Mr, J. H. Alexander on the Tensian of Vapour of Water. 5 



nearly with the actual observations. For instance, comparing 

 it with the experiments of Dniong and Arago, 

 Temp. 335°-3 F. gave a pressure of 7'391 atmos. ; by formula 7-478 atin. 



... 371°3F 11-660 ... ... 11-938 ... 



Nevertheless, as the object I had in view was not to find an 

 equation that merely fits any particular series of observations, 

 or is exact only for the higher ranges of temperature, I aban- 

 doned this theoretical expression and preferred to deduce a 

 formula for pressure in atmospheres from the original one, in 

 the ordinary analytic way. This results in the alternative 

 expressions — 



^"VsiT'Ts'^ 1695 / ~\317-1 3 2986 33./ ' 

 either of which may be adopted, according as we prefer to re- 

 tain in view the factor of the latent heat, or that of the expan- 

 sion at the unitary atmospheric pressure. In practice, the 

 constant fraction may be substituted by the number 0-33151. 

 For the original formula, the similar constant was carried no 

 further xhvinfour places of decimals ; in this, where the unit ol' 

 pressure is thirty times larger, both the attainment of equal 

 precision requires, and the facility of calculation allows, an- 

 other decimal place to be taken. 



These decimal constants might, indeed, have been given in 

 both formulae at once, instead of the fractional expression 

 from which they originate, were it not that I thought it de- 

 sirable to preserve those factors which, besides solving the 

 equation, indicate also, in part accidentally and in part essen- 

 tially, certain elementary relations between pressure and tem- 

 perature (or rather certain epochs in those relations), which 

 are important in the future complete theory of the subject. 

 For example, the denominator (1695) which expresses the 

 number of volumes of steam under the atmospheric pressure 

 and at a temperature of 212°, developed from an unitary vo- 

 lume of water at its maximum density, shows also the number 

 of atmospheres, the equivalent of whose pressure will, below a 

 certain temperature, prohibit the development of steam beyond 

 the sphere of said unitary volume. On the other hand, the nu- 

 merator (990°) gives this limiting temperature ; and shows the 

 degree on Fahrenheit's scale, where the force of steam becomes 

 equivalent to 1695 atmospheres. Its density, therefore, would 

 be equal to that of water ; if its behaviour in other regards were 

 like that of water too, this temperature would be the limit to its 

 useful effect. But as there would most probably still remain the 

 elasticity due to its expansion at that temperature, it does not 

 appear that we are warranted in supposing any such limit. 



At this point, 990°, the ratio of the latent and sensible heats 

 has just inverted itself from what it was under the unitary at- 



