256 



MANUAL OF AGEICULTURE. 



vacant place being occupied by a fresli supply possessed of the 

 normal coldness and density. Hence the origin of the winds, 

 and the constant atmospheric movements round the earth. The 

 constant ascent in this manner of the body of air superincumbent 

 upon the tracts of ocean, having a breadth of belt extending to 

 one or two degrees on either side the equator, and the supply 

 of colder air to fill up the vacuum, combine to effect the bene- 

 ficial phenomenon of the steady trade winds. And their pre- 

 vailing direction is consequent upon a supplementary fact, viz., 

 the greater speed of terrestrial revolution at the equator, arising 

 from the earth's increased diameter at that j)art ; for this swifter 

 easterly motion than at other parts of the earth's surface makes 

 the trade winds fall behind, so to speak, and to seem blowing 

 from the north-east and south-east. To the irregular occurrence 

 of partial aerial rarefaction is due the phenomena of monsoons, 

 and all winds great and small. From water, moisture evaporates 

 at all temperatures, even when it is in the form of ice or snow ; 

 and the air has the property of being able to hold a large quan- 

 tity of such aqueous vapour in suspension. The higher the 

 temperature of the atmosphere, the larger the proportion of 

 aqueous vapour it can absorb ; therefore it possesses various 

 degrees of saturation, ix., points beyond which it can hold no 

 more in suspense. As air becomes cooler, so does its power of 

 saturation also become lowered ; whereupon all the watery 

 vapour in excess is condensed, and falls as rain, or is deposited 

 as dew. Dew is produced by the rapid radiation of heat from 

 the warm surface-ground and herbage after sunset, These 

 becoming cooler than the immediately surrounding air, the latter 

 in turn parts with its heat to them by radiation, whereby it has 

 its saturation point lowered, and the excess of vapour becomes 

 deposited as dew. And so with fogs and mists ; they result 

 from the radiation of heat from land and water, taking with it 

 aqueous vapour, which becomes visible upon encountering cooler 

 air. Similarly, rain is produced when heated volumes of air 

 are deprived of their heat, through the fall of condensed vapour, 

 which assumes, according to the temperature it encounters, the 

 form of rain, hail, or snow. The following table gives the weight 

 in grains of a cubic foot of vapour at successive ascents of 10°, 

 from 0° to 90° Fahrenheit, clearly demonstrating the increase of 

 the saturation point with the rise of temperature : — 



