TRANSACTIONS OF SECTION A. 441 



slide on the rail was nearly 27,000 lbs. ; whilst at 20 miles an hour, a pressure of 

 about 10,300 lbs. was found sufficient to obtain the same result. 



The strain on the drawbar 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 50 miles an hour, which shall exert a retarding force on the train equal to that 

 at 20 miles an hour, the pressure applied to the brake blocks at 50 miles an hour 

 must be nearly two and a half times as great as that required at 20 miles an hour ; 

 and a still greater pressure is required for higher velocities. 



Therefore, whilst a comparatively low pressure woidd make the wheels slide at 

 low velocities, it was difficult to obtain any sufficient pressure to make the wheels 

 slide at velocities over 60 miles an hour. 



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 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 Thomas 

 Westinghouse, and constructed under his immediate supervision, and he assisted 

 mainly in the experiments, &c. The earlier experiments were also made with 

 the sanction of Mr. Horace Darwin. 



3. Oji a Spectroscope of unusually large Aperture.* By G. J. Stoney. 



4. On the Support of Spheroidal Drops and allied Phenomena. By G. 

 Johnstone Stoney, M.A., F.B.S., George F. Fitzgerald, M.A., 

 F.T.C.D., and Richard J. Moss, Keeper of the Minerals in the Museum 

 of Science and Art, Dublin. 



The authors gave an account of several investigations and experiments! made by 

 them during the past year upon the unequal stresses which arise in gas when 

 polarised, that is, in which the molecular motions are not alike in all directions; 

 and especially in the case where the polarisation arises from heat traversing the 

 gas, as within radiometers, and in the gaseous layers which support spheroidal 

 drops, those which protect the hand from a dangerous burn when dipped into 

 melted metal, and many others. In such cases the molecular motions are not 

 alike in all directions, and cause the stress within the gas directed across the layer 

 to exceed the transverse stresses by an amount which may be called the Crookes's, 

 or polarisation stress. 



The investigation of this stress by a direct mechanical consideration of the 

 molecular motions had been much facilitated by employing the conception of a 



* The instrument was exhibited in the Chemical Laboratory of Trinity College. 



+ See the following memoirs : — ' On some Kemarkable Instances of Compressed 

 Strata of Polarized Gas at Ordinary Atmospheric Tensions,' by G. Johnstone Stoney 

 (' Scientific Proceedings of the Royal Dublin Society,' vol. i. p. 53, and • Philos. 

 Mag.' June, 1878, p. 457) ; ' On the Spheroidal State,' by Richd. J. Moss (do. vol. i. 

 p. 83) ; ' On the Mechanical Theory of Polarization Stress in Gases,' by G. Johnstone 

 Stoney (' Scientific Transactions of the Royal Dublin Society,' vol. i. Memoir 5) ; 

 ' On the Mechanical Theory of Crookes's Force,' by George F. Fitzgerald (do. vol. i. 

 Memoir 6). 



