THE FARMER AT HOME. 191 



surface to the air ; for, in very cold weather, the air conducts the heat 

 from the surface faster than combustion renews it, so that, if the 

 amount of surface exposed be large, the fire will go out. This kind 

 of coal yields no visible smoke. The chimney, however, should be 

 large enough to transmit smoke, otherwise some of the carbonic acid, 

 which is formed during the combustion, will be sent into the room. 

 This gas is the suffocating vapor of burning charcoal. 



GRAVEL . Stones from the size of a pin's head to those of two 

 or three pounds in weight, are termed gravel, and the greater or less 

 quantity of them in the soil, as well as the kind of rock of which they 

 are composed, has a great influence on the fertility and cultivation of 

 land. If the gravel is coarse, and the particles slightly connected, 

 the soil will be what is called hungry, as manures put upon it sink 

 among the porous materials, and produce little or no effect on the 

 crops. On the contrary, if the gravel is finer, or sufficiently filled 

 with other earths and vegetable and animal matter, it constitutes one 

 of the most valuable soils, arid is particularly excellent for wheat and 

 clover. Gravelly soils are apt to be too dry ; but unless very coarse, 

 the incorporation of clay will have a beneficial effect, by rendering it 

 more retentive of moisture, and consequently giving it more support to 

 the crop. Under the influence of the doctrine promulgated by Tull, 

 that the finer the particles of the soil the better it would be, some farmers 

 sifted some of their soils, and carried off the gravel so collected ; but 

 the effect was found to be most injurious, as the soils speedily became 

 too compact for profitable vegetation ; the experiment was abandoned, 

 and the pebbles returned to their original place. 



GRAVITATION. The principle of gravity, of so much import- 

 ance in the sciences of mechanics and astronomy, was first explained 

 by Sir Isaac Newton. His attention was fi r t directed to the subject 

 by seeing an apple fall to the ground, and lie began to consider what 

 could be the cause. The same phenomenon everywhere presents itself. 

 A stone dropped from the hand comes to the ground. A sand-bag let 

 fall from a balloon does the same. The same cause acting in all these 

 cases is called gravity ; and its influence is exerted over all bodies 

 whatever upon our earth. Smoke and vapor indeed arise from the 

 surface of the earth, but that is for the same reason that wood rises 

 to the surface of water, or a balloon in the air. In a vacuum pro- 

 duced by the air-pump, smoke and vapor do not ascend. Specific 

 gravity, is the relative gravity of any body or substance, considered 

 Adth regard to some other body which is assumed as a standard of 

 comparison, and this standard, by universal consent and practice, is 

 rain-water, on account of its being less subject in variation in different 

 circumstances of time and place, than any other body, whether solid 

 or fluid. And, by a very fortunate coincidence, at least to English 

 philosophers, it happens that a cubic foot of rain-water weighs one 

 thousand ounces avoirdupois ; and, consequently, assuming this as the 



