-406 MASSACHUSETTS AGRICULTURE. 



which are always more or less imbued with the fertilizing gases 

 of the atmosphere, to be deposited among the absorbent parts of 

 soil, and given up to the necessities of plants. 



19. It prevents the formation of so hard a crust on the sur- 

 face of the soil as is customary on heavy lands. 



20. It in a great measure obviates the washing away of soil 

 by heavy rains. 



All of these twenty propositions are susceptible of the fullest 

 demonstration ; but for want of time we will merely examine 

 the first one which may give us an insight into the course 

 of reasoning to be applied to the others. 



Underdraining prevents drought, because it renders the soil 

 porous, and thereby gives more freedom to the circulation of air 

 among its particles. There is always the same amount of water 

 in and about the surface of the earth. In winter there is more 

 in the soil than in summer, while in summer that which has 

 been dried out of the soil exists in the form of a vapor in the 

 atmosphere. Its vapory form is maintained by heat which acts 

 as braces to keep it distended. "When this vapor comes in con- 

 tact with substances sufficiently colder than itself, it loses its 

 heat, thus losing the braces which sustained its vapory condi- 

 tion, contracts and thus becomes liquid water. 



This may be observed in many common cases. 



It is well known that a cold pitcher in summer robs the vapor 



in the atmosphere of its heat, and causes it to be deposited on 



■ its own surface. It looks as though the pitcher were sweating; 



but the water all comes from the atmosphere, not, of course, 



through the material of the pitcher. 



If one breathe on a knife-blade, it condenses in the same 

 manner the moisture of the breath, and becomes coated with a 

 film of water. 



Stone houses are damp in summer, because the inner surfaces 

 of the wall being cooler than the atmosphere, cause its moisture 

 to be deposited in the manner described. By leaving a space, 

 however, between the wall and the plaster this moisture is 

 prevented from being troublesome. 



Nearly every night, in the summer time, the cold earth re- 

 ceives moisture from the atmosphere, in the form of dew, by 

 the same principle of condensation. 



