1338. 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,' says Dr. Wells, that 'the warmth of grass, sheltered by a cambric handkerchief raised a few inches in the air, was 3ï¾¦ 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 sky, was 4ï¾¦. Possibly, 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.'