158 
MR. GLAISHER ON THE RADIATION OF HEAT, 
The mean reading of a thermometer placed on short grass was less than that 0 
under short grass by 5 2 
The mean reading of a thermometer placed under short grass was less than 
that one inch below the surface of the ground under short grass by . . 3 5 
The mean reading of a thermometer placed on short grass was less than that 
one inch below the surface of the ground under short grass by ... . 8 •/ 
The mean rate of decrease in the reading of a thermometer placed under long 
grass was, per hour 0 ~ 
The mean rate of decrease in the reading of a thermometer placed under short 
grass was, per hour 08 
And the mean rate of decrease in the reading of a thermometer placed one inch 
below the surface under short grass was, per hour 0 */5 
In this investigation the excess of the reading of a thermometer placed one inch 
below the surface of the ground under long grass, above that placed on the surface 
of the ground, has not been deduced ; from a few experiments it appeared to be ex- 
actly the same as that of short grass ; and assuming such to be the case, the mean 
difference on clear and calm nights between the reading of two thermometers, the 
one placed on long grass and the other placed one inch below the ground immediately 
underneath the other, was 13 o- 0 ; and the same difference with respect to short grass 
was 11 0, 5. As the results contained in this table were deduced from observations 
taken at times when the temperature of the long grass was above 30° generally, and 
occasionally a little below 30°; and as it appears from the remarks following Table 
XLV. that at temperatures much below 30° very great differences occurred with 
grass of different lengths, it w T as desirable to investigate observations similar to the 
above at low temperatures: such observations I took only on one night, viz. that 
of March 13, 1845 ; during this night the thermometers were read hourly for eight 
hours, and during the time — 
o 
The mean reading of the thermometer in air at the heignt of four feet was . 15d 
The mean reading of the thermometer on long grass was 56 
The mean reading of the thermometer on the surface of the soil under long 
grass was 26 9 
The mean reading of the thermometer one inch beneath the surface of the 
ground under long grass was 33T 
The mean reading of the thermometer on short grass was 1T1 
The mean reading of the thermometer on the surface of the ground under 
short grass was 2T4 
The mean reading of the thermometer one inch beneath the surface of the 
ground under short grass was 28*2 
The results of these experiments exhibit in a marked manner the badness of the 
conducting power of grass for heat, the reading on long grass being less than that on 
