NATURAL PHILOSOPHY. 151 



CURIOUS APPLICATION OF AIR EXPANSION THROUGH HEAT. 



It is we'll known that the air confined under glass, if it receive the 

 direct rays of the sun, will become much heated, far beyond the tem- 

 perature of the rays, owing- to the action of the glass in absorbing 

 these rays and conveying the absorbed heat to the air within. Prof. 

 Mouchot, of Alencon, has made the following application of the heat 

 thus acquired, lie takes a bell of silver, very thin and covered with 

 lampblack, and places over it two bells of glass, and exposes the 

 whole to the rays of the sun. Two curved tubes, furnished with 

 stopcocks, pass under the black bell, one of them to supply water 

 when it is required, the other to give exit to the water; the latter ter- 

 minating outside in an ordinary jet d^au orifice. Being now exposed 

 to the solar rays, whose heat is transformed into non-luminous heat 

 in its passage through the walls of the bells, an effect that goes on 

 accumulating without cessation, the air situated above the water 

 dilates, and by its pressure causes a jet to rise, attaining sometimes in 

 Mouchot's trials a hight of nearly b'3 feet. When the water is ex- 

 hausted, a screen placed before the sun will cool the interior and cause 

 the water to return, or a new supply may be introduced through the 

 supply-pipe. Many times the shade thrown over the apparatus by spec- 

 tators caused it to stop, much to their surprise. Les Mondes, Sept. 22. 



EFFECT OF THE SUN ON SOIL AND All!. 



The relative heating of the soil and air by the solar rays on a high 

 mountain and in a plain has been examined by the distinguished trav- 

 eler, M. Ch. Marteus, who has reported on the subject to the Acad- 

 emy of Sciences at Paris. We give a few notes : A solar ray, it is 

 said, falling on an elevated summit, should be hotter than one which, 

 after traversing the lower and denser strata of the atmosphere, de- 

 scends into a plain, since these strata necessarily absorb a notable 

 quantity of the heat of the ray. All travelers who have ascended 

 high mountains have been surprised at the extraordinary heat of the 

 sun and the soil compared with the temperature of the air in the shade, 

 or with that of the soil during the night. The observations of MM. 

 Peltier, Bravais, and Marteus (two series, 125 in the whole), made on 

 the Fauthorn, Switzerland, between G A. M. and 6 P. M., continued 

 indifferently in fine and bad weather, give, nevertheless, for the mean 

 temperature of the soil during the day, 11'75 per cent, that of the air 

 being only o*oO. It became evident that heating of the soil during 

 the day was twice that of the air ; but the observers were not aware 

 what had been the relative heating of the earth and air in the plain 

 below during the same period. To obtain this knowledge, M. Mar- 

 teus selected the summit of the Pic du Midi, in the Pyrenees, and a 



temperatun 



at Bagneres, was 22'o ; from the same on the Pic du Midi, IU'1 only. 

 The mean temperature of the surface of the soil at Bagneivs was oG'l ; 

 on the Pic, oo'b. The mean excess of the temperature of the soil over 

 that of the air at the two stations is then as 10 : 171, nearly double 

 that on the mountain. These experiments put out of doubt the great- 

 er calorific power of the sun upon the mountain than upon the plain. 



