ON THE INTERACTION OF NATURAL FORCES. 159 



wcmld, therefore, be quite fused, and for the most part converted 

 into vapour. If, then, the earth, after having heen thus brought 

 to rest, should fall into the sun which, of course, would be the 

 case the quantity of heat developed by the shock would be 400 

 times greater. 



Even now from time to time such a process is repeated on a 

 small scale. There can hardly be a doubt that meteors, fireballs, 

 and meteoric stones are masses which belong to the universe, 

 and before coming into the domain of our earth, moved like the 

 planets round the sun. Only when they enter our atmosphere 

 do they become visible and fall sometimes to the earth. In 

 order to explain the emission of light by these bodies, and the 

 fact that for some time after their descent they are very hot, 

 the friction was long ago thought of which they experience in 

 passing through the air. We can now calculate that a velocity 

 of 3,000 feet a second, supposing the whole of the friction to be 

 expended in heating the solid mass, would raise a piece of 

 meteoric iron 1,000 C. in temperature, or, in other words, 

 to a vivid red heat. Now the average velocity of the 

 meteors seems to be thirty to fifty times the above amount. To 

 compensate this, however, the greater portion of the heat is 

 doubtless carried away by the condensed mass of air which 

 the meteor drives before it. It is known that bright 

 meteors generally leave a luminous trail behind them, which 

 probably consists of severed portions of the red-hot surfaces. 

 Meteoric masses which fall to the earth often burst with a 

 violent explosion, which may be regarded as a result of the 

 quick heating. The newly-fallen pieces have been for the most 

 part found hot, but not red-hot, which is easily explainable by 

 the circumstance, that during the short time occupied by the 

 meteor in passing through the atmosphere, only a thin superficial 

 layer is heated to redness, while but a small quantity of heat 

 has been able to penetrate to the interior of the mass. For 

 this reason the red heat can speedily disappear. 



Thus has the falling of the meteoric stone, the minute rem- 

 nant of processes which seem to have played an important part 

 in the formation of the heavenly bodies, conducted us to the 



