338 



SCIENCE 



[N. S. Vol. XXXIX. No. 1001 



temperature of the water to be well ascer- 

 tained. The instrument was greatly im- 

 proved by Tyndall, who substituted mer- 

 cury for water, and, in order to contain the 

 mercury, used iron in the making of the 

 box. 



Crova Alcohol Actinometer. — A large 

 spherical bulb thermometer containing 

 alcohol is enclosed in a nickel-plated metal 

 chamber 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 ob- 

 serving. The method of observing is the 

 same as that adopted by Herschel and by 

 Pouillet. 



Violle Actinometer. — A large spherical 

 double-walled enclosure filled with water is 

 kept at a known constant temperature. A 

 spherical blackened-bulb thermometer lies 

 at the center of the enclosure, and the sun- 

 light is introduced to it through a suitable 

 vestibule in the double-walled chamber. 

 Violle 's method of reading 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 posi- 

 tion 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 in- 

 strument is open to the objection that the 

 water equivalent of the bulb of the ther- 

 mometer is very small, and difficult to 

 measure, and that several corrections rather 

 difficult of determination should be ap- 

 plied. It was used by Dr. Langley in his 

 expedition to Mount Whitney in 1881. 



Angstrom Electrical Compensation Pyr- 

 heliometer. — This instrument has had the 

 most extensive adoption in recent years ot 



any form of instrument for measuring the 

 solar radiation. It was invented about the 

 year 1895. Two metal strips exactly 

 similar to one another, and blackened upon 

 the front, are exposed alternately to heat- 

 ing by the sun. Arrangement is provided 

 for passing an electrical current through 

 the strip which is not at the moment being 

 heated by the sun. Thermo-elements fast- 

 ened to the back of each strip indicate 

 when the temperature of the exposed strip 

 is equal to that of the strip which is elec- 

 trically heated. Under these circumstances 

 it is assumed that the energy of the electric 

 current is equal to the energy received 

 from the sun. About 160 copies of this 

 electrical compensation pyrheliometer have 

 been sent out from Upsala to different parts 

 of the world. 



Several other kinds of pyrheliometers 

 have been used in recent years, among them 

 two forms which have been devised by the 

 writer. We shall have occasion to speak 

 of these later. 



EAELY OBSERVATIONS 



Forbes observed with the Herschel actin- 

 ometer in the year 1832 at Brientz and 

 the Faulhorn. He showed that the trans- 

 missibility of sun rays continually increases 

 as the length of path of the ray in air in- 

 creases. Forbes rightly attributed this to 

 the non-homogeneity of the solar radiation, 

 and the inequality of transmission of the 

 different component parts of it. Under 

 such circumstances Bouguer's formula of 

 course can not apply. Forbes concluded 

 that equal barometric columns of air give 

 equal transmission, whether taken from the 

 high or low station. In this he was wrong. 

 He formed an empirical curve to represent 

 all his observations at both stations, em- 

 ploying air masses as abscissae and actinom- 

 eter readings as ordinates. Instead of 

 extrapolating this curve directly to air 



