THE FARMER AT HOME. J2l 



constantly taking place in all bodies, it is obvious that the tempera- 

 ture of any body can remain the same only by its receiving from 

 another source as many rays as it emits. In the case of the earth's 

 surface, so long as the sun remains above the horizon, it continues to 

 receive as well as to emit heat ; but when the sun sinks below the 

 horizon, no object is present in the atmosphere to exchange rays with 

 the earth, which still emitting heat into free space, must, consequently, 

 experience a diminution in its temperature. 



Thus the earth becomes not only many degrees cooler than the 

 superincumbent air ; and, as the atmosphere always contains watery 

 vapor, this vapor becomes condensed on the cold surface ; hence, the 

 origin of dew, and, if the temperature of the earth is below thirty-two 

 degrees, of hoar frost. And since the projection of heat into free 

 space takes place most readily in a clear atmosphere, it is under tho 

 former condition that dew and hoar frost are formed ; for if the radi- 

 ant caloric, proceeding from the earth, is intercepted by the clouds, 

 an interchange is established, and the ground retains nearly, if not 

 quite, the same temperature, as the adjacent portions of the air. 

 Whatever circumstances favor radiation favor also the production of 

 dew ; and, accordingly, under the same exposure, dew is much more 

 copiously deposited on some surfaces than on others. Gravel walks 

 and pavements project heat and acquire dew less readily than a 

 grassy surface. Rough and porous substances, as shavings of wood, 

 take more dew than smooth and solid wood. Glass projects heat 

 rapidly, and is as rapidly coated with dew. Bright bodies attract 

 dew much less powerfully than other bodies. 



Dew acts an important part in the processes of agriculture, and in 

 the nutrition and growth of plants. Large quantities of the most 

 active agents escape from the earth during the processes of decompo- 

 sition and evaporation in the shape of gases, and these combined with 

 the aqueous vapor are deposited with the dew on the earth, or on the 

 plants, and in either case are available to nutrition. Hence the 

 advantages of frequently stirring the earth, and keeping the surface in 

 a pulverised and absorbing state. In some parts of the world it 

 rarely rains, but the dews are so copious, that vegetation does not 

 seem to suffer from the want of water. Spreading a substance, no 

 matter how flimsy, as a thin cloth, over vegetables will preserve 

 them from severe frosts, if it is not allowed to touch them ; acting by 

 intercepting the heat. Every one has observed that plants liable to 

 destruction by frost, remain green much longer under the shade of 

 trees than when exposed. Thus potatoes or any thing else planted in 

 an orchard, will be unhurt by frost as far as the branches of the trees 

 extend, while the tops in the uncovered spaces will be wholly 

 prostrated. 



DIBBLE R. This is an instrument used in gardening for making 

 holes in the ground to receive roots in transplanting. It is generally 

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