WHAT IS KNOWN OF THE EARTH. 819 



about the restoration of equilibrium, which, is being constantly 

 disturbed from below. The principal periodical winds — such as 

 the trade-winds, the monsoons, the land and sea breezes — are 

 found to be essentially dependent on periodical variations of 

 atmospheric pressure, accompanying variations of temperature 

 due to geographical position or surface conditions. The proxi- 

 mate causes of the more characteristic winds have also been 

 well made out. These, too, are due to atmospheric disturbances 

 producing areas of high or low pressure ; the rapidity and inten- 

 sity of the development of which, with the direction of their paths 

 and their position, determine the force of the wind, the direction 

 in which it blows, and the manner in which it veers or backs, that 

 is, changes its direction. But how the changes of pressure are 

 determined, and what causes the transfer of the disturbed area, 

 commonly under the form of an atmospheric eddy or vortex, in a 

 definite direction, usually from west to east, is still to be ascer- 

 tained ; though here, too, it is obvious that the distribution of the 

 land and sea areas, and of the ocean-currents, on which the tem- 

 perature of the superincumbent air so immediately depends, com- 

 bined with the rotatory motion of the earth, are among the 

 principal agencies at work. 



■ Among the most intricate problems of meteorology are those 

 relating to the evaj)oration of water, the formation of vapor and 

 its diffusion and suspension in the air, and its condensation as 

 cloud, rain, or snow. The low specific gravity of aqueous vapor, 

 and the consequent evaporation that releases it at the earth's sur- 

 face, tend to diffuse it in accordance with the mechanical laws 

 which govern elastic fluids. But the reduction of the temperature 

 of the air in ascending above the surface renders this diffusion 

 impossible beyond a certain point ; and observation shows that 

 the quantity of vapor actually existing in the upper parts of the 

 atmosphere is mainly dependent on temperature, and amounts to 

 not more than one fourth part of what would be present if it were 

 diffused freely and simply obeyed the law of hydrostatic pressure. 

 It follows that a height in the atmosphere is at length necessarily 

 reached where condensation must take place and clouds or rains 

 be formed, and that, speaking generally, the vapor in the upper 

 strata of the air is constantly tending to a condition of unstable 

 equilibrium, from which it may readily be once more restored to 

 the earth in the shape of water. This sufiiciently accounts for 

 the rarity of a perfectly cloudless sky, which indeed can hardly 

 exist excepting where such a movement of the air is going on as 

 will carry off the aqueous vapor, as fast as it is formed by evap- 

 oration, to a region where the temperature is high enough to pre- 

 vent its condensation. 



The great activity of the air in discharging the functions of 



