99 



be absolutely independent of the temperature of the air and the veloc- 

 ity of the wind, as well as of the quantity of liquid in the bulb A, and 

 should depend wholly on the heat received from the sun and sky. In 

 its present form it can not bo recommended as a simple means of meas- 

 uring the daily sum total of radiation from the sun and sky. A sec- 

 ond and improved form of Bellani's apparatus has been brought out 

 under the title " Vaporization lucimeter " (see Marie-Davy, Annuaire, 

 1888, p. 207), but further improvements are necessary, especially the 

 maintenance of a uniform constant temperature in the condensation 

 bulb and tube, as, for instance, by immersing both in a bath at melting 

 point of ice. 



ARAGO'S CYANOMETER AND DESAINS' THERMO-ELECTRIC 

 ACTINOMETER. 



Other methods of observing the condition of the sky and solar 

 radiation have been devised by physicists. Thus the cyanometer of 

 Arago, especially in the modified form made by Dubosc, of Paris, or 

 the thermo-electric actinometer of Desains (both of which are in 

 occasional use at Montsouris) give useful indications. The cyanom- 

 eter gives the blueness of the sky, which is largely dependent on the 

 number and size of the particles of moisture, while the actinometer 

 gives the quantity of heat that penetrates directly from the sun 

 through this moist air to the ground. These instruments are comple- 

 mentary to each other, but can only give good results in the hands of 

 those accustomed to the use of delicate apparatus. They serve as checks 

 upon the records of the Arago-Davy actinometer, Avhich latter has 

 been made by Richard in such form as to keep a continuous register. 

 Thus during the years 1879-1886 the Arago-Davy instruments, both 

 in France and in India, showed a steady, progressive dimimition in 

 the intensity of the solar radiation received at the ground, followed, 

 however, by a recovery, subsequently, which is not likely to have been 

 due to any instrumental peculiarity. This peculiar fluctuation may 

 have had its cause either in the sun or in the earth's atmosphere." 



DURATION OF SUNSHINE. 



Those who can not undertake the labor of observing the heating or 

 chemical effects of the solar radiation can easily keep a photographic 

 register of the number of hours of sunshine, as in the apparatus 

 devised by Jordan, of England, and modified by Marvin for use at 

 Signal Service stations, or can keep a record of the hours of full 

 hot sunshine, as in the Campbell, or Campbell-Stokes, sunshine 

 recorder used in Canada. The Marvin photographic sunshine reg- 



oThis paragraph, written in 1801, is of special interest in connection with the 

 general interest in the subject awakened in 1903 by the observations of Kimball, 

 Dufour, and Abbott. 



