Boo!v III. ROTATION OF CllOrS. 331 



a calcareous soil, are injurious on soils which do not effervesce with acids ; and that cal- 

 careous waters, which are known by the earthy deposit they afford when boiled, are of 

 most use on silicious soils, or other soils containing no remarkable quantity of carbonate 

 of lime." 

 SuBSECT. 6. Changing the Condition of Lands, in respect to Atmospherical Influence. 

 '2213. The influence of the weather on soils may be affected by changing the position 

 of their surface and by sheltering or shading. 



2214. Changing the condition of lands, as to solar influence, is but a limited means of 

 improvement; but is capable of being turned to some account in gardening. It is' 

 effected by altering the position of their surface, so as that surface may be more or less 

 at a right angle to the plane of the sun's rays, according as heat or cold is to be increased 

 or diminished. The influence of the sun's rays upon any plane are demonstrated to be 

 as their number and perpendicularity to that plane, the effects of the atmosphere being 

 excepted. Hence one advantage of ridging lands, provided the ridges run north and 

 south ; for on such surfaces the rays of the morning sun will take effect sooner on the 

 east side, and those of the afternoon will remain longer in operation on tiie west side ; 

 whilst at mid-day his elevation will compensate, in some degree, for the obliquity of his 

 rays to both sides of the ridge. In culture, on a small scale, ridges or sloping beds for 

 winter-crops may be made south-east and north-west, with their slope to the south, at an 

 angle of forty degrees, and as steep on the north side as the mass can be got to stand ; 

 and on the south slope of such ridge, cceteris paribus, it is evident much earlier crops 

 may be produced than on level ground. The north side, however, will be lost during 

 this early cropping ; but as early crops are soon gathered, the whole can be laid level in 

 time for a main crop. Hence all the advantage of grounds sloping to the south south- 

 east, or south-west, in point of precocity, and of those sloping to the north for lateness and 

 diminished evaporation. Another advantage of such surfaces is, that they dry sooner 

 after rains, whether by the operation of natural or artificial drainage ; or, in the case of 

 sloping to the south, by evaporation. 



2215. Shelter, whether by walls, hedges, strips of plantation, or trees scattered over 

 the surface, may be considered, generally, as increasing or preserving heat, and lessening 

 evaporation from the soil. But if the current of air should be of a higher temperature 

 than the earth, screens against wind will prevent the earth from being so soon heated ; 

 and from the increased evaporation arising from so great a multiplication of vegetable 

 surface by the trees, more cold will be produced after rains, and the atmosphere kept in 

 a more moist state, than in grounds perfectly naked. When the temperature of a 

 current of air is lower than that of the earthy screens will prevent its canying off so 

 much heat ; but more especially scattered trees, the tops of which will be chiefly cooled 

 whilst the under surfaces of their lower branches reflect back the rays of heat as they 

 radiate from the surface of the soil. Heat, in its transmission from one body to another, 

 follows the same laws as light ; and, therefore, the temperature of the surface in a forest 

 will, in winter, be considerably higher than that of a similarly constituted soil exposed 

 to the full influence of the weatlier. The early flowering of plants, in woods and hedges, 

 is a proof of this : but as such soils cannot be so easily heated in summer, and are cooled 

 like others after the sinking in of rains, or the melting of snows, the effect of the reflec- 

 tion as to the whole year is nearly neutralised, and the average temperature of the year of 

 such soils and situations will probably be found not greater than that of open lands. 



2216. Shading the ground, whether by umbrageous trees, spreading plants, or cover- 

 ing it with tiles, slates, moss, litter, or other materials, has a tendency to exclude atmo- 

 spherical heat and retain moisture. Shading dry loose soils, by covering them with litter, 

 slates, or tiles, laid round the roots of plants, is found very beneficial. 



SuBSECT. 7. Rotation of Crops. 



2217. Growing different crops in succession is a practice which every cultivator knows 

 to be highly advantageous, though its beneficial influence has not yet been fully accounted 

 for by chemists. The most general theory is, that though all plants will live on the same 

 food, as the chemical constituents of their roots and leaves are nearly the same, yet that 

 many species require particular substances to bring their seeds or fruits to perfection, as 

 the analysis of these seeds or fruits often afford substances different from those which 

 constitute the body of the plant. A sort of rotation may be said to take place in 

 nature, for perennial herbaceous plants have a tendency to extend their circumference, 

 and rot and decay at their centre, where others of a different kind spring up and succeed 

 them. This is more especially the case with travelling roots, as in mint, strawberry, 

 creeping crowfoot, &c. 



2218. The rationale of rotation is thus given by Sir H. Davy : " It is a great advan- 

 tage in the convertible system of cultivation, that the whole of the manure is employed ; 

 and that those parts of it which are not fitted for one crop, remain as nourishment for 

 another. Thus, if the turnip is the first in the order of succession, this crop, manured 



