Book III. CHANGING THE CONDITION OF LANDS. 325 



country, it is generally found impregnated with carbonate of lime ; and in this state it 

 tends, in many instances, to ameliorate the soil. Common river water also generally 

 contains a certain portion of organisable matter, which is much greater after rains than 

 at other times ; or which exists in the largest quantity when the stream rises in a 

 cultivated country. Even in cases when the water used for flooding is pure, and free 

 from animal or vegetable substances, it acts by causing a more equable diffusion of 

 nutritive matter existing in the land ; and in very cold seasons it preserves the tender 

 roots and leaves of the grass from being affected by frost. Water is of greater specific 

 gravity at 42 Fahrenheit, than at 32, the freezing point ; and hence, in a meadow 

 irrigated in winter, the water immediately in contact with the grass is rarely below 40", 

 a degree of temperature not at all prejudicial to the living organs of plants. In 1804, 

 in the month of March, the temperature in a water meadow near Hungerford was 

 examined by a very delicate thermometer. The temperature of the air at seven in the 

 morning was 29. The water was frozen above the grass. The temperature of the soil 

 below the water in which the roots of the grass were fixed, was 43." Water may also 

 operate usefully in warm seasons by moderating temperature, and thus retarding the 

 over-rapid progress of vegetation. The consequence of this retardation will be greater 

 magnitude and improved texture of the grosser parts of plants, a more perfect and 

 ample developement of their finer parts, and, above all, an increase in the size of their 

 fruits and seeds. We apprehend this to be one of the principal uses of flooding rice- 

 grounds in the East ; for it is ascertained that the rice-plant will perfect its seeds in 

 Europe, and even in this country, without any water beyond what is furnished by the 

 weather, and the natural moisture of a well constituted soil. " In general, those 

 waters which breed the best fish are the best fitted for watering meadows ; but most of 

 the benefits of irrigation may be derived from any kind of water. It is, however, a 

 general principle, that waters containing ferruginous impregnation, though possessed of 

 fertilising effgcts when applied to a calcareous soil, are injurious on soils that do not 

 effervesce with acids ; and tliat calcareous waters, which are known by the earthy deposit 

 they afford when boiled, are of most use on siliceous soils, or other soils containing 

 no remarkable quantity of carbonate of lime." 



SuBSECT. 6. Changing the Condition of Lands, in respect to Atmosjiherical Iriflv^nce. 



2150. The injluence of the weather on soils may be affected by changing the position of 

 their surface and by sheltering or shading. 



2151. Changing the condition of lands, as to solar influence, is but a limited means of 

 imjirovement ; 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 right angles 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, 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, ceeteris 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. 



2152. S/teZier, 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 current 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 : and, therefore, the temperature of the surface in a forest 



Y 3 



