150 
MR. GLAISHER ON THE RADIATION OF HEAT, 
0°, 10°, 20° and at 30°, and above 30°, will be respectively e, ^ e, e, and 
for any intermediate reading the difference can be easily calculated from these terms. 
The very singular and unexpected facts now detailed, merit attention and suggest the 
necessity of carefully noting the position of a thermometer in any investigation in which 
such instrument is needed, as indeed do all the experiments that I have made, though 
not in so marked a manner as does this. I now proceed to the results of those expe- 
riments in which grass was covered by different substances, which are as follows: — 
bo 
bo 
C 
o 
&C 
.5 
<u 
o 
o 
O) 
JS 
H 
raw wool, flax, hare- 
skin or rabbit-skin 
caused the thermo- 
meter to read higher 
than one placed on 
uncovered grass by 
a sheet of white tin .. 
a sheet of white tin! 
one inch high ... J " 
a sheet of blackened tin 
a sheet of lead 
a sheet of lead 6 in 
ches high 
a sheet of colourless , 
glass J" 
a sheet of colourless "1 
glass 1 inch high] " 
the whole 
five- eighths 
five-eighths 
three-eighths 
three-eighths 
five-eighths 
one-fourth ... 
five- eighths 
of the difference be- 
tween the thermo- 
meter in air at the 
height of 4 feet and 
that on long grass 
or in its mean, caused 
the covered grass to 
be warmer by 
8-0 
5-0 
5-4 
3-6 
3-6 
5-4 
2-4 
5-0 
The following are some of the particulars that we may collect from these results. 
The filamentous substances and skins did not allow any heat to escape from the grass 
they covered, thus proving them to be very bad conductors of heat. The raising any 
substance above the grass caused the thermometer reading to increase, although in 
this case it must have been exposed to a portion of the sky : this in the case of tin 
was a small quantity; in the cases of lead and of glass the quantity was large, with 
the former amounting to 1°7, and with the latter to 2 °*7 in their means. The amount 
of heat transmitted through a colourless medium, as glass, is remarkable, the mean 
amount of radiation from grass thus covered being less by only one-fourth part of 
that of uncovered grass. 
The next investigation connected with grass was that of placing thermometers at 
different distances from it, with the view of determining the cooling effect of a body, 
as cooled by radiation, upon the air in contact with it ; the results of these experi- 
ments as contained in the table, will probably be more clearly seen by arranging 
the numbers on the supposition that the reading of a thermometer on long grass was 
0°, and combining with them those of 1 inch below and on the surface of the soil as 
follows : — 
o 
At one inch below the surface of the soil the mean reading would be . . . 10’24 
On the surface of the soil under long grass the mean reading would be . . 7‘8 4 
On long grass fully exposed to the sky the mean reading wmidd be ... . 0 - 00 
One inch above long grass fully exposed to the sky the mean reading would be 2 '76 
Two inches above long grass fully exposed to the sky the mean reading would be 3’61 
