METEOROLOGICAL INSTRUMENTS. 139 



TO FIND THE QUANTITY OF VAPOR BY WEIGHT EXISTING IN 

 THE ATMOSPHERE. 



PROBLEM. The temperature of the atmosphere in the shade, 

 and of the Dew Point, being given, to find the quantity of vapor in 

 a cubic foot of air. 



If the temperature of the air and the Dew Point correspond, which 

 is the case when both thermometers are alike, and the air conse- 

 quently saturated with moisture, then in the table of quantity oppo- 

 site to the temperature will be found the corresponding weight of a 

 cubic foot of vapor expressed in grains. 



EXAMPLE Let the temperature of the air be 70 deg. Fah.. and 

 the Dew Point the same then opposite the temperature you have 

 the weight of a cubic foot of vapor 8.392 grains. 



But if the temperature of the air be different from the Diw Point, 

 a correction is necessary to find the exact weight 



EXAMPLE. Suppose the Dew Point be 70 deg. Fah. as before, but 

 the temperature of the air in the shade be 80 deg., then the vapor 

 has suffered an expansion due to an excess of 10 deg , which requires 

 a correction. 



We Hud in the table of corrections for 10 deg. 1.0208. 



Then divide 8.392 grains at the Dciv Point, viz. 70 deg. by the 

 correction corresponding to the degrees of absolute dryness, viz. 

 10 deg , and you have the actual weight of vapor existing. 

 8.3920 



EXAMPLE. 8.221 grains existing, which subtracted from 



1.0208 



weight of vapor, corresponding to the temperature of 80 Fah., gives 

 the number of grains required for saturation at that temperature. 



EXAMPLE. 11.333 grains at the temperature of 80 deg. Fah. 

 8.221 grains contained in the air. 



3.112 grains required for saturation. 



To find the relations of these conditions on the natural scale of 

 humidity (complete saturation being 1.000), divide the weight of 

 vapor at the Dew Point by the weight at the temperature of the air, 

 the quotient, gives the parts of 1.000 the degrees of saturation. 

 8.392 grains at the Dew Point 70 



EXAMPLE. =.740 deg. of humidity 



11. 333 grains at the temp, of the air 80 saturation being 1.000 



The principles of these calculations will be found in Professor 

 Daniell's Meteorological Essays Mr. Anderson's Essays on Hygro- 

 metry, in the Edinburgh Encyclopedia, vol. XL, and in the Edin- 

 burgh Journal of Science, vol. VII., page 43, in an excellent article 

 on the Dew Point Hygrometer, by Mr. Foggo from which the taMe 

 of corrections has been partly subtracted. The Table of Quantity 

 by Weight has been taken from Professor DanielPs Work on Meteor 

 ology ; to which the reader is referred for further particulars. 



