2G8 WELLS's NATURAL PHILOSOPHY. 



temperature of a place would depend only on its geographical latitude, and 

 cousequently all places having the same latitude would have a hke climate. 

 Owing, however, to various disturbing causes, such as the elevation and form 

 of the land, the proximity of the sea, the direction of the winds, etc., places 

 of the same latitude, aud comparatively near each other, have very diflerent 

 temperatures. 



In warm climates the proximity of the sea tends to diminish the heat ; in 

 cold climates, to mitigate the cold. Islands and peninsulas are warmer than 

 continents ; bays and inland seas also tend to raise the mean temperature. 

 Chains of mountains which ward off cold winds, augment the temperature; 

 but mountains which ward off south and west winds, lower it. A sandy soil, 

 which is dry, is wanner tlian a marshy soil, which is wet and subject to great 

 evaporation. 



602. Air absorbs moisture at all tempera- 

 capacity of air tures, and retains it in an invisible state, 



for moisture? i-m • /• i i • • ± i 'i v 



This power oi the air is termed its capacity 

 for absorption. 



The capacity of air for moisture increases with the tem- 

 perature. 



A volume of air at 32° can absorb an amount of moisture equal to the hun- 

 dred and sixtieth part of its own weight, and for every 27 additional degrees 

 of heat, the quantity of moisture it can absorb at 32° is doubled. Thus a body 

 of air at 32° P. absorbs the 160th part of its own weight ; at 59" R, the 80th; 

 at 8G° F., the 40th ; at 113° F., the 20th part of its own weight in moisture. 

 It follows from this that while the temperature of the air advances in an arith- 

 metical series, its capacity for moisture is accelerated in a geometrical series. 



When is ai.- -^^^ ^^ ^^i*^^ ^^ ^^ Saturated with moisture 

 Mtedf^^ *'''''' when it contains as much of the vapor of water 

 as it is capable of holding with a given tem- 

 perature. 



"We say that air is dry when water evaporates quickly, or any wetted sur- 

 face dries rapidly ; and that it is damp when moistened surfaces dry slowly, 

 or not at all, and the sliglitest diminution of temperature occasions a deposit 

 of moisture in the form of mist and rain. These expressions do not, however, 

 convey altogether a correct idea of the condition of the atmosphere, since air 

 which we term " dry," may contain much more moisture than that which we 

 distinguish as " damp." For indicating the true condition of the atmosphere 

 in reference to moisture, we therefore use the terms " absolute" and " relative" 

 humidity. 



"When we speak of the absolute humidity of the air, we 

 br^blolii'tlfand ^^^^ reference to the quantity of moisture contained in a given 

 relative humid- volume. By relative humidity, we refer to its proximity to 

 * ^ saturation. Relative humidity is a state dependent upon the 



mutual influeuce of absolute humidity and temperature ; for a given volume 



