83 



observatory of Montsouris ever since 1873) is an apparatus that is 

 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 Marie-Davy, but the proper method of calculating its 

 results was first developed with exactness by Ferrel, in Professional 

 Papers of the Signal Service, No. XIII (1884), and subsequently in 

 his Recent Advances in Meteorology (Annual Report, Chief Signal 

 Officer, p. 373). 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 Avith 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 

 difi'erence 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 



