abbot: solar constant of radiation 93 



rear, extending away from the direction of the sun. The in- 

 strument, hke that of Herschel, was exposed to the influence of 

 the surroundings while shaded for a certain interval 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 practice it was found that the 

 water within the box could not be well enough stirred in order 

 to allow the average temperature of the water to be well ascer- 

 tained. The instrument was greatly improved by Tyndall, who 

 substituted mercury for water, and, in order to contain the mer- 

 cury used iron in the making of the box. 



Crova alcohol actinometer : A large spherical bulb thermome- 

 ter containing alcohol, is enclosed in a nickel pated metal cham- 

 ber with a vestibule for the entrance of the rays. The stem of 

 the thermometer runs back, directly away from the sun, and is 

 enclosed in a nickel plated tube with a side opening for reading 

 the thermometer. A short mercury thread is introduced in the 

 alcohol column at a suitable point for observing. The method 

 of observing is the same as that adopted by Herschel and by 

 Pouillet. 



Violle actinometer: A large spherical double walled enclo- 

 sure filled with water is kept at a known constant temperature. 

 A spherical blackened bulb thermometer lies at the center of the 

 enclosure, and the sunlight is introduced to it through a suitable 

 vestibule in the double walled chamber. Violle 's method of read- 

 ing was static, as opposed to the dynamic methods we have just 

 considered. He observed the total rise of the thermometer and 

 its fall after the cutting off of the sun rays, noting the position 

 of the column at fixed intervals after exposure and after closure. 

 The theory of the instrument as developed by Violle is simple 

 and elegant. As a standard the instrument is open to the ob- 

 jection that the water equivalent of the bulb of the thermometer 

 is very small, and difficult to measure, and that several corrections, 

 rather difficult of determination, should be applied. It was used 

 by Dr. Langley in his expedition to Mount Whitney in 1881. 



Angstrom electrical compensation pyrheliometer : This instru- 

 ment has had the most extensive adoption in recent years of any 



