126 THE PRINCIPLES OF SCIENCE. 



of science, no precise laws have yet been detected. The 

 tension of aqueous vapour at different temperatures has 

 been determined by a succession of eminent experimenta- 

 lists, Dalton, Ksemtz, Dulong, Arago, Magnus, and Eeg- 

 nault, and by the last mentioned the measurements were 

 conducted with all accuracy apparently attainable at pre- 

 sent. Yet no incontestible general law has been esta- 

 blished. Several functions have been proposed to express 

 the elastic force of the vapour as depending on the tempe- 

 rature. The first general form is that of Young, namely 

 F = (a + 1} t) m , in which a, l> and m are unknown quanti- 

 ties to be determined by comparison with observation. 

 Roche has proposed, on theoretical grounds, a complicated 

 formula of an exponential form, and a third form of func- 

 tion is that of Biot, as follows log ~F = a + ba t + c /3' u . 

 I mention these formulae particularly, because they well 

 illustrate the feeble powers of empirical inquiry. None 

 of the formulae can be made to correspond exactly with 

 experimental results, and the last two forms correspond 

 nearly equally well. But there is very little probability 

 that the real law has been reached, and it is highly 

 unlikely that it will be discovered except by deduction 

 from mechanical theory. 



The same remarks may be made upon any other laws 

 except those of the most simple character. A vast amount 

 of the most ingenious labour has been spent upon the 

 discovery of some general law of atmospheric refraction. 

 Tycho Brahe and Kepler commenced the inquiry : Cassini 

 first formed a table of refractions, calculated on theoretical 

 grounds : Newton entered into some profound investiga- 

 tions upon the subject : Brooke Taylor, Bouguer, Simpson, 

 Bradley, Mayer, and Kramp successively attacked the ques- 

 tion, which is of the highest practical importance as regards 

 the correction of astronomical observations. Laplace next 



u Jamin, ' Cours de Physique,' vol. ii. p. 1 38. 



