404 Experimental Researches on the Depressions [July, 



There would have been little satisfaction in carrying this train of 

 research further, because of the difficulty of measuring the tempera- 

 ture ; otherwise it is evident that the coated thermometer might be 

 safely trusted in a much greater heat, ere it would itself reach even 

 the boiling point of water under the ordinary pressure ; an illustration 

 of which will be hereafter mentioned, but, not being strictly experi- 

 mental, it cannot be introduced here. 



Having however accumulated abundant data for the formation of an 

 experimental curve, I may proceed to throw them together in the 

 form of a diagram (fig. 4.), and to compare at once the results with the 

 various formulae that have been proposed by different philoso- 

 phers. 



As, however, each author has employed different algebraic charac- 

 ters for working out the problem, it will be better first to bring them 

 to common terms, adopting the most simple expressions : thus let 



t = the temperature of the air. 



V = the temperature of a wet-bulb, or of an evaporating surface. 



t" = the temperature of saturation, or the dew-point, 

 then/,/', and/" may be conveniently used to represent the force of 

 aqueous vapour, at t, V, and t" respectively, d, the depression, is of 

 course = t — t', and not absolutely wanted, but it is frequently a more 

 convenient expression ; and D may be also used to distinguish the 

 maximum depression in dry air, when/" = 0. 



Now supposing the increasing temperatures, t, to be represented 

 by the abscissae of the divided line TT, the observed depressions 

 may be laid off as ordinates, through the apices of which a dotted 

 line being drawn, will form an experimental curve of maximum de- 

 pression, for which a mathematical expression is required. 



Next, to collect the materials for the theoretical curves to be entered 

 in the same diagram, we must take a cursory view of the existing 

 theories. 



Leslie, who must be regarded as the inventor of the wet-bulb 

 hygrometer, deserves the precedence in this inquiry. His experi- 

 ments were conducted by approaching a dry and a wet thermometer 

 together gradually towards a heated furnace in a closed chamber. 

 The Professor calculated the hygrometric conditions of the air as its 

 heat rose ; and on comparing his results, he was led to the conclusion, 

 that as the caloric necessary to convert water into steam was = 6000 

 degrees of his instrument, and the capacity of air was f ths that of 

 water, the same measure of heat would raise an equal mass of air, 

 16000 degrees ; and consequently that at the temperature of the wet- 

 bulb, V, air would take up the 16000th part of its weight for each 



