138 THE WORT, P. 



For instance, an increase of temperature 10, of air already heated 

 to 70, will increase its capacity for water much more than the 

 same increase to air heated to only 40. The converse of this 

 is also true, the cooling of hot air diminishes its capacity for heat, 

 much faster than the cooling of air already cold. Air in mount- 

 ing upwards becomes colder, and since every increase of cold is 

 attended with a dimunition of capacity for moisture, it becomes 

 proportionally damper, and thus the middle regions of the atmos- 

 phere become soon charged with moisture, and were it not for a 

 conservative principle which we will now mention, the heavens 

 would be perpetually shrouded with clouds ; this principle is, air 

 in expanding has its capacity for moisture increased, and there- 

 fore, as the air which is ascending, continually expands from the 

 dimunition of pressure, it becomes consequently drier and drier. 



Clouds are formed either by a watery vapor rising so high as to 

 reach a degree of cold sufficient to condense it, or from a mix- 

 ture of warm air with cold, the moisture being derived from the 

 warmer portion. Fog is nothing more than a cloud formed upon, 

 or near to, the surface of the earth, and is due to a mixture of 

 warm and cold air. Thus in a cold morning we see moisture 

 deposited from the warm breath of animals, when it comes in 

 contact with the air. The course of that remarkable body of wa- 

 ter which is called the Gulf Stream, and which flows in a warm 

 current from the Gulf of Mexico as far up as the banks of New- 

 foundland, is marked by a fog, produced by the colder air sweep- 

 ing over it. Fogs are not common in hot climates, the air being 

 too warm near the surface of the earth to condense the moisture 

 faifficiently to form a cloud. 



Dew is formed when air charged with moisture comes in con- 

 tact with a surface in a certain degree colder than itself. The 

 formation of dew may be very prettily illustrated by bringing a 

 tumbler of cold water into a warm room, the outside of the tum- 

 bler will soon be covered with a coat of moisture, deposited from 

 the warm air which has come in contact with it. In order to 

 have a copious deposite of dew it is essential that the ground 

 should be considerably colder than the air above it, and, it is act- 

 ually found, that upon those nights when the most copious dews 



