and the joining of the bonds in such a way as clearly to 

 define the atomicities, and therefore the natures of the several 

 atoms. Formulae so drawn became reduced to a sort of geo- 

 metrical diagram, and conversely any geometrical curve represented 

 some real or imaginary chemical compound. 



Abstract of a Paper on the Action of Heat upon the Selenate of 



