IN PASSING THROUGH THE ATMOSPHERE. 
231 
16. The former of these objections applies to all thermometric instruments consi- 
dered as light measurers ; but the latter has been ingeniously got over by Sir John 
Herschel, in a way more convenient in practice than that used by Lambert. 
17. Herschel’s actinometer consists of a thermometer with a large cylindric bulb, 
containing - a deep-blue fluid (the ainmonio-sulphate of copper), and inclosed in a 
wooden case, blackened interiorly and covered with a piece of thick plate glass. The 
capacity of the bulb may be caused to vary, by screwing in or out a plunger which 
enters parallel to the axis of the cylinder, and the use of which is to retain the top of 
the column of fluid within the range of the tube, which is connected Avith the cylin- 
der as in the common thermometer, and which it would otherwise be liable to exceed, 
owing to the great variations of temperature to which it is exposed. The velocity of 
heating during exposure to the sun is ascertained by limiting the exposure to one 
minute , during which the rise of the liquid is accurately observed. But since during 
this minute, the rise was not that due to the solar influence alone, but to the direct 
solar influence plus or minus all the cooling or heating influences simultaneously 
acting on the actinometer, these indirect influences are ascertained and allowed for 
by exposing the instrument for one minute behind a screen, which merely stops the 
solar rays, but allows all other actions to go forward. If the instrumental readings 
fall during the shade observation (owing to the coolness of the atmosphere and the 
high temperature of the liquid), it is plain that the solar action was not only to raise but 
to maintain the temperature, and that the fall during the shade observation must be 
added to the rise during the sun observations to give the effect due to the sun. On 
the other hand, if the temperature continue to rise during the shade observation 
(which may be due to indirectly reflected heat, or to the communication of heat from 
the parts of the instrument), it is plain that the rise in the sun was not wholly due to 
the immediate solar influence, and therefore that the rise in the shade must be sub- 
tracted from it. 
18. This instrument gives very constant and satisfactory results: it is the one 
with which the following observations were entirely made ; examples of the mode 
of using it will therefore be given when I come to describe my own experiments. In 
the mean time I may refer for the first description of the actinometer to the Edin- 
burgh Journal of Science for 1825* ; and for a very full account of it, and the method 
of using it by Sir John Herschel himself, in the e Instructions’ lately published by 
the Royal Society^. 
time, t the time, and S the subtangent of the logarithmic curve which expresses the Newtonian law of cooling, 
having the excess of temperature above the medium for its ordinate, and the time for its abscissa. This sub- 
tangent varies (as he observes § 282) with the state of the atmosphere. It is very remarkable that Leslie 
himself pointed out in his very earliest published composition (an Essay written in 1792 or 1793, but first 
printed in Thomson’s Annals of Philosophy in 1819, vol. xv. p. 7.), that “ the initial change” (or rate of heat- 
ing of a black surface exposed to the sun) “ on the thermometer, is in every case the only certain and accurate 
measure of the communication of heat”: — a principle which, however, he practically abandoned, as we have seen. 
* Vol. iii. p. 107. f p. 58. 
