Maech 6, 1914] 



SCIENCE 



337 



millimeters of mercury, then p in our form- 

 ula corresponds to the vertical transmission 

 coefficient of the atmosphere above sea 

 level, and for any station where the baro- 

 metric pressure is B the intensity of the 

 ray from the sun as it reaches the earth's 

 surface, which we call A, may be expressed 

 by the formula 



A = A^p(snmseo!:. 



Some writers have preferred to use the 

 formula as a formula of "absorption" 

 rather than of transmission. In that way 

 the expression reduces to a somewhat 

 different form, but its fundamental prin- 

 ciples are the same. The investigations of 

 Hersehel, Forbes, Pouillet and others up 

 to the time of Langley had reference to this 

 exponential formula based upon the hy- 

 pothesis of BougTier, which was to the effect 

 that successive equal layers of transparent 

 material transmit equal fractions of the 

 incident ray. 



A convenient method of applying the 

 atmospheric transmission formula is to take 

 logarithms of both members of the equa- 

 tion so as to reduce the expression to the 

 form of the equation of a straight line. 

 Thus 



log ^ = — sec z log p -t- 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 in- 

 clination 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 far obtained is given 

 only in illustration of the work of the 

 earlier investigators, and it must be hedged 

 about with certain conditions and limita- 

 tions 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 



Hersehel' s Actinometer. — This instru- 

 ment consists of a thermometer with a large 

 cylindric bulb, containing a deep blue fluid 

 (the ammoniacal sulphate of copper) and 

 enclosed in a wooden case blackened inte- 

 riorly and covered with a piece of thick 

 plate glass. The thermometer has a very 

 large bulb, and it is adjusted in volume by 

 means of a screw, so as to regulate the posi- 

 tion of the column of liquid on the ther- 

 mometer scale. Hersehel introduced what 

 is termed the dynamical method of observ- 

 ing the solar radiation, for he obtained not 

 the total rise of temperature of the instru- 

 ment when long exposed to the sun, but its 

 initial rate of rise, corrected for the cooling 

 or warming of the thermometer due to ex- 

 ternal 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 deter- 

 mining the rise or fall after such exposure 

 to the sun is completed. The mean rate of 

 warming or cooling, due to the surround- 

 ings, 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, had a thermometer inserted at the 

 rear, extending away from the direction of 

 the sun. The instrument, like that of Her- 

 sehel, was exposed to the influence of the sur- 

 roundings while shaded for a certain inter- 

 val of time the shade was then removed for 

 a similar interval so as to allow the solar 

 radiation to fall upon the blackened box, 

 after which the instrument was again 

 shaded. In practise it was found that the 

 water within the box could not be well 

 enough stirred in order to allow the average 



