THE SMITHSONIAN INSTITUTION. 



217 



From this it appears that the minimum dryness or greatest damp- 

 ness of the air also occurs at four o'clock in the morning, and the 

 maximum dryness at two o'clock in tl)e afternoon. 



Tlie degree of humidity, however, of the air, may be more definitely 

 expressed by the result of a more laborious process, namely, by 

 dividing the weight of water in the air at a given time by the whole 

 quantity the air could hold if it were saturated. By making the 

 necessary calculation, and considering the point of saturation as unity, 

 we shall have the following table : 



TABLE V. 



MEAN DIURNAL VARIATION OF THE HUMIDITY OF THE AIR AT GREENWICH 



+ 



Maximum. 



Minimum. 



Wiien the quantity of water is in the least degree greater than the 

 atmosphere can contain at a given temperature, condensation takes 

 place, which may be either in the form of fogs or dew^ in the lower 

 strata or clouds, and rain in the upper. The formation of clouds will 

 therefore depend on the amount of moisture in the atmosphere. This 

 is shown by 



TABLE VI. 



MEAN DirUNAl, VARIATION OF THE CLOUDINESS OF THE ATMOSPHERE AT GREENWICH. 



4- 



Maximum. 



Minimum. 



The actual weight of a given portion of the air is also affected by the 

 variation of temperature. This is shown in the following table, in 

 which the minimum of weight occurs at the hour of maximum tem- 

 perature. 



TABLE VII. 



MEAN DIURNAL VARIATION OF THE WEIGHT OF A CUBIC FOOT OF AIR AT GREENWICH. 



Maximum. 



Minimum. 



