BOOK II. ROTATION OF CROPS. 233 



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

 as their number and perpendicularity to that plane, neglecting the effects of the atmo- 

 sphere. 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 the 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, cfBteris 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 di- 

 minished 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. 



1105. S/ielter, 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 that of 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 cur- 

 rent of air is lower than that of the earth, screens will prevent its carrying 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 j 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 weather. 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 reflection 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. 



1106. Shading the ground, whether by umbrageous trees, spreading plants, or covering 

 it with tiles, slates, moss, litter, &c. has a tendency to exclude atmospherical heat and 

 retain moisture. Shading dry loose soils, by covering them with litter, or slates, or tiles, 

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



SUBSECT. 7. Hotation of Crops. 



1 107. 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 affords substances different from those which 

 constitute the body of the plant. (736.) 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. 



1 108. The rationale of rotation, is thus given by Sir H. Davy. " It is a great advantage 

 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 with recent 

 dung, immediately finds sufficient soluble matter for its nourishment ; and the heat pro- 

 duced in fermentation assists the germination of the seed and the growth of the plant. 

 If, after turnips, barley with grass-seeds is sown, then the land, having been little 

 exhausted by the turnip crop, affords the soluble parts of the decomposing manure to the 

 grain. The grasses, rye-grass, and clover remain, which derive a small part only of 

 their organised matter from the soil, and probably consume the gypsum in the manure 

 which would be useless to other crops : these plants, likewise, by their large systems 

 of leaves, absorb a considerable quantity of nourishment from the atmosphere ; and when 

 ploughed in at -the end of two years, the decay of their roots and leaves affords manure 

 for the wheat crop; and at this period of the course, the woody fibre of the farm-yard 

 manure, which contains the phosphate of lime and the other difficultly soluble parts, is 



