PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 317 



few inches in the air, was 3 greater than a neighboring piece 

 of grass, which was sheltered by a similar handkerchief, which 

 was actually in contact with it. On another night the difference 

 between the temperatures of the two portions of grass, sheltered 

 in the same manner as the two above mentioned, from the influ- 

 ence of the sky, was 4. Possibly," says he, " experience has 

 long ago taught gardeners the superior advantages of defending 

 tender plants from the cold of clear and calm nights, by means 

 of substances not directly touching them, though I do not recol- 

 lect ever having seen any contrivance for keeping mats, and such 

 like bodies, at a distance from the plants which they were meant 

 to protect." We know this to be a fact ; for gardeners seldom 

 take any thought whether the plant is protected or not, provid- 

 ing it be covered, with mats or something else, from the external 

 atmosphere. 



Straw, and corn stalks, afford good protection to trees and half 

 hardy shrubs, when properly arranged, so that the covering may 

 be water-tight. The air that lodges among the straw, and in 

 the interstices of the stalks, keeps the plant within, always at a 

 regular temperature, and prevents sudden freezing and thawing, 

 which prove the destruction of tender plants. 



Bodies, however, capable of absorbing heat during the day, 

 and parting with it at night, when the temperature of the atmos- 

 phere falls, are also useful as a means of protecting plants, &c. 

 Among such bodies may be classed the walls of houses, which 

 may be regarded useful in two ways ; namely, by the mechani- 

 cal shelter they afford against cold winds, and by giving out 

 the warmth, during the night, which they had absorbed during 

 the day. It appears, however, that on clear and calm nights, 

 those, on which plants frequently receive much injury from cold, 

 walls must be beneficial in another 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 by Dr. Wells, for the purpose of deter- 

 mining the justness of this opinion. A cambric handkerchief 

 having been placed, by means of two upright sticks, perpendicu- 

 larly 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 



