198 
MR. GLAISHER ON THE RADIATION OF HEAT, 
("The mean state of 1 o f and the mean excess of the reading') o 
In the year 1841j cloudiness during > . . 5 ’8, j of the thermometer in the air above l . . 4* 7 
the nights was J l, that in the reflector was J 
In the year 1842 „ . . 4*4, „ . . 5*7 
In the year 1843 „ . . 4'9, „ . . 6 4 
In the year 1844 „ . . 5*0, „ . . 5*7 
And the mean state of cloudiness during the nights of the four years was 5 (or the 
shy upon the average had been one-half covered by cloud), and the corresponding 
mean excess of the reading of the thermometer in air above that in the reflector was 
5°*65. 
In the year 1841 the number of clear and cloudy nights were equal; and they 
were nearly the same in number in the year 1844 ; in the year 1842 the number of 
clear nights exceeded the number of cloudy nights by thirteen ; and in 1843 the cloudy 
exceeded the clear nights by five from this it appears that during these four years 
there were one clear and one cloudy night out of every four nights. 
In Table XLV. the relative less reading of a thermometer placed on raw wool and 
one in the focus of a parabolic reflector, than that in the air was found to be as 
1221 : 888 from 889 simultaneous observations. 
In Table CVI. this ratio is found to be as 1280 : 962 from 992 observations, de- 
duced from self-registering minimum thermometers. As these results are so nearly 
identical, and as they have been deduced by entirely different instruments, there can 
be but little doubt of the correctness of their mean, viz. 1251 : 910, or in other words, 
the results as derived from the observations of the thermometer in the reflector, 
would be converted into results as deduced from placing a thermometer on raw wool 
by multiplying the former by T375, and in this way we should derive 7°‘76 (*• e. 
5°*65 X T375) as the mean deduced from the four years’ observations; and we should 
find 1 1°*5 ( i . e.8°*33X T375) as that deduced from cloudless nights. I have preferred 
exhibiting these results in terms of that derived from raw wool in consequence of it 
being free from the effects of heat conducted from the earth, and therefore free from 
one of the many sources from which the reading of a thermometer placed on the 
ground is affected, some of which are as follows :■ — 
From the heat of the earth upon which it is placed. 
From the heat radiated to it from lateral objects. 
From the heat communicated to the substance from the air in contact with it. 
From the heat evolved during the change of the watery vapour in the atmosphere 
into dew. 
From the heat received from the radiation from clouds. 
From the heat received from the upper regions of the atmosphere, and 
From the heat received from space. 
By placing a thermometer on wool the effect of the first of these causes is evaded, 
as appears in a previous section ; in a wide and open plain the second would be 
evaded, as there would be but few objects to emit heat; I fear, however, that all my 
