42 THE CULTURE OF FARM CROPS. 



538 grains, or more than an ounce, and the air con- 

 tained in a room 40 feet square and 18 feet high will weigh 

 a ton. The weight of the air, the ease with which a gas is 

 moved, and the fact that the pressure of the weight is ex- 

 erted in all directions, tend to force it into every vacancy, 

 and to diffuse itself everywhere. Every pore and interstice 

 of the soil is filled with it. There it yields up its oxygen to 

 dead matter and quickly converts it into plant food; it 

 carries with it its carbonic acid upon which the roots seize 

 and convey it into the tissues of the plants, where it is elab- 

 orated into new cells and the starch which fills them. Like 

 all other gases it expands with heat and contracts with cold. 

 Every change of temperature therefore expels a portion of 

 the air from the soil or impels a portion to enter it. Thi& 

 causes a constant current and change by Avhich the air is 

 renewed and that vitiated by the Joss of its useful properties 

 is replaced by a fresh supply. Every shower of rain expels 

 it from the soil, and as the water sinks to lower depth, the 

 atmospheric pressure forces air again into the vacant spaces. 

 All this has a most beneficial effect upon vegetation, the re- 

 sults of which are to be considered hereafter. 



This vast body of air, like the watery ocean, has its tides,, 

 its great currents, and its storms, which keep it in perpetual 

 motion. When the farmer hies to his field in the summer 

 morning, the gentle zephyr fans his cheek. In the season 

 of storms the boisterous gale beats him from his path and 

 forces him to take shelter. All these motions serve to mix 

 the air; to purify the centers of corruption; and bring ta 

 them renewed springs of health and vigor. If one could 

 float upon the surface of the atmosphere, great waves would 

 be seen coursing over the vast aerial ocean. These extend 

 for thousands of miles; have many miles of elevation and 

 their courses extend across oceans and continents. These 

 enormous waves and the following depressions, necessarilly 

 changs the pressure of the air on the -surface of the earth, 

 and are marked by a change of the barometer. The result 

 is an increased pressure under the wave with a rise in the 

 barometer; and a decreased pressure under the depression 



