142 
MR. GLAISHER ON THE RADIATION OF HEAT, 
In addition to the tabulated experiments, a variety of others were always in 
progress. I may mention the results of a few of these. 
I placed a large sheet of pasteboard vertically on the grass plat, and laid a ther- 
mometer close to its lower edge on each side of it ; the readings of both these ther- 
mometers thus placed were found to be identical and intermediate to that placed in 
air at the height of four feet and protected from the effects of radiation, and that 
placed on grass fully exposed to the sky ; the same relation was found to exist in 
what azimuth soever the board was placed. 
I then placed the pasteboard at an angle of 45° nearly with the horizon, and laid 
the thermometers as before. In this situation their readings were found to be inter- 
mediate as before, but that which was exposed to three-fourths of the sky, read most 
nearly to that on the grass fully exposed to the sky, and the one which was exposed to 
one-fourth part of the sky only, read most nearly to that in air. The amount of these 
differences of readings was as nearly as could be determined exactly proportional to 
the amount of the exposed sky. 
Hence, as a general fact it may be considered, that whatever diminishes the view 
of the sky as seen from an exposed body, causes its temperature to decrease less 
than it would if the exposure to the sky be complete. 
Various experiments were made to ascertain the effect of covering plants at night 
by matting, or other thin substances; and it was always found that when the protect- 
ing substances touched the plant, much heat was conducted away from it, and such 
plant w r as at a lower temperature than when the substance was merely interposed 
between it and the sky ; the thinnest substance thus interposed, at any distance from 
the plant, was found to be effectual in preventing the loss of its heat by radiation. 
It was found, however, that when a plant was thus itself protected, but yet was ex- 
posed to any body which was exposed to the sky, more heat radiated from the former 
to the latter, than from the exposed body to the plant, and thus it lost some heat. 
The several thermometers at the different distances from the earth were for some 
time read at short intervals during the night and day, and it was found that except 
after noon, the reading of the thermometer at twelve feet from the earth was very 
nearly identical with the true temperature of the air. 
The bulb of the thermometer thus exposed to the full rays of the sun was situated 
nine inches west of the plank which carried it, and whose width was three inches ; 
and the cause of its readings being about 1° too high during the afternoon, was 
owing to the heat reflected from the plank to it. 
During' the summer of 1844 and the year 1845, the reading of this thermometer 
was frequently examined, and found always thus to agree. Hence there is no doubt 
that if a thermometer be freely suspended in the air with its bulb at the height of 
thirteen feet above the soil, and far from any object to reflect heat to it, its readings 
will represent the true temperature of the air at the time, and much more truly than 
those of any one placed near the ground, or within a few feet of walls or buildings. 
