150 WRITINGS OF JOSEPH HENRY. [1855- 



sisted of a flat cylindrical vessel, the top of which was of thin 

 silver, of about four inches in diameter and six-tenths of an 

 inch in height or thickness. It was filled with 100 grammes 

 of distilled water, and in the middle of this liquid was placed 

 the bulb of a thermometer with a fine bore and a long stem 

 projecting downward in the direction of the axis of the 

 cylinder through its lower surface. 



The observations were made in the following manner: 

 The upper surface of the vessel, coated with lampblack to 

 render it absorbent of heat, was turned directly towards the 

 sun, the water being kept in a state of constant agitation in 

 order to equalize the heat. The increase of temperature re- 

 ceived from minute to minute in the course of five minutes 

 was noted. The vessel was then placed in the shade while 

 its face was exposed to a portion of clear sky near the sun, 

 and the loss of temperature from minute to minute during 

 five minutes was again noted. A Httle reflection on the 

 principles of the interchange of heat, according to which 

 bodies are constantly radiating even while they are receiving 

 heat from other bodies, will render it evident that in order 

 to find the amount of temperature communicated by the sun 

 in a minute of time we must add the loss of temperature 

 during the shading of the instrument to the gain of tem- 

 perature noted during the direct exposure to the sun, for 

 while the instrument was receiving heat from the sun it was 

 at the same moment radiating heat to that body. To find 

 from the indications thus obtained the absolute amount of 

 heat which falls on the face of the vessel in one minute of 

 time we must make a correction for the absorption of heat 

 by the metal, and allow for the specific heat of the water, that 

 is, the relative quantity necessary to elevate a pound of this 

 liquid one degree of Fahrenheit's scale. In this way the 

 quantity of heat which falls on a given surface, (say a square 

 foot,) perpendicular to the solar beam at the surface of the 

 earth is determined. But this quantity is not all that would 

 be given to the same surface were the atmosphere removed, 

 or if the same experiment were made at the outer limits of 

 the aerial covering of the globe. A portion of the heat is 



