83 
observatory of Montsouris ever since 1873) is an apparatus that 1s 
intended to determine the total solar plus sky radiation that is needed 
in agricultural physics. A theory of the action of this instrument 
was devised by Marié-Davy, but the proper method of calculating its 
results was first developed with exactness by Ferrel, in Professional 
Papers of the Signal Service, No. XTIT (1884), and subsequently in 
his Recent Advances in Meteorology (Annual Report, Chief Signal 
Officer, p. 8373). His formula will be given on page 88. 
The Arago-Davy actinometer is composed of two mercurial ther- 
mometers with very fine tubes, and having spherical reservoirs of 
equal dimensions, one colorless and the other covered with lamp- 
black. In the empty space above the mercury in the thermometer 
tubes there is a small quantity of hydrogen or other inert gas. The 
small quantity of gas left in the tubes of these thermometers has no 
other object than to prevent the mercury from falling in the tube by 
the force of gravity when the bulb is turned upward toward the sky. 
Each thermometer is inclosed in a larger glass tube or cylinder, ter- 
minated by a spherical enlargement, in the center of which is placed 
the center of the bulb of the thermometer. This tube and enlarge- 
ment constitute the inclosure, and it is exhausted of air as perfectly 
as possible. The immovability of the thermometer, relative to the 
walls of its inclosure, is assured by a soldering at the upper extremity 
of the tube and, at the opposite end toward the reservoir, by two rings 
of cork held by friction between the interior tube and exterior cylinder. 
These thermometers, with their respective glass inclosures, are turned 
up with their bulbs toward the sky, and by means of double clamps 
fixed parallel to two metallic rods, arranged in the form of a V and 
turned, the one toward the east, the other toward the west. These 
metallic rods make an angle with each other of 60°—that is to say, of 
30° with the vertical—and are fastened to a support of wood or iron 
1.20 or 1.30 meters in height above the earth. The support is solidly 
planted in the ground in an open place, remote from buildings, plants, 
or any other obstacle capable of intercepting the direct radiation of 
the sun. The two thermometers, the envelopes of which are exposed 
near each other, have necessarily the same temperature and mark the 
same degree as long as they remain in perfect darkness; but hardly 
does day begin to break than the thermometer with the black bulb 
marks a higher temperature than that with a plain glass bulb. The 
difference in temperature of these two thermometers gives the * acti- 
nometric degree” for the moment of observation; that is to say, it 
serves to measure the intensity with which the radiation strikes the 
two thermometers and is absorbed by the black bulb; consequently, at 
least approximately, it serves to measure the intensity with which the 
