=87. Pouillet's Pyrheliometer.=--"This instrument is composed of a shallow cylinder of steel, _A_, fig. 67, which is filled with mercury. Into the cylinder a thermometer, _D_, is introduced, the stem of which is protected by a piece of brass tubing. We thus obtain the temperature of the mercury. The flat end of the cylinder is to be turned towards the sun, and the surface, _B_, thus presented is coated with lamp black. There is a collar and screw, _C_, by means of which the instrument may be attached to a stake driven into the ground, or into the snow, if the observations are made at considerable heights. It is necessary that the surface which receives the sun's rays should be perpendicular to the rays; and this is secured by appending to the brass tube which shields the stem of the thermometer, a disk, _E_, of precisely the same diameter as the steel cylinder. When the shadow of the cylinder accurately covers the disk, we are sure that the rays fall, as perpendiculars, on the upturned surface of the cylinder. [Illustration: Fig. 67.] "The observations are made in the following manner:--First, the instrument is permitted, not to receive the sun's rays, but to radiate its own heat for five minutes against an unclouded part of the firmament; the decrease of the temperature of the mercury consequent on this radiation is then noted. Next, the instrument is turned towards the sun, so that the solar rays fall perpendicularly upon it for five minutes; the augmentation of heat is now noted. Finally, the instrument is turned again towards the firmament, away from the sun, and allowed to radiate for another five minutes, the sinking of the thermometer being noted as before. In order to obtain the whole heating power of the sun, we must add to his observed heating power the quantity lost during the time of exposure, and this quantity is the mean of the first and last observations. Supposing the letter _R_ to represent the augmentation of temperature by five minutes' exposure to the sun, and that _t_ and _t^1_ represent the reductions of temperature observed before and after, then the whole force of the sun, which we may call _T_, would be thus expressed:--_T = R + 1/2(t + t^1)_. "The surface on which the sun's rays here fall is known; the quantity of mercury within the cylinder is also known; hence we can express the effect of the sun's heat upon a given area, by stating that it is competent, in five minutes, to raise so much mercury so many degrees in temperature."--_Dr. Tyndall's "Heat considered as a Mode of Motion."_ [Illustration: Fig. 68.]