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THE POPULAR SCIENCE MONTHLY. 



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 ap- 

 paratus is mounted upon what is called an equatorial stand, like a tele- 

 scope, and provided with clock-work, the whole labor of observation 

 will consist merely in reading the thermometer. The difference be- 

 tween its temperature and that of the water in the surrounding shell 

 gives the necessary data for calculating the intensity of the solar radi- 

 ation at the time of reading ; since the heat received by the thermome- 

 ter 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. Cbova's Ptrheliometer. 



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 be- 

 came 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 



