524 



THE FARMERS' REGISTER. 



be regarded the power of absorbing heat eo 

 emilted from other bodies. Polished inetais and 

 the fibres of vegetables may be considered as 

 placed at ihe two exiremilies of the scale upon 

 which these properties in difiierent substances 

 may be measured. If a body be so situated that 

 it may receive just as much radiant heat as itself 

 projects, its temperature remains the same; if the 

 surrounding bodies emit heat of greater intensity 

 than the same body, its temperature rises, till the 

 quantity which it receives exactly balances its 

 expenditure, at which point it again becomes 

 stationary ; and if the power of radiation be 

 exerted under circumstances which prevent a 

 return, the temperature ol' the body declines. 

 Thus, if a thermometer be placed in the focus 

 of a concave metallic mirror, and turned towards 

 any clear portion of the sky, at any period of 

 the day, it will fall many degrees below the tem- 

 perature of another thermometer placed near it, 

 out of the mirror; the power of radiation is ex- 

 erted in both thermometers, but to the first all 

 return of radiant heat is cut off, while the other 

 receives as much Irom the surrounding bodies, 

 as itself projects. This interchange amongst 

 bodies takes place in transparent media as wellas 

 in vacuo ; but in the former case, the effect is 

 modified by the equalizing power of the medium. 



" Any portion of the surlace of the globe which 

 is fully turned towards the sun receives more 

 radiant heat than it projects, and becomes heated ; 

 but when, by the revolution of the axis, this por- 

 tion is turned from the source of heat, the radia- 

 tion into space still continues, and, being uncom- 

 pensated, the temperature declines. In conse- 

 quence of the different degrees in which different 

 bodies possess this power of radiation, two con- 

 tiguous portions of the system of the earth will 

 become of different temperatures; and if on a 

 clear night we place a thermometer upon a grass- 

 plat, and another upon a gravel walk or the bare 

 soil, we shall find the temperature of the former 

 many degrees below that of the latter. The 

 fibrous texture of the grass is favorable to the 

 emission of the heat, but tlie detjse surface of 

 the gravel seems to retain and fix it. JBut this 

 unequal effect will only be perceived when ttie 

 atmosphere is unclouded, and a free passage is 

 open into space ; lor even a light mist will arrest 

 the radiant matter in its course, and return as 

 much to the radiating body as it emi\s. The 

 intervention of more substantial obstacles will 

 of course equally prevent the result, and the ba- 

 lance of temperature will not be disturbed in any 

 substance which is not placed in the clear aspect 

 of the sky. A portion of a grass-plat under the 

 protection of a tree or hedge, will generally be 

 found, on a clear night, to be eight or ten degrees 

 warmer than surrounding unsheltered parts; and 

 It is well known to gardeners, that less dew and 

 frost are to be found in such situations, than in 

 those which are wholly exposed." {/Jort. Trans., 

 vi. 8.) 



These laws plainly direct us to the means we 

 are to employ to moderate atmospherical tempera- 

 ture. A screen, of whatever kind, interposed be- 

 tween the sun and a plant, intercepts the radiant 

 heat of the sun, and returns it into space ; and 

 thus, in addition to the diminution of perspiration 

 by tfie removal o( a part of the stimulus that causes 

 it, actually tends to lower the temperature that 



surrounds the plant. In like manner, the inter- 

 position of a screen, however slight, between a 

 plant and the sky, intercepts the radiant heat of 

 the earth ; and, instead of allowing it to pass off 

 into space, returns it to the ground, the tempera- 

 ture of which is maintained at a liigher point than 

 it otherwise would be. Hence it is that plants 

 growing below the deep projecting leaves of 

 houses, or guarded by a mere coping of thatched 

 hurdles, sufier less in winter than if they were 

 fully exposed to the sky. 



It is also obvious from what has been stated, 

 that plants growing upon grass will be exposed to 

 a greater degree of cold in winter than such as 

 grow upon gravel : but it does not therefore follow 

 that hard gravel is, with respect to vegetation, a 

 better coating lor the surface of the ground than 

 turf; it has its disadvantages as well as its ad- 

 vantages, and the former probably outweigh the 

 latter. Its superior heating power is its only ad- 

 vantage ; the objections to it are, its dryness in 

 summer, and its comparative impermeability to- 

 rain, eo that it causes the force of perspiration to 

 be inversely as the absorbing power of the roots. 

 It is well known that blackened surfaces absorb 

 heat much more than those of any ottier color ; 

 and it has been expected that the effect of black- 

 ening garden walls, on which fruit trees are train- 

 ed would be to accelerate the maturation of the 

 fi uit : but, notwithstanding a lew cases of apparent 

 advantages, one of which, of tlie vine, is mention- 

 ed in the Horticultural Transactions, vol. iii. p. 

 830, this has been, in general, found either not to 

 happen at all, or to so small an extent as not to be 

 worth the tiouble. It is true that so long as tlie 

 wall is but little covered by the branches and 

 leaves of a plant, the absorbent power of the black- 

 ened surlace is brought into play ; but this effiect 

 is lost as soon as the vvall becomes covered with 

 foliage. Ill the early spring, however, before the 

 leaves appear, the flowers are brought rather more 

 forward than would otherwise be the case; and in 

 the autumn the wood certainly becomes more 

 completely ripened, a result of infinite conse- 

 quence in the northern parts of the country- 

 It is rather to a judicious choice of soil and situ- 

 ation that tlie gardener must look for the means of 

 softening the rigor of climate. Wet tenacious 

 soils are found the most difficult to heat or to drain, 

 and ihey will, therefore, be the most unfavorable 

 to the operations of the gardener ; extremely 

 light sandy soils, on the other hand, part with their 

 moisture so rajjidly, and absorb so much heat, 

 that they are equally unfavorable ; and it is the 

 light loamy soils, which are intermediate between 

 the two extremes, ttiat, as is well known, form 

 the best soil for a garden. Situation is, however, 

 of much more consequence than soil, for the lat- 

 ter may be changed or improved, but a bad (that 

 is. Cold) situation is incurable. Cold air is heavi- 

 er than warm air, and, consequently, the stratum 

 ol the atmosphere next the soil will be in general 

 colder ilian those above it. VVlien, therefore, a 

 garden is placed upon the level ground of the bot- 

 tom of a valley, whatever cold air is formed upon 

 its surface remains there, and surrounds the herb- 

 age ; and, moreover, the cold air that is formed 

 upon the sides of lo\v hills rolls down into the val- 

 ley as quickly as it is formed. Hence the fact 

 which to many seems surprising, that what are 

 called sheltered places are, in spring and autumn, 



