S62 



Theory of Dew — Change in Plants. 



Vol. IV, 



Tlieory of De-w. 



Notwithstanding the researches of Dr. 

 Wells, and others, upon the subject of dews, 

 there are many who still remain either in 

 total ignorance of the principles of the de- 

 position of the moisture, called dew, or hold 

 to the old theory that it is caused by the air 

 alone becoming colder. The ancient Greeks 

 observed the tact, that dew was deposited in 

 clear nights, and not in windy or cloudy ones, 

 and every barefooted boy who goes out early 

 in the morning, knows that it is much more 

 abundant on the grass by the wood side, than 

 it is upon the sand or gravel in the road it- 

 self Dr. Wells, of England, made many ex- 

 periments upon the subject; and from the 

 facts thus ascertained, explained the cause 

 of the phenomenon, in a treatise which he 

 published some time ago. He ascertained 

 the cause of the deposit of moisture in the 

 form of dew, to be the radiation or throwing 

 of the heat imbibed by the sun, which thus 

 cools the particles of air that come in contact 

 with it, and causes the moisture which was 

 in the warm air, to be condensed. It may be 

 well, for the better understanding of the 

 theory, to state, in the first place, that as all 

 bodies receive heat more or less easily, so 

 they part with it, or radiate it more or le.ss 

 easily. Some bodies will part with, or radi- 

 ate the heat which they have received, as 

 fast again as other bodies. Hence they be- 

 come cooled before the others. Warm air 

 will bold more moisture than cold air : there- 

 fore, the body which radiates heat fastest, be- 

 comes cool first, and cools the particles of air 

 which surrounds it, which deposits the mois- 

 ture or dew upon it first. Grass parts with 

 its caloric, or heat, much faster than sand or 

 gravel — hence it has more dew upon it. 



The following experiments may not be un- 

 interesting to some of our readers. After a 

 long period of drought, when the air was 

 very still, and the sky serene, Doctor W^ells 

 exposed to the sky, twenty-eight minutes be- 

 fore sunset, previously weighed parcels of 

 wool and swandown, upon a smooth, unpaint- 

 ed and perfectly dry fir table, five feet long, 

 three broad, and three feet in height, which 

 had been placed an hour before, in the sun- 

 shine, in a large, level grass field. 



The wool, twelve minutes after sundown, 

 was found to be fourteen degrees colder than 

 the air, and to have acquired no weight. 

 The swandown, the quantify of which was 

 much smaller than that of the wool, was at 

 the same time thirteen degrees colder than 

 the air. In twenty minutes more, the swan- 

 down was fburteon and a half degrees colder 

 than the surrounding air. At the same time, 

 the grass was fifteen degrees colder than the 

 air four feet above the ground. F'rom such 

 experiments, he established the proposition, 



that bodies must become colder than thesur. 

 rounding air, before they become dewed, 



He also explained the reason why tiiere 

 was no dew in cloudy nights. It is because 

 the clouds act as reflectors, and throw tic 

 heat back again; in the same manner as t)i« 

 bright tin top of a tin baker throws dov 

 reflects the heat down upon the bread, A 

 blanket, or umbrella, put over any bodt, 

 would prevent the dew settling on that bod; 

 for some time, although it was falling ai; 

 around it. 



On this principle, vines, and other things, 

 are protected from frosts, by putting a blank?, 

 or covering over them, at night. 



Frost is dew frozen. By putting the co 

 vering over the body, it reflects heat baci, 

 and prevents, for sometime, thedewbeiij 

 deposited upon what it covers. 



\_Maine paper. 



Change of Soil Effectiiig a change ia 

 Plants. 



A change of soil may be effected either It 

 removing a plant from one spot of earth li 

 another, difl^ering from it in fertility, or ij 

 the addition of manure, producing a cbanji 

 in the character of the soil in wh:.ch thepl»i 

 grows, without changing the location of ii( 

 plant. The effect of removing a plant fin 

 a comparatively barren to a more fertile s^ 

 is to increase the size of all its parts, ui 

 often to convert its organs of one kind idI 

 those of another. Experience has taiiyhtu 

 that it is advantageous to supply food t 

 plants artificially. Where increase in tbf 

 size of vegetables, without reference to tia 

 number, is desired, it can almost alw 

 accomplished by affording an increased suf 

 ply of all the ingredients of the food i 

 plants, distributed in a well pulverized soi 

 in such a manner that the roots of the plui 

 can easily reach it. The effect thus [» 

 duced can be greatly increased byadditim 

 heat and moisture, and by a partial exclusH 

 of the direct rays of the sun, so as tomodij 

 the evaporation of fluids from the plant, a 

 perience alone can determine to wliatexi* 

 this may profitably be carried in tlie case* 

 such species of vegetable. Tlie results whid 

 have been produced in some instances, «^ 

 truly remarkable. Loudon etafes, that »k 

 bages have been produced, wcighin" ht"' 

 hundred weight, apples a pound ami a W 

 and cabbage-roses of tour inches in dianieW 

 or more than a foot in circumference. ». 

 cultivation and a change of soil, the ap}'* 

 ancc of manv trees lias been entirely a''''"* 

 Tlie wild crab-apple, the original stock W 

 which all our vast variety of apples i»* 

 sprung, lias its stem and brandies scttfi» 

 with thorn.s. On removing it to a more W 

 tile soil and more favourable circumstsiCT 



