Book III. HEAT AND LIGHT. 351 



which they had lost by radiation. A slight agitation of the air is sufficient to produce 

 some effect of this kind ; though, as has already been said, such an agitation, Avhen the 

 air is very pregnant with moisture, will render greater the quantity of dew ; one requisite 

 for a considerable production of this fluid being more increased by it, than another is 

 diminished. 



2319. It has been remarked that the hurtful effects of cold occur chiejly in holloiv places. 

 If this be restricted to what happens on the serene and calm nights, two reasons from 

 different sources are to be assigned for it. The first is, that the air being stiller in such 

 a situation, than in any other, the cold, from radiation, in the bodies contained in it, 

 will be less diminished by renewed applications of warmer air ; the second, that from the 

 longer continuance of the same air in contact with the ground, in depressed places than 

 in others, less dew will be deposited, and therefore less heat extricated during its 

 formation. 



2320. An observation closely connected with the preceding, namely, that, in clear and 

 still nights, frosts are less severe upon the hills, than in the neighbouring plains, has excited 

 more attention, chiefly from its contradicting what is commonly regarded an established 

 fact, that the cold of the atmosphere always increases with the distance from the earth. 

 But on the contrary the fact is certain, that, in very clear and still nights, the air near to 

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



2321. A very slight covering will exclude much cold. I had often, observes Dr. Wells, 

 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 imme- 

 diately 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 handkerchief. In this dis- 

 position 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 handker- 

 chief. 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 un- 

 sheltered grass did not exceed 5, the sheltered grass was about as warm as the air. If 

 that difference, however, exceeded 5, 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, v/hen 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 exposed 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. 



2322. The covering has most effect when placed at a little distance above the plants 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 



