212 HEAT. 



and be content with an approximation to the truth. In Hazen's Tables * 

 the formula given is 



using a constant barometer reading of 29 '4 inches. A variation of two 

 or three inches in the barometer does not seriously affect the result. 



If the pressures are in millimetres and the temperatures are centi- 

 grade the formula may be replaced by 



F=/- -00068(^-0750 



As an example of the first formula suppose = 65F. and ' = 50F. f 

 so that t-t' = 15. At 50F. /= '3598 in., whence 



F = -3598 - -165 = -1948 inch. 



which is the vapour- pressure at 34, and this is the dew-point. 



The relative humidity is obtained by dividing the actual pressure 

 1948 by the vapour-pressure at 65 which is -6163. This gives the 

 f&lue '31. 



To save time and arithmetic, tables are constructed in which the dew- 

 point and relative humidity are given for every depression read to half a 

 degree below the temperature of the dry bulb. Thus opposite 65 in the 

 table 34 and -31 are entered under t-t' '= 15. 



The Chemical Method. In this method a measured quantity of 

 air is drawn through drying tubes which take from it all the water 

 vapour, and weighing the tubes before and after the passage of the air 

 through them, the gain in weight gives the amount of vapour which was 

 present in the air. For details of the method we refer the reader to 

 Glazebrook and Shaw's Practical Physics, p. 233. 



In Fig. 125 A is the aspirator from which the water sipnons out; 

 CO are the drying tubes ; B an intercepting drying bottle to prevent 

 any vapour passing back from the aspirator to CO. The volume of 

 water drawn through from the beginning to the time when the aspirator 

 is empty is measured. Suppose it to be V. Let the mean temperature 

 of the air entering at a be t and let the final temperature of the air in 

 the aspirator be t'. Let the barometric height be H. Then we have a 

 volume V of air at if saturated with vapour at t'. If the saturation 

 pressure at t' is P' the pressure of the dry air is H P' and the weight 

 of the dry air drawn through the tubes 



W _. V H-F X -001293 

 760 l+at' 



If the gain in the weight of the tubes is w, the weights of water- 

 vapour and dry air present in any volume of air are in the ratio w : W. 



Since, according to the results given below, a given volume of water- 

 vapour weighs 0'622 as much as an equal volume of dry air at the same 

 temperature and pressure, the pressures exercised by these are in the 



an 



ratio --- : W, their sum making up the barometric pressure H. 



"D tit 



If then P is the actual pressure of water-vapour 



* Handbook of Meteorological Tables (Washington). 



