Notices respecting New Books. 223 



[V.] vol. i. pp. 177 & 305, I offered an exphination of the 

 mechanical stresses within Crookes's radiometers based upon 

 this very consideration: see in particular page 178, where the 

 following words occur: — " I cannot refrain from observing 

 here how entirely remote such a chamber" [viz. a Sprengel 

 vacuum indicated by one tenth of a millimetre of mercury] 

 " is from being empty. It follows from what we know of the 

 number of molecules in gases at ordinary pressures, that the 

 number remaining in this so-called vacuum will be somewhere 

 about a unit-fourteen, i. e. one hundred millions of millions, in 

 every cubic millimetre." After which I quote, in proof of 

 this assertion, the determinations of the mean interval at which 

 the molecules of gases are spaced, by Professor Loschmidt in 

 1865, by myself in 1867, and by Sir William Thomson in 

 1870. 



It is plain, however, that Mr. Preston has done good service 

 by recalling attention to the immense number of the molecules, 

 and the consequent shortness of the excursions of each mole- 

 cule between its successive encounters with other molecules ; 

 since the subject was new to himself, and had been overlooked 

 by some of the writers upon Crookes's radiometer. 

 I am, Grentlemen, 



Yours faithfully, 



G. Johnstone Stoney. 



XXIX, Notices respecting New Books. 



The WhitiuortJi Measuring Machine, including Descriptions of the 

 Surface-plates, Gauges, and other Measurinq Instruments made hy 

 ISir Joseph Whitwoiith, Bart., O.E., F.R.S., D.C.L., LL.D., 4-c. 

 By T. M. GoODEVE, M.A., Barrister-at-Law, Lecturer on Applied 

 Mechanics at the Royal School of Mines, and C. P. B. Shelley, 

 Civil Engineer, Honorary Fellow of, and Professor of Manufac- 

 turing Art and Machinery in, King's College, London. With 4 

 plates and 44 ivoodcuts. London : Longmans, Grreen, and Co. 

 1877. (Fcp. 4to, pp. 84.) 



4 S an instance of the importance to mechanical engineers o£ 

 ■^-^ very small differences of size, Sir J. Whitworth, in a paper 

 read in 1857, took the case of "an internal gauge having a cyhu- 

 drical aperture '5770 inch diameter, and two external gauges, or 

 solid cylinders, one being -5769 inch and the other "5770 inch dia- 

 meter. The latter is 1-lOOOOth of an inch larger than the former, 

 and fits tightly in the internal gauge when both are clean, and 

 dry ; while the smaller •5769-iuch gauge is so loose in it as to ap- 

 pear not to fit at all." He remarked, that it was plain from this 

 instance, " that the difference between these two cylinders is an 

 appreciable and important quantity," and he added that " when a 



