170 Mr. T. R. Edmonds on the Elastic Force of 



sity appropriate to the given temperature is attained. The 

 elastic force of such steam is measured by the height of a column 

 of mercury contained in a tube of which the open end is inserted 

 in the boiler containing both water and steam. At a given tem- 

 perature, when the elastic force ceases to increase, the contained 

 steam must be of maximum density. It is to be noted that when, 

 by increasing the temperature, the whole of the contained water 

 has been converted into steam, any further addition to the tem- 

 perature cannot further increase the density of the steam, which 

 then becomes "dry" or " superheated " steam, a kind of steam 

 which is not here taken into consideration. 



The problem offered for solution is this : — Given the tempe- 

 rature of the water and steam contained in the boiler, it is 

 required to find, either by experiment or calculation, the corre- 

 sponding elastic force when such force is constant and at its 

 maximum. This problem has been completely solved by M. 

 Regnault so far as experiment is concerned, and this for all 

 ordinary ranges of temperature and pressure — extending from 

 130° Centigrade above to 130° Centigrade below the boiling-point 

 of water, or from a pressure of twenty-seven atmospheres to a 

 pressure of only the two-thousandth part of one atmosphere. 

 What remains to be desired is, the knowledge of the law which 

 connects together the experimental results, and which will enable 

 a person to determine by calculation the elastic force at every other 

 temperature from the known elastic force at any one temperature. 



At different times during the last fifty years there have ap- 

 peared various empirical formulae intended to express approxi- 

 mately the elastic force of steam of maximum density in terms 

 of the temperature. The comparative merits of several of these 

 formulae have been discussed by the two commissions (on steam, 

 &c.) appointed by the French Government, viz. by MM. Dulong 

 and Arago in their Report of the year 1829, and by M. Regnault 

 in his Report made in the year 1847. The conclusion arrived 

 at by M. Regnault is, that one only of these empirical formulae 

 yields results sufficiently near the truth to render it capable of 

 being usefully applied to purposes of interpolation. The favoured 

 formula is well known as that of M. Roche, who was formerly 

 professor in a college or military school at Toulon. It was pub- 

 lished in 1829, or before that year. This formula, when reduced 

 to its simplest terms by making P = l when £ = (the former 

 quantity representing elastic force, and the latter temperature 

 measured on the thermometric scale), is the following : 



lo s P '= A TTM? 



