152 BOARD OP AGRICULTURE. 



the Arabs, who now have their liuts built upon these irrigated 

 portions, and attend to their cultivation. So that what I hinted 

 at last night as possible or probable, is already being realized. 



Mr. Sears. Perhaps I may be allowed to state a fact and ask 

 an explanation of the Professor. A few years ago I was upon 

 high lands in a town northeast of here. Westward from me laj?^ a 

 valley of some 12 or 15 miles extent. Presently a small dense 

 cloud arose in the west. When I observed it, it cast a shadow on 

 a few acres of this basin. That little cloud came towards me, and 

 before it reached me, I saw that rain was coming from it, and be- 

 fore I could remove my traps from the wagon, I was completely 

 drenched. It was one of the heaviest showers upon record. The 

 question is, from whence came that water ? 



Prof. Fernald. Most of the variable winds that prevail over 

 this continent, are to a greater or less degree charged with mois- 

 ture. As was stated last evening, the prevailing southwesterly 

 winds and the westerly winds, blow over a large extent of land, 

 and hence are constantly losing moisture ; for any circumstances 

 which shall produce condensation, or a reduction of #the tempera- 

 ture, will cause a deposition of it. It is sometimes the case, that 

 a wind blowing from the south coming in contact with elevated 

 portions of land is deflected from its course, and moves on, as a 

 rain wind in a direction contrary to the prevailing wind, and thus 

 we may occasionally have rains when the wind is from a very dif- 

 ferent quarter from that from which rains genei'ally come. It was 

 mentioned last night by Mr. Brackett that there was a storm last 

 season when the wind was from the northwest, and doubtless this 

 was due to some such circumstance. 



Mr. Sears. Do you suppose that the water was held in the at- 

 mosphere, and that the cooling of the atmosphere condensed the 

 moisture and caused the shower ? 



Prof. Fernald. I do. Vapor is held in the atmosphere in an 

 invisible state until by condensation a cloud is formed. The little 

 vesicles, when first formed, ai'o very minute indeed, but increase 

 in size until they possess sufficient weight to fall. There is a 

 curious fact in connection with the falling of rain, which was not 

 noticed last evening, although the general fact was stated that the 

 velocity of rainfall is so regulated that it seldom does injury to 

 vegetation, liuin drops descend with an accelerated motion, until 

 they have acquired a velocity at which the resistance of the air 

 is just equal to the downward pull of gravity ; thereafter they de- 



