388 Solar Radiation 



The fundamental principle of the actinometer is analogous 

 to Newton's second law of motion ; when a body is engaged 

 in the exchange of heat between itself and any number of 

 other bodies, each exchange takes place independently of the 

 others. The rate of exchange in each case depends on the 

 difference of temperature between the two bodies and takes 

 place on the principle that equal fractions of heat are lost 

 or gained in equal times. A body cooling in the air is 

 always subject to at least two quite independent sources 

 of loss of heat, namely, radiation between itself and the 

 surrounding objects and conduction between itself and the 

 contiguous air. Under ordinary circumstances the rate of 

 loss of heat by radiation is subject to but little variation, but 

 that due to conduction is subject to continual variation owing 

 to the varying rate at which the air actually in contact with 

 the thermometer is renewed. It is not to be expected that 

 a body subject to at least two independent sources of loss of 

 heat will cool in the same way as it would if exposed to only 

 one, any more than it is to be expected that a body acted 

 on by two forces will move in the same way as if it were 

 impelled by only one of them. The composition of rates of 

 cooling is like that of velocities in the same straight line ; 

 the resultant rate is the nett, or algebraic, sum of all the 

 rates. When the actinometer is exposed to the sun, its 

 temperature rises at first rapidly, and then more slowly 

 until, if the experiment is sufficiently prolonged, it becomes 

 stationary. The temperature is noted at equal intervals of 

 time. The sun is screened off, either after the temperature 

 has become stationary or beforehand, and the temperature is 

 observed at equal intervals during cooling. Whenever the 

 thermometer is at a higher temperature than its enclosure, 

 it is cooling. Therefore when it is exposed to the sun's 

 rays, and its temperature rises ever so little above that of 

 the enclosure, cooling begins ; and what is observed in the 

 first operation is, not the rate of heating by the sun's 

 rays, but that rate diminished by the rate at which the 

 thermometer is cooling. Hence, when the two series of 

 observations have been made and tabulated, the rate of rise 



