498 Mr. J. J. Waterston on Solar Radiation. 



where the sun rules in a cloudless sky, the presumed law might 

 soon be put to the test, and the heating-power of the sun's rays 

 before entering the atmosphere ascertained with precision. 

 Having determined this for the earth's mean distance from the 

 sun, its value for any other planetary distance may be deduced 

 by the law of the inverse square. 



§ 3. When a thermometer is exposed to the sun with its bulb 

 blackened, it is presumed to absorb all the heat that impinges 

 on a plane surface equal to the transverse section of the bulb ; 

 it rises and is maintained at a certain temperature ; and when 

 this balanced condition is attained, we can with certainty assert 

 that the amount that issues from the bulb is precisely equal to 

 the quantity that enters. The elevation of its temperature above 

 surrounding bodies due to the sun's radiant power (which is 

 denoted by the symbol r) would be an exact measure of that 

 power if no heat issued from it except by radiation, and if the 

 rate at which heat was emitted from it increased exactly in pro- 

 portion to r. Now I find that if the bulb of a thermometer is 

 enclosed in a vacuum, the walls of which are brass coated with 

 lampblack, the rate at which it cools is exactly proportional to 

 the value of r, and this rate has exactly the same value whether 

 the glass bulb is uncoated or coated with lampblack*. When 

 enclosed in air, the rate of cooling increases faster than r. The 

 mode of measuring the rate in both cases, and of reducing the 

 values of r observed in air to what they would be in a vacuum, 

 are described at the end of this paper. 



§ 4. The instrument employed was designed so that the 

 thermometer exposed to the sun's rays should radiate against an 

 enclosing metallic surface coated with lampblack, and so that 

 the temperature of that surface should always be known. 



Fig. 1, Plate V., with the description that accompanies it, 

 gives the details. The rays of the sun were admitted, to strike 

 upon the bulb of the thermometer X, through a hole but little 

 larger than its diameter, and were entirely screened off the brass 

 tube, against the blackened inner surface of which the radiation 

 of the bulb took place. The thickness of the sides of the brass 

 tube was | inch ; and the thermometer Y, that indicated its 

 temperature, was lodged in a hole cut in the upper side. The 



* This unexpected result is confirmed by repeated observations. While 

 the emitting-power of a mercurial thermometer is thus the same, coated 

 or uncoated, the absorbing -power of the uncoated surface is only six-tenths 

 of the same coated with lampblack. This apparent anomaly is no doubt 

 caused by the incident rays of heat, after passing through the glass, being 

 reflected from the surface of the mercury, because I find that, with spirit- 

 thermometers (in which the spirit is largely impregnated with red vegetable 

 matter), the uncoated reservoir absorbs radiant heat exactly the same as 

 when coated w r ith lampblack. 



