92 abbot: solar constant of radiation 



tion so as to reduce the expression to the form of the equation 

 of a straight line. Thus 



log A = TjQQ sec z log p + log Ao 



By this equation the intersept of the best straight line on the 

 axis of ordinates is the logarithm of the intensity of solar radia- 

 tion outside the atmosphere, and the inclination of the line to 

 the horizontal is the logarithm of the atmospheric transmission 

 for vertical rays. 



The reader must bear in mind that the simple expression thus 

 obtained is given only in illustration of the work of the earlier 

 investigators, and it must be hedged about with certain condi- 

 tions and limitations in order to apply it, as we shall see later, 

 to the determination of the solar constant of radiation by the 

 most approved methods. 



Instruments. Herschel's actinometer: This instrument con- 

 sists of a thermometer with a large cylindric bulb, containing a 

 deep blue fluid (the ammoniacal sulphate of copper) and en- 

 closed in a wooden case blackened interiorly and covered with 

 a piece of plate glass. The thermometer has a very large bulb, 

 and it is adjusted in volume by means of a screw, so as to regu- 

 late the position of the column of liquid on the thermometer 

 scale. Herschel introduced what is termed the dynamical meth- 

 od of observing the solar radiation, for he obtained not the total 

 rise of temperature of the instrument when long exposed to the 

 sun, but its initial rate of rise, corrected for the cooling or warm- 

 ing of the thermometer due to external conditions when the sun 

 is shaded. The determination of the cooling correction is done 

 by observing the rise or fall of the temperature for a certain time 

 interval before exposing to the sun, and again determining the 

 rise or fall after such exposure to the sun is completed. The 

 mean rate of warming or cooling, due to the surroundings, is 

 applied as a correction to the rate of warming due to the exposure 

 to the solar radiation. 



Pouillet's pyrheliometer : A flat metal box, blackened on the 

 front, and filled with water, has a thermometer inserted at the 



