Marsh.] lio [March 6, 



small. But there is some evidence in favor of a larger value of Joule's 



equivalent, and, consequently, in favor of a larger value of x. 



In his final paper,* Joule announces the following results from his 



experiments : 



From 1st Series, consisting of 40 experiments from friction of 



water, vaAue of equivalent. 772.692 

 "2 " " 20 "mercury " 772,814 



" -8 " " 30 " " " 775.352 



" 4 " *' 10 "cast-iron " 77G.045 



" 5 " " 10 " " " 773.930 



Whence we see that the mean result from the whole 110 ex- 

 periments was 773.857 



Joule adds, "I consider that 772.692, the equivalent derived from the 

 friction of water, is the most correct, both on account of the number of 

 experiments tried, and the great capacity of the apparatus for heat ; and 

 since, even in the friction of fluids, it was impossible entirely to 

 avoid vibration, and the production of a slight sound, it is probable that 

 the above number is slightly in excess ;" and he concludes by adopting 

 772 as the most probable value. 



Now, inasmuch as, in the case of cast-iron, he had made an experimental 

 determination of the heat expended in the production of sound, and had 

 allowed for it ; and since no further explanation is given, we must look 

 upon his final conclusion as based upon the 40 experiments with water 

 alone ; the fractional part being rejected in consideration of probable 

 loss from the noise produced in that series of experiments. 



Although Joule thus ignored nearly two-thirds of a series of experi- 

 ments, all of which had been conducted with equal care, and each of which 

 would therefore seem to be entitled to some weight, he placed their results 

 upon record ; and we may certainly be permitted to inquire what conse- 

 quences would have followed the adoption of the purely experimental 

 value which the whole series indicated — 773.857. 



Adopting this as the mechanical equivalent of heat, we find the force 

 competent to lift 2.8 X 773.857 = 2166.8 lbs. one foot. But the weight 

 actually lifted was 2160.0 lbs. one foot. Hence, a force competent to lift 

 6.8 lbs. one foot has become latent, having been employed in producing 

 and maintaining the expansion ; but, inasmuch as the quantity of heat 

 necessary to augment the temperature of one cubic foot of air (weighiug 

 1.29 oz.) at a constant volume, 490°, would heat 6.7 lbs. of water 1°, it 

 would be competent to lift 6.7 X 773.857 = 5184.8 lbs. one foot high ; 

 whence we have the statement : 



Foot-pounds. Foot-pounds. 

 as 5184.8 : 6.8 : : 490O : 0.642° = x, showing that 

 sufficient heat was rendered latent, to raise the temperature of the whole 

 mass of air (1.29 oz.) at a constant volume, 0.642 degrees. 

 » Philosophical Transactions 1850. 



