REPORT ON RADIANT HEAT. 
285 
Most of these observations were made by comparing two ther¬ 
mometers, one of which was kept in the shade, whilst the other, 
having its bulb blackened, was exposed to the direct rays of the 
sun; but, as Dr. Ritchie observes, no correction seems to have 
been made for the variable causes which abstract caloric from 
the blackened ball of the exposed thermometer. (. Edinb . Journ. 
of Science , v. 107.) 
In the same paper is described the method proposed by 
Sir J. F. W. Herschel : his object was to ascertain, by direct 
experiment, the relative heating power of the sun’s rays ; this 
he did by exposing in a glass vessel, or large thermometer, at 
different times and places, a deep blue liquid , for a given time, 
to the direct rays of the sun,—noting the increase of temperature, 
which was purposely rendered very small by properly adjusting 
the capacity of the instrument, then shading the sun’s direct 
rays, and leaving it exposed for an equal time to the free in¬ 
fluence of all the other heating and cooling causes, radiation, 
conduction, wind, &c., and again noting the effect of these. 
The same difference of these, according to their signs, was the 
effect of the mere solar radiation. Dividing this by the time of 
exposure, he had the momentary effector differential co-efficient, 
which is the true measure of the intensity of radiation. 
Professor Gumming has been engaged in researches, the ob¬ 
ject of which was to obtain a measure of the total heating 
effect of the sun’s rays. He has communicated for this Report 
an account of his investigations, of which the following is the 
substance. 
His instrument consists of a bent tube in the form one side 
terminating in a black bulb containing ether, or sulphuret of 
carbon; the other a graduated tube closed at the bottom ; into 
this, on exposure to the sun, some of the liquid is distilled over 
from the bulb; and the quantity measured on the scale is pro¬ 
portional to the amount of radiation, when all interfering causes 
are allowed for ; and these are estimated by comparative obser¬ 
vations. 
The experiments have been varied by exposing the bulb and 
screening the other part, or by exposing the whole instru¬ 
ment equally to the sun ; and by making contemporaneous ob¬ 
servations with the instrument wholly uncovered, or covered 
totally or partially by a glass to protect it from currents of air. 
The Professor has endeavoured to make a standard scale by 
registering the sun’s radiation on clear days every half hour, or 
hour, in the usual manner, and comparing them with the con¬ 
temporary distillation ; or by placing the two sides of the instru¬ 
ment in two vessels of water at unequal temperatures, and 
