placed in the center of the inner sphere, the stem reaching outside through a tubulure provided for the purpose. Two opposite openings, shown in the figure, allow a beam of sunlight to pass through the globes. A perforated screen at D limits its diameter so that none of it shall touch the walls of the vessel, though the thermometer-bulb is entirely covered by it. A small screen at M allows the observer to see the shadow of the thermometer-bulb, and so to perceive whether the tube through which the light enters is properly directed. If the apparatus is mounted upon what is called an equatorial stand, like a telescope, and provided with clock-work, the whole labor of observation will consist merely in reading the thermometer. The difference between its temperature and that of the water in the surrounding shell gives the necessary data for calculating the intensity of the solar radiation at the time of reading; since the heat received by the thermometer from the sun and shell together must just equal that radiated back by the thermometer-bulb to the shell, after allowing for the orifices.
Fig. 2.—Crova's Pyrheliometer.
Violle found that at noon, on a fair day, the thermometer of this apparatus generally stood, when exposed to the sun, from 10∙5° to 12∙5° centigrade (i. e., 18∙9° to 22∙5° Fahr.) above the temperature of the shell when the latter was filled with ice-water. If it were filled with boiling water, as in some of his experiments, the difference became about 1° C. less.
The results obtained with instruments of this class of course agree very closely with those reached by the dynamic method.
It need hardly be said that the amount of heat received from the sun in a minute by a given area exposed to its radiation varies widely
