So/ar Radiation 389 



of temperature when that of the thermometer is, say, 2, 4 

 or 6 above that of the enclosure is found. Similarly, the 

 rate of fall of temperature when the temperature of the 

 thermometer is 2, 4 or 6 above that of the enclosure 

 during cooling is found. Three pairs of rates are thus 

 obtained. The sums of all three pairs of rates should be 

 alike, and each gives a value of the rate at which the 

 temperature of the actinometer would rise when exposed to 

 the sun if there were no cooling. The rule is the same 

 whether the temperature is allowed to rise to the stationary 

 point or not. A distinction is often made between the static 

 method, when the experiment is continued until the stationary 

 temperature is arrived at, and the kinetic method, when it is 

 interrupted before that temperature is reached. This dis- 

 tinction rests on no substantial difference ; at the same time 

 it is convenient to retain the designations to distinguish the 

 manipulative processes. 



Were the protecting enclosures, such as the double 

 spherical shell packed \vith melting ice, used by Violle, or 

 the thick metal shell used by Crova, perfectly efficient, 

 then it would not be necessary to make a separate cooling 

 experiment in connection with every heating one. The 

 necessity for it is due to the fact that, when the sun's rays 

 are introduced, the temperature of the air in the enclosure 

 no longer is, and it cannot be, at the temperature of the 

 enclosing shell ; nor can it remain motionless, as it is when 

 at a constant temperature in the shade. These perturba- 

 tions, which cannot be avoided, so long as there is air in the 

 enclosure, make it impossible to apply a rate of cooling 

 determined beforehand. It is necessary on each occasion 

 to determine the actual integral rate of cooling during the 

 particular experiment. 



If the actinometer could be so arranged that the rate of 

 cooling should not be affected by the introduction or exclu- 

 sion of the sun's rays, the static method could be adopted 

 without hesitation, and the instrument would become a valu- 

 able one for continuous self-recording observations. Their 

 value would be mainly relative. The absolute value of the 



