THE FARMER'S MAGAZINE. 



143 



the atmosplieric air can freely penetrate, there the 

 deposition of dew under favourable circumstances 

 takes place. This also often occurs in the 'interior 

 of a soil when evaporation is taking place from the 

 surface. The pulverization of the soil, therefore, 

 by promoting its power of absorbing the atmos- 

 pheric aqueous vapour, increases its fertility. Dry 

 soils, even the driest contain about 10 per cent, of 

 moisture. We know that when soils are dried in 

 a temperature of '212 degrees, and exposed on their 

 surfaces to air saturated with moisture, they ab- 

 sorb very considerable portions of water. Suppose 

 a soil which weighs about 1,000 tons per acre was 

 j)ulverized so as to be freely permeable by the at- 

 mosphere, and that such a soil, after being 

 thoroughly dried, was exposed to the air, then we 

 find from the experiments of Schubler that it would 

 absorb of water in twenty-four hours — 



If a sandy clay 2G tons. 



Loamy clay 30 „ 



Stiff' clay 36 „ 



Garden mould 45 „ 



If, however, the efforts of the husbandman in 

 this may tend to increase the supply of atmospheric 

 moisture, his labours in another direction some- 

 times diminish it. 



" It is evident," says Dove, "that a vigorous 

 vegetation produces rain, which, on the other 

 hand, again nourishes that vegetation ; and that 

 the senseless destruction of forests has thus often 

 destroyed the fertility of a district." 



Previous to 1821, Provence and the Department 

 of the Var possessed an abundance of brooks and 

 springs. In that year the olive trees, which there 

 almost formed forests, were killed by the frost, and 

 they were cut down to the roots in 1S22 ; since 

 which time the springs dried up, and agriculture 

 suffered. In Upper Egypt, eighty years ago, 

 springs were still abundant, but they have ceased 

 since the Arabs cut down the trees in the Valley of 

 the Nile, towards Libya and Arabia. A contrary 

 effect has been produced in Lower Egypt through 



the extensive plantations of trees by the Pasha, 

 In Alexandria and Cairo, where rain was formerly 

 a great rarity, it has since that period become 

 much more frequent. 



The somewhat different way in which the leaves 

 of plants appear to act in the receiving and dis- 

 posal of dew was noticed by Mr. J. Parkes {Journal 

 R.A.S., vol. V, p. 133). "A blade of grass," he 

 observes, "is sometimes spangled over with dew- 

 drops, but it usually becomes wetted through its 

 whole surface, by the running together of the 

 drops, and thus conducts the water to the earth in 

 minute streamlets ; whereas the leaves of the clover, 

 nasturtium, and many other plants, will be found 

 to collect it in distinct globules, which may be 

 rolled about on the leaf without appearing to 

 moisten it. These drops, in fact, do not touch 

 the leaf, but rest and roll upon a pillow of air in- 

 terposed between them and the substance of the 

 leaf. On very translucent nights I have seen 

 the tender clover leaf bend beneath the weight of 

 its crystal load, discharge it on the ground, and 

 immediately begin to accumulate another globule. 

 Cup-formed and horizontal, leaves and flowers, 

 seem to retain all or very nearly all their collected 

 dew for their special use, as if it were more benefi- 

 cial to them when so applied than to their roots." 



We have thus traced the vapour of the atmos- 

 phere from its various sources, till it again appears 

 as cloud, or descends as the fog or dew. We 

 have marked that dew gradually accumulating till 

 it became united in drops, and then either rolhng 

 off the surface of the leaves of plants in crystal 

 globules, or in streams of dew-water. But these 

 bright drops are almost as often formed at, and 

 descend upon us from considerable elevations ; and 

 here, again, we find in the rain-drop other sources 

 of interest and instruction. But these are too 

 copious to be merely viewed as the concluding 

 portion of our present essay ; I propose, therefore, 

 to recur to the raindrop on a future occasion. 



THE ROTATION OF CROPS, AND THE REASON. 



The practice of agriculture consists of a series of 

 operations, by which we endeavour to raise from the 

 land the most valuable produco it is capable of yielding, 

 with as little cost as possible, and with the least injury 

 to the soil. An ignorant person might manage to 

 draw from the land very heavy crops of corn for a short 

 time, and in doing so he might seriously damage the 

 property. Whereas, another, possessing a practical 

 knowledge of good systems, might succeed in realizing 

 the same value of produce without injuring the charac- 

 ter of the land. It has thei'efore been accepted as an I 

 established principle, that a judicious succession of i 

 crops is advantageous for aiding the farmer to produce ' 

 the best crops he can with the least detriment to theland. 

 Experience proves to every observant person, that, 

 under our ordinary practice, the repeated growth of 

 any crop upon the same piece produces a gi-adual de- 



crease in the quantity which the land is capable of 

 yielding. This takes place moi e rapidly in some soils 

 and under some crops than others, but the same fact 

 stands out in all the light of an accepted truth. Now 

 to what causes can we trace this ? The only two 

 changes which the plant can have produced in the 

 soil are the addition of noxious-matter and the ab- 

 straction of fertiiiziug-matter. It has been well 

 argued that a plant, during the period of its growth 

 having received from various powers its supplies of 

 food, appropriated for the purposes of its own de- 

 velopment such matter as it required, and ejected by 

 its roots, as excremeutitious matter, that which was 

 of no further use. It was further believed, that as 

 the soil became more and more fully charged with such 

 excrementitious matter, the growth of the crop be- 

 came less luxuriant because of the soil being charged 



