164 
MR. GLAISHER ON THE RADIATION OF HEAT, 
Section II . — Results of Observations made by Self-registering Minimum 
Thermometers. 
These observations extend over a period of time of nearly four years’ duration, viz. 
from 1841 February to the end of 1844*. 
The observations consist of the daily reading of a self-registering minimum ther- 
mometer with its bulb placed at the height of four feet above the ground, protected 
from the effects of radiation and rain, but in other respects freely exposed to the air, 
and the daily reading of a similar thermometer placed with its bulb in the focus of a 
metallic parabolic reflector, and fully exposed to the sky. 
The kind and average amount of cloud by which the sky was covered were also 
noted every night. The average direction and strength of the wind were also 
determined every night. 
From 1843 April and extending to the end of 1844, several similar thermometers 
were placed on or near different substances, and their readings were taken daily. 
The first process in the reduction of the observations was the taking the difference 
between the reading of the thermometer in air and the simultaneous reading of every 
other thermometer. 
The second process was the forming all those differences into groups, according 
to the variable state of the sky, depending only on the kind and amount of cloud, 
but independently of its height. The result of this investigation was found to be 
that the amount of the difference varied with every variation of the amount of the 
clouds. 
The third process was the forming groups of the differences derived from the 
same cloudy state of the sky, but with the clouds at different distances from the 
earth. The result was found to be that the amount of the numbers was different 
according to the variable distance that the clouds were from the earth. 
The fourth step was the forming groups of numbers derived from observations on 
different nights with the same state of the sky, but with the wind blowing from 
different quarters, independently of its strength. The result of this investigation was 
that no certain difference existed depending on the quarter from whence the wind blew. 
The fifth step was the forming groups of the numbers found from observations on 
different nights with the same state of the sky, on calm nights, and on nights when 
the wind has been blowing strongly, at times amounting to a gale, independently of its 
direction. The result was found to be that no certain difference existed, depending 
on the strength or velocity of the wind, showing clearly that on every windy night 
a portion of time had been calm of sufficient duration for the instruments to register 
the loss of heat by radiation, and to the same amount as would have been shown had 
the air been in a calm state during the night. 
Having thus explained the manner in which the preliminary steps in the reduction 
* The observations are placed in the Archives of the Royal Society. 
