1835.] 



Meteorological Observations. 



49 



Hygrometer. Atmospheric air always contains a certain 

 quantity of moisture in the" state of vapour, whiqh varies 

 under different circumstances. It is capable of holding a 

 much larger quantity at a high, than at a low temperature ; 

 and this power increases in a more rapid ratio than 

 the temperature. Thus at 0 of the centigrade scale, a cubic 

 mass of air 40 inches every way can hold 100 grains of 

 moisture ; at 5 of the same scale it can hold 126 grains; 

 at 10, 158.7 grains; at 15,200 grains; at 20,252.0 

 grains, and so forth. It follows that if air charged with 

 moisture have its temperature reduced, part of this mois- 

 ture must be precipitated ; and this, in fact, is one of the 

 most frequent causes of clouds and of rain. By the accom- 

 panying table. No. 3, we find that air at the temperature 

 of 25'* centigrade is capable of holding 317.5 grains of humi - 

 dity in the state of vapour, and at 15^ centigrade, that it 

 Can retain only 200 grains. Accordingly, if air at the tem- 

 perature of 25o and having its maximum of humidity cooled 

 down to 15, it will discharge 117.5 grains of w^ater, which 

 will appear under the form of dew, fog, rain, or clouds, ac- 

 cording to the situation and circumstances in which it may 

 be placed. Again, if two currents of air having different 

 temperatures, and both fully charged with aqueous vapour, 

 meet and be mixed, a quantity of their moisture must be 

 precipitated ; for, since the humidity increases in a more rapid 

 ratio than the temperature, the mean of the former must 

 be greater than that of the latter. For example, let us sup- 

 pose that a current of air of the temperature of 25«, meet 

 another current of the temperature of 15° and that both are 

 fully charged with moisture— when mixed their mean tem- 

 perature will be 20°. Now, if we consult the table, we shall 

 find that air at 25° can hold 317.5 grains of moisture, and 

 at 20°, 200 grains, the mean of which is 258.75 ; but at the 

 mean temperature of the two currents, viz. at 20°, air can 

 only contain 252 grains ; therefore 6.75 grains must be 

 precipitated ; and it is in this way that clouds and ram are 



frequently formed. . 



It may be remarked that there are two ways m which 

 air may be rendered absolutely humid, viz. either by adding 



