relating to Hygrometers and Evaporation. I8l 



perature of the air t, and u the maximum at the temperature m 

 of the moist bulb ; also t — m being = ^, we have w=zu — cj. 



But if the temperature t, at which w grains would saturate 

 the original volume, be wanted, it may be found from the ther- 

 mometers only, without the aid of any tables, by the following 

 approximate formula, which, however, comes very close to the 

 foregoing, between the temperatures of 25° and 90° Fahrenheit. 

 Put k for the temperature at which the variation in the weight 

 of moisture in the given volume for a change of 1° is c grain, 

 then the temperature sought will be 



If the volume be a cubic foot, and if, as appears from a mean of 

 various experiments, c= .15, then A; = 53° Fahr., and 

 ,= <_3(£±io«°). 



If the centigrade thermometer be used, c — ,27, and both k 

 and m must be increased about 18°. Hence 



The maximum forces of vapour for different temperatures fol- 

 low a law very similar, and nearly related, to the law of the den- 

 sity. So that the actual force of vapour in the air may be re- 

 presented by y=: F — g^\ where F =: maximum force at the 

 temperatures, and^a constant, which will = .0125 or -^^ when 

 c = .15. Hence the temperature at which aqueous vapour ha- 

 ving the force j^ would be in a state of saturation, and which 

 temperature is usually called the dewing point, will be 



The number substituted for k in this case being 49°.5 Fahr, 

 the temperature at which the variation of force for 1° is .0125. 

 By means of this formula, the point of deposition, or dewing 

 point, may be readily obtained without the aid of tables. With 

 the centigrade thermometer, 



These formulae are adapted to the ordinary pressure, and are 

 by much the simplest I have ever seen for the purpose. 



The dewing point, or point of deposition, is the temperature 

 of saturation under the original pressure. The temperature r is 



