Book II. 



HEAT AND LIGHT. 251 



earth is colder than that which is more distant from it, to the height at least of 220 feet, 

 this being the greatest to which experiments relate. If then a hill be supposed to rise 

 from a plain to the height of 220 feet, having upon its summit a small flat surface 

 covered with grass ; and if the atmosphere, during a calm and serene night, be admitted 

 to be 10 warmer there than it is near the surface of the low grounds, which is a less 

 difference than what sometimes occurs in such circumstances, it is manifest that, should 

 both the grass upon the hill, and that upon the plain, acquire a cold of 10 by radiation, 

 the former will, notwithstanding, be 10 warmer than the latter. Hence also the tops 

 of trees are sometimes found dry when the grass on the ground's surface has been found 

 covered with dew. 



1204. A very slight covering will exclude much cold. I had often, observes Dr. >\ ells, 

 in the pride of half knowledge, smiled at the means frequently employed by gardeners, 

 to protect tender plants from cold, as it appeared to me impossible, that a thin mat, or 

 any such flimsy substance, could prevent them from attaining the temperature of the 

 atmosphere, by which alone I thought them liable to be injured. But, when I had 

 learned, that bodies on the surface of the earth become, during a still and serene night, 

 colder than the atmosphere, by radiating their heat to the heavens, I perceived 

 immediately a just reason for the practice, which I had before deemed useless. 

 Being desirous, however, of acquiring some precise information on this subject, I 

 fixed, perpendicularly, in the earth of a grass-plot, four small sticks, and over their 

 upper extremities, which were six inches above the grass, and formed the corners of 

 a square, the sides of which were two feet long, drew tightly a very thin cambric hand- 

 kerchief. In this disposition of things, therefore, nothing existed to prevent the free 

 passage of air from the exposed grass, to that which was sheltered, except the four 

 small sticks, and there was no substance to radiate heat downwards to the latter grass, 

 except the cambric handkerchief. The temperature of the grass, which was thus 

 shielded from the sky, was, upon many nights afterwards examined by me, and was 

 always found higher than that of neighbouring grass which was uncovered, if this was 

 colder than the air. When the difference in temperature, between the air several feet 

 above the ground and the unsheltered grass, did not exceed 5, the sheltered grass was 

 about as warm as the air. If that difference, however, exceeded 5 J , the air was 

 found to be somewhat warmer than the sheltered grass. Thus, upon one night, when 

 fully exposed grass was 11 colder than the air, the latter was 3 warmer than the 

 sheltered grass ; and the same difference existed on another night, when the air was 

 14 warmer than the exposed grass. One reason for this difference, no doubt, was 

 that the air, which passed from the exposed grass, by which it had been very much 

 cooled, to that under the handkerchief, had deprived the latter of part of its heat; 

 another, that the handkerchief, from being made colder than the atmosphere by the 

 radiation of its upper surface to the heavens, would remit somewhat less heat to the 

 grass beneath, than what it received from that substance. But still, as the sheltered 

 grass, notwithstanding these drawbacks, was upon one night, as may be collected 

 from the preceding relation, 8, and upon another 1 1 , warmer than grass fully ex- 

 posed to the sky, a sufficient reason was now obtained for the utility of a very slight 

 shelter to plants, in averting or lessening injury from cold, on a still and serene night. 



1 205. 1 lie covering has most effect when placed at a tilth', distance above the pla/its or objects 

 to be sheltered. A difference in temperature, of some magnitude, was always observed on 

 still and serene nights, between bodies sheltered from the sky by substances touching them, 

 and similar bodies, which were sheltered by a substance a little above them. I found, for 

 example, upon one night, that the warmth of grass, sheltered by a cambric handkerchief 

 raised a few inches in the air, was 3 greater than that of a neighbouring piece of grass 

 which was sheltered by a similar handkerchief actually in contact with it. On another 

 night, the difference between the temperatures of two portions of grass, shielded in the 

 same manner, as the two above mentioned, from the influence of the *ky, was 4. Pos- 

 sibly, continues Dr. Wells, experience has long ago taught gardeners the superior ad- 

 vantage of defending tender vegetables, from the cold of clear and calm nights, by means 

 of substances not directly touching them ; though I do not recollect ever having seen 

 any contrivance for keeping mats, or such like bodies, at a distance from the plants 

 which they were meant to protect. 



1206. Heat produced by walls. Walls, Dr. Wells observes, as far as warmth is con- 

 cerned, are regarded as useful, during a cold night, to the plants wliich touch them, or 

 are near to them, only in two ways ; first, by the mechanical shelter which they afford 

 against cold winds, and secondly, by giving out the heat which they had acquired during 

 the day. It appearing to me, however, that, on clear and calm nights, those on which plants 

 frequently receive much injury from cold, walls must be beneficial in a third way, 

 namely, by preventing, in part, the loss of heat, which the plants would sustain from 

 radiation, if they were fully exposed to the sky : the following experiment was made for 

 the purpose of determining the justness of this opinion. A cambric handkerchief having 



