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 be 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 Marié-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, which 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 diminution 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- 
a This paragraph, mitten in 1891, is of gnecial interest in Connection Ww ith the 
general interest in the subject awakened in 1903 by the observations of Kimball, 
Dufour, and Abbott. 
