588 Mr. Rankine on the Centrifugal Theory of Elasticity , 



they would be scarcely distinguishable from those which he has 

 himself drawn to exhibit the mean results of his experiments. 

 Annexed is- a table of the values of the constants a, log fi, 



^^S yy Q~y ~A73.i ^or *^® fluids for which they have been calculated. 



As the existing experiments on mercury, turpentine and petro- 

 leum are not sufficiently extensive to indicate any precise value 

 for the coefficient 7 (which requires a great range of tempera- 

 tures to evince its effect), I have used for these fluids, as an ap- 

 proximation, the first two terms of the formula only, a— — . 



For different measures of pressure, the contact a evidently 

 varies equally with the complement of the logarithm of the unit 

 of pressure. 



For different thermometric scales, ^ varies inversely as the 

 length of a degree, 7 inversely as the square of that length, 



— directly as the length of a degree, and 7-2 directly as the 



square of that length. 



For all the fluids except water, it will probably be found neces- 

 sary to correct more or less the values of the constants, when 

 more precise and extensive experiments have been made, espe- 

 cially those for the more volatile sether, and for turpentine, pe- 

 troleum and mercury, which have all been determined from data 

 embracing but a small range of pressures. 



In reducing the constants for the Centigrade scale to those 

 for Fahrenheit's scale, 180° of the latter have been assumed to 

 be equal to 100° of the former. In order that this may be the 

 case, the boiling-point of Fahrenheit's scale must be adjusted 

 under a barometric pressure of 760 millimetres, or 29*922 inches, 

 of mercuiy, whose temperature is 0° Centigrade. 



In the ninth and tenth columns of the table are given the 

 limits on the scales of temperature and pressure between which 

 the formulae have been compared with experiment. It is almost 

 certain that the formula for the pressure of steam may be em- 

 ployed without material error for a considerable range beyond, 

 and probably also that for the pressure of vapour of alcohol ; but 

 none of the formulae are to be regarded as more than approxima- 

 tions to the exact physical law of the elasticity of vapours, for 

 the determination of which many data are still wanting, that can 

 only be supplied by extensive series of experiments. 



