216 



NINTH ANNUAL REPORT OF 



series of calculations from the record of the observations of the wet 

 and dry bulb ihermometer. 



It will be seen by a comparison of table No. 11 with table No. I that 

 as the temperature increases, the amount of water which exists in the 

 air as vapor also increases. The two elements heat and moisture 

 mutually uifluence each other as to the quantity present in the atmos- 

 phere at a given time. With an increase of elevation of the sun above 

 the horizon its rays pass to the earth through the atmosphere less 

 obliquely and impinge more perpendicularly on the surface. This 

 produces an increased amount of vapor, with an increased elastic force, 

 which enables the air in turn to absorb and retain a larger quantity of 

 heat. 



'The maximum quantity of moisture is at 2 p. m. The increase in 

 weight is from about 3J grains to near 4 grains. 



This table, however, does not giv^e the amount of vapor which the 

 air could hold if sufficient moisture were present to entirely saturate it. 

 Indeed the air is very seldom fully saturated, and in order to begin to 

 precipitate the vapor it contains into water, it is generally necessary to 

 lower the temperature quite a nmnber of degrees. The point of tem- 

 perature at which the moisture begins to settle — for example, on the 

 surface of a bright tin cup partly filled with water which is slowly 

 cooled down by gradually adding ice water — is called the dew jwint. 

 The greater number of degrees the water is obliged to be lowered 

 before dew begins to be deposited, or the greater the difference 

 between the temperature of the air and the dew point the greater is 

 the dryness of the air, or the greater is the tendency of vapor to exhale 

 from the skin and from all bodies containing moisture. 



Table No. Ill gives the mean diurnal variation in the dew point. 



TABLE in. 



MEAN DIURJIAL VARIATION OF THE DEW POINT AT GREENWICH. 



+ 



Maximum. 



Minimum. 



If we subtract the numbers in this table from those in Table No. 1, 

 we shall have approximately the relative dryness, or evaporating 

 power of the air ; this is given in No. IV. 



TABLE IV. 



Minimum. 



Maximum. 



