Mr. W. J. M. Rankine on the Mechanical Action of Heat. 13 



represents the depth of fall, which is equivalent to one degree of 



r^'IV- «'-^ ^- ^^y ^^^^^ ""S^' '^ '^' ^'^''"" = 



consideration. • ■ ii,„ „„+i«= 



We know, to a greater or less degree of precision, the ratios 

 of the specific heats of many substances to each other, and they 

 are commonly expressed bytaking that of water at the temperature 

 of meltins; ice as unity. , ■ e 



2 Mr Joule has made several very mteresting series of ex- 

 periments, m order to ascertain the quantity of heat developed 

 in various substances by mechanical power employed m different 

 wavs viz. by electric currents excited by the rotation of a 

 magnet, by the forcing of water through ^^^row ^ubes, by the 

 Sation of water, mei^ury and od with a paddle by the friction 

 of cast iron by the compression of air, and by the friction of air 

 rLhmg through a narrow orifice. In his latest and best series 

 of experiments (Phd. Trans. 1850), the mechamcal equiva ent 

 of hea^t in liquid water was determined by means of the friction 

 of water, mercury, and cast iron, the mean results bemg as 



^'^One 'cTntigrade degree in liquid water = the action of gravity 

 through 1389-6 feet = 432-54 metres. 0^« ^fSf^f ^^^^''^"^^^^^^ 

 in liquid water = the action of gravity through 772 feet. So far 

 as I can judge from the mutual agreement of the experiments, 

 the probable error of this result does not exceed ^l^dth of its 



^'"3'' Means of determining the mechanical equivalent of heat in 

 ail and other gases are furnished by experiments on the ve ocity 

 of sound, which, according to the received and well-known theory 

 of Laplacc'is accelerated by the heat developed by the compres- 



^^Thf ac^u^cy 7this theory has lately been called in question. 

 Theit can be no doubt that it deviates from absolute exactness, 

 in far that the magnitude of the displacements of the particles 

 of an- is neglected m'comparison with the length of a wave It 

 appears to me, however, that the Astronomer Royal, m his re- 

 ason the sUjeet m the London and Edinburgh Phdosophical 

 Zcazine for July 1849, has shown, in a satisfactory manner, 

 It^a hough the- effect 'of the appreciable magnitude of those 

 displacements, as compared with the length of a wave of sound, 

 fst alter slo;iy the form of the function representing the wave, 

 Sill that effect is not sufficiently great to make Laplace's theory 



P^f^^;:;;:SS^' already given for the real specific heat of 

 unky of w'ght of a given substance may be resolved into two 

 factors, thus :— JQ 1 ^^ (1) 



