192 Mr. A. E. Dunstan on the Coefficient of 



entropy comes out 

 (.,MC + ^) log «J±J + L ( 2 ^ - ^\ + .MRlo g . l .=0 



where, in addition to the symbols already define, s is the 

 specific heat of water, T the temperature before expansion, 

 and R the gas constant for one gram of air. 



I have plotted this curve for the particular case discussed 

 in detail by Prof. Thomson, viz., starting from an initial tem- 

 perature of 16° C, and giving an expansion of 1*36. The 

 graph comes out practically a straight line running parallel 

 to the irreversible line, both lines being nearly perpendicular 

 to the saturation-density curve with which their intersections 

 are sought. The numerical values come out on the reversible 

 hypothesis, 



p = 4'87 X 10- 6 , * = 0°-5, mass of cloud per cm. 3 5'06 X 10- 6 grm. 



as compared with the values obtained on the accepted 

 assumption by Prof. Thomson, 



p = 5'15 x 10- 6 , t=l°% mass of cloud per cm. 3 4-77 x 10~ 6 grm. 



So that even on the extreme assumption here made we 

 arrive at a value differing by only 6 per cent, from the 

 other. We can infer that no error, comparable with the 

 other inaccuracies attached to the methods of measurement, 

 will be involved if the assumption of complete adiabatic 

 cooling of the air does not exactly correspond with the facts. 



27th May, 1908. 



XII. The Coefficient of Viscous Traction of Lead and Tin 

 Alloys. By Albert Ernest Dunstan, B.Sc* 



FROM results obtained in experiments where pitch, shoe- 

 makers' wax, and the like materials were subjected to 

 torsional stresses, Trouton (Phil. Mag. 1904, xix. p. 34) 

 found that the rate of twisting was approximately propor- 

 tional to the driving couple, and the proportionality between 

 the shearing stress and the induced flow was sufficiently 

 exact to render useful the introduction of a coefficient of 

 viscous flow for such materials. The viscous flow was shown 

 also by the same author (Proc. Roy. Soc. 1906, A. lxxvii. 

 p. 426) in the case of drawn-out rods of the same materials 



* Communicated by the Author. 



