322 



NATURE 



{yuly 31, 1879 



copper, are themselves acted on by the variable tempera- 

 tures which complicate the results. A second twisted tube, 

 similar to the first, provided with its capillary tube, serves 

 to give the necessary corrections. The two twisted tubes 

 are arranged parallel to each other in a trough, E F, filled 

 with glycerinated water ; the two capillary tubes are 

 soldered to each other throughout their length. The two 

 needles trace two curves, one very slightly sinuous, the 

 other very undulating ; the difference of their ordinates is 



measured. We thus obtain the temperatures of ground 

 exposed to the sun during day, to nocturnal radiation 

 during night, to rain and to evaporation. 



The actinometer is also composed of two twisted tubes 

 resting parallel to each other in the above-mentioned 

 trough, E F, and of two capillary tubes. To each of these 

 tubes corresponds a reservoir placed at the top of the roof 

 of the kiosk, and inclosed in an envelope of glass on 

 which a dry vacuum has been made. One of the reser- 



1 



Fig. 6. — Specimen of curves made by the registers of Montscuris. 



^oirs is silvered, the other is blackened. Placed symme- 

 trically at a little distance from each other, and far from 

 any shelter, they give the same indications during the 

 •night; but during the day the black thermometer registers 

 higher than the white one. The differences of the ordinates 

 of the curves traced by the two needles serve to measure 

 'the actinic degree. Every cloud which passes over the 

 «un'gives rise to a rapid and considerable lowering of the 



temperatures of the white and black thermometers, and of 

 that on the surface of the ground. 



The atmograph is a third registering instrument in- 

 tended to mark the variations of weight of a mass of 

 earth whose surface is flush with the summit of the plat- 

 form of the roof of the kiosk, and is exposed to the sun, 

 to evaporation, rain, snow, and dew. At a little distance 

 is the registering rain-gauge. The comparison of the 



