160 ANNUAL OF SCIENTIFIC DISCOVERY. 



results obtained by Prof. Tyndall, estimating its absorbing power at 

 60 times that of air. 



MECHANICAL VALUE OF THE SUN'S HEAT UPON THE EARTH. 



Mr. J. 11. Mayer, well known for his researches in celestial dynamics, 

 estimates the effect of the solar radiation in mechanical work on the 

 total surface of the earth, as equal to 180 billions of horse-power per 

 minute, or in other words, the surface of our globe receives every 

 minute an addition of vis viva equivalent to this amount. A not in- 

 considerable portion of this enormous quantity of vis viva is consumed 

 in the production of atmospheric actions, in consequence of which 

 numerous motions are set up in the earth's atmosphere. Sill inlands 

 Journal. 



BREAKING ROCKS BY FIRE. 



The ancient method of breaking rocks by fire has lately been re- 

 vived at the Rammelsberg mine, in the Hartz Mountains. In a port- 

 able furnace about one and a half bushels of coke burn from 4 P. M. 

 to 5 A. M., when the furnace is removed and the rock left to cool, 

 after being sprinkled with water. About noon the face of the rock 

 spontaneously detaches itself; and after that a further portion is 

 broken. The effect of the fire extends to 8 inches, and with 1^ 

 bushels of coke from 16 to 2-4 cwt. of ore is obtained at half the cost 

 of the gunpowder process. 



COOLING OF SOLIDS BY TENSION. 



Mr. James Croll, in the Philosophical Magazine, directs attention 

 to the experiments of Dr. Joule, which proved that the quantity of 

 cold produced by the application of tension to solids was sensibly 

 equal to the heat evolved by its removal ; and further, that the ther- 

 mal effects were proportional to the weight employed. The probable 

 explanation given by Mr. Croll is this : If the molecules of a body 

 are held together by any force, of whatever nature it may be, which 

 prevents any further separation taking place, then the entire heat of 

 such a body will appear as temperature ; but if this binding force 

 become lessened, so as to allow further expansion, then a portion of 

 the heat will be lost in producing expansion. All solids, at any 

 given temperature expand until the expansive force of their heat 

 exactly balances the cohesive force of their molecules ; after which 

 no further expansion at the same temperature can possibly take place 

 while the cohesive force remains unchanged. But if by some means 

 or other the cohesive force of the molecules become reduced, then 

 instantly the body will expand under the heat which it possesses, 

 and of course a portion of the heat will be consumed in expansion, 

 aud a cooling effect will result. 



CONDUCTION OF HEAT. 



In relation to TyndalPs researches, M. L'Abbe Laborde states that 

 he heated to redness one end of a thin iron bar so long that the 

 other end could be held without burning. When the red end was 

 plunged into water the other end became so hot he was compelled to 

 drop it. The rapid compression of the hot metal, he says, is no 

 doubt the cause of the elevation of temperature ; but he asks if 

 another cause may not be suspected : for instance, the creation of 

 thermo-electric currents ? 



