Book III. HEAT AND LIGHT. 345 



cambric handkerchief raised a few inches in the air, was 3 greater than that of a neighbor- 

 ing 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 sky, 

 was 4. Possibly, continues Dr. Wells, experience has long ago taught gardeners the 

 superior advantage 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. 



2256. 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 which 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 

 been placed, by means of two upright sticks, perpendicularly to a grass-plot, and at right 

 angles to the course of the air, a thermometer was laid upon the grass close to the lower 

 edge of the handkerchief, on its windward side. The thermometer thus situated was 

 several nights compared with another lying on the same grass-plot, but on a part of it 

 fully exposed to the sky. On two of these nights, the air being clear and calm, the grass 

 close to the handkerchief was found to be 4 warmer than the fully exposed grass. On 

 a third, the difference was 6. An analogous fact is mentioned by Gersten, who says, 

 that a horizontal surface is more abundantly dewed, than one which is perpendicular to 

 the ground. 



2257. Beat from a covering of snow. The covering of snow, the same author ob- 

 serves, which countries in high latitudes enjoy during the winter, has been very com- 

 monly thought to be beneficial to vegetable substances on the surface of the earth, as 

 far as their temperature is concerned, solely by protecting them from the cold of the 

 atmosphere. But were this supposition just, the advantage of the covering would be 

 greatly circumscribed ; since the upper parts of trees and of tall shrubs are still exposed 

 to the influence of the air. Another reason, however, is furnished for its usefulness, by 

 what has been said in this essay ; which is, that it prevents the occurrence of the cold, 

 which bodies on the earth acquire, in addition to that of the atmosphere, by the radiation 

 of their heat to the heavens during still and clear nights. The cause, indeed, of this 

 additional cold, does not constantly operate ; but its presence, during only a few hours, 

 might effectually destroy plants, which now pass unhurt through the winter. Again, as 

 things are, while low vegetable productions are prevented, by their covering of snow, 

 from becoming colder than the atmosphere in consequence of their own radiation, the 

 parts of trees and talL shrubs, which rise above the snow, are little affected by cold from 

 this cause. For their uttermost twigs, now that they are destitute of leaves, are much 

 smaller than the thermometers suspended by me in the air, which in this situation very 

 seldom became more than 2 colder than the atmosphere. The larger branches, too, 

 which, if fully exposed to the sky, would become colder than the extreme parts, are, in 

 a great degree, sheltered by them ; and, in the last place, the trunks are sheltered both by 

 the smaller and larger parts, not to mention that the trunks must derive heat, by con- 

 duction through the roots, from the earth kept warm by the snow. In a similar way is 

 partly to be explained, the manner, in which a layer of earth or straw preserves vegetable 

 matters in our own fields, from the injurious effects of cold in winter. (^Essay on Dew, 

 &c. 1819.) 



2258. The nature of light is totally unknown : the light which proceeds from the sun 

 seems to be composed of three distinct substances. Scheele discovered that a glass mir- 

 ror held before the fire reflected the rays of light, but not the rays of caloric ; but when 

 a metallic mirror was placed in the same situation, both heat and light were reflected. 

 The mirror of glass became hot in a short time, but no change of temperature took place 

 on the metallic mirror. This experiment shows that the glass mirror absorbed the rays 

 of caloric, and reflected those of light ; while the metallic mirror, suffering no change 

 of temperature, reflected both. And if a plate glass be held before a burning body, the 

 rays of light are not sensibly interrupted, but the rays of caloric are intercepted ; for no 

 sensible heat is observed on the opposite side of the glass ; but when the glass has reached 

 a proper degree of temperature, the rays of caloric are transmitted with the same facility 

 as those of light. And thus the rays of light and caloric may be separated. But the 

 curious experiments of Dr. Herschel have clearly proved that the invisible rays which 

 are emitted by the sun, have the greatest heating power. In those experiments, the dif- 

 ferent colored rays were thrown on the bulb of a very delicate thermometer, and their 



