HEAT. 61 



cular cases have been given. Theoretically it should be the 

 same that is to say, if a fall of 772 Ibs. through a space of 

 one foot will raise the temperature of I Ib. of water through 

 one degree of Fahrenheit, then the fall in the temperature of 

 I Ib. of water through one degree of Fahrenheit should be 

 able to raise 772 Ibs. through a space of one foot. The calcu- 

 lations of M. Seguin are not far from this, but since the elabo- 

 rate experiments of M. Regnault he has expressed some 

 doubt of the correctness of his former estimate, as by these 

 experiments it appears that, within certain limits, for elevating 

 the temperature of compressed vapour by one degree, no more 

 than about three-tenths of a degree of total heat is required ; 

 consequently, the equivalent multiplied in this ratio would be 

 1,666 grammes instead of 500. Other investigators have 

 given numbers more or less discordant ; so that, without 

 giving any opinion on their different results, this question may 

 be considered at present far from settled. M. Regnault 

 himself does not give the law by which the ratio of heat 

 varies with reference to the pressure, a subject involving 

 questions of which experiments on the mechanical effects 

 of elastic fluids seem to offer the most promising means 

 of solution. In a former edition I inclined to this opinion 

 from the greater visible effects of small changes in tempe- 

 rature on gases than on solids. When we read on a 

 mercurial thermometer to the i,oooth or even looth of a 

 degree, and aim at quantitative results, errors may well 

 arise from the minuteness of the measurement, but with 

 gases minute differences of temperature are cognizable and 

 measurable. Messrs. Joule and Thomson, however, in a 

 paper on the thermal effects of fluids in motion, 1854, thus 

 express themselves on the relation between the heat evolved 

 and the work spent in compressing a gas kept at a constant 

 temperature : ' This relation is not a relation of simple 

 mechanical equivalence, as was supposed by Mayer in his 

 " Bemerkungen ueber die Krafte der Unbeleben Natur," 

 in which he founded on it an attempt to evaluate numerically 

 the mechanical equivalent of the thermal unit. The heat 

 evolved may be less than, equal to, or greater than the 

 equivalent of the work spent, according as the work pro- 



