On the Hygrometer hij Evd^wration. 87 



will express how much the vapour really contained In tlie air 

 falls short of saturation ; and this seems to be the proper hy- 

 grometric quantity, or the just measure of the drying quality 

 of the atmosphere. 



It is proper to observe that the formula for computing M 

 when T is given, is 



Log. H = At + Br- + &c.; 



whence it follows that H— // is not proportional to 8r, but to 

 H X St. The measure of the dryness of the air is therefore a 

 quantity which varies both with the observed depression and 

 the actual humidity of the atmosphere. 



It appears therefore that the experiment of Dr. Hutton will 

 fully disclose the condition of the atmosphere with regard to 

 moisture. Nothing more is necessary for this purpose than 

 to provide a table containing, in one column, the maximum 

 tensions of tlie vapour for all degrees of temperature, such as 

 is found in voL 1. of Biot's Daite de Physique; and, in an- 

 other column, the weights of vapour in a given volume, that 

 correspond to the different tensions; quantities which, for 

 every tension g and temperature t, are computed by the for- 

 mula, 5x0 X — - — . 



' l + mr 



4. There is a great analogy between the experiment of 

 Dr. Hutton and the method employed by Dalton for finding 

 the Dewing point. In the latter method the air is brought to 

 a state of saturation by the agency of cold alone ; in the former 

 the same effect is produced by the joint operation of cold 

 and the addition of humidity. But Dalton's process cannot 

 be used without some complication of machinery and the 

 employing of artificial means for producing cold; whereas 

 Dr. Huttou's experiment requires no more than the exposing 

 of the wet bulb of a thermometer to the action of the atmo- 

 sphere. In the one case, great nicety of observation P5 re- 

 quired for detecting the precise moment when tlie dew begins 

 to be deposited ; and from this cau'-e considerable uncertainty, 

 it is natural to think, must often arise, more especially at low 

 temperatures, when the air contains only a very small portion 

 of humiditv. In the other case, the observer has only to read 

 off the degree on the scale of a thermometer; and it ap})ears 

 that any want of precision in the experiment will cause only 

 a pro})ortional inaccuracy in the result; insomuch tluit the 

 (|uantity of moisture in the atm()s))here maybe known with an 

 exactness sufficient for most practical purj)oses, even it we sup- 

 pose an uncertiiinty of half a degree, or a whole degree, in the 

 quantity of the depression. The simplicity of the one ])roci.ss 

 fits it for being useful in many situations, (in an astronomical 



obhcivalorv 



