82 Hough — Mechanical Equivalent of the 



determined the mean specific beat of: water from 12° to 100° 

 in terms of the calorie at 20° using the following relation : 



wL—Ws(t 1 —t 2 ) 



observing w, W, t„ and £ a , and taking Regnault's value of L, 

 536*5. Callendar substitutes in this relation Joly's observa- 

 tions of w, "W, t„ and £ 2 , and Barnes'* determination of the 

 mean specific heat of water from 12° to 100° in terms of the 

 calorie at 20° and solves for L. This gives the value of 540*2 

 in terms of the calorie at 20°. Callendar prefers this value to 

 that of Regnault and uses it in his work on the properties of 

 steam. It is probable that even this value is low, since Barnes'f 

 values for the specific heats of water from 40° to 100° are 

 almost parallel to but much lower than those of Regnault. 

 There is much uncertainty as to the value of L in calories. 



There is as yet no absolute determination of L in ergs. It 

 may be expressed in ergs, however, as the product of L in 

 calories into the mechanical equivalent of heat. Using 540*2 

 as the most probable value of L in calories at 20° and 

 4*184 XlO 7 as the most probable value of the mechanical 

 equivalent at 20°, this being an average of the values due to 

 Barnes, Rowland, Griffith, Schuster, and Moorby, gives 

 2*26 XlO 10 against 2*24 XlO 10 from Regnault's value. 



The sources of error for this value are many. It can not be 

 stated that the mechanical equivalent of the heat of vaporiza- 

 tion of water is known with certainty to one per cent. 



In any method of calorimetry involving the use of the 

 calorie, no greater degree of accuracy can be attained than that 

 of the calorie itself. But the determination of the value of C 

 involves the use of the thermometer and all the errors incident 

 to the measurement of temperature. That these are greater 

 and more varied than is commonly supposed, and can only be 

 corrected for by the exercise of the greatest care and skill, is 

 definitely shown by Rowland;]; in his work on thermometry. 

 He sees visions of careful, painstaking observers conscientiously 

 reading with telescope and micrometer eye-piece to the thou- 

 sandth part of a degree, unconscious of the fact that variations 

 due to internal and external pressure, apparent friction and 

 previous history, to say nothing of those due to the sectional 

 calibration and the fundamental points, are many times as great 

 as the usual errors of parallax and estimation. 



Griffith, § who is not so caustic though quite as vigorous, 

 says : " The difficulties with regard to the measurement of 

 temperature are not peculiar to the electrical method of inves- 



* Brit. Assoc. Rep., 1889. f Phil. Trans. A., 1902, vol. cxcvii. 



X Rowland, Proc. Am. Acad, of Arts and Sciences, vol. xv. • 

 § Griffith, Phil. Trans., 1893. 



