350 Mr. J. P. Joule on Shooting Stars. 



dark sky of night, pursue a short and rapid course, burst, and 

 are dissipated in shining fragments. From the velocity with 

 which these bodies travel, there can be little doubt that they 

 are small planets which, in the course of their revolution round 

 the sun, are attracted and drawn to the earth. Reflect for a 

 moment on the consequences which would ensue, if a hard 

 meteoric stone were to strike the room in which we are assem- 

 bled with a velocity sixty times as great as that of a cannon- 

 ball. The dire effects of such a collision are effectually pre- 

 vented by the atmosphere surrounding our globe, by which 

 the velocity of the meteoric stone is checked, and its living 

 force converted into heat, which at hist becomes so intense as 

 to melt the body and dissipate it in fragments too small pro- 

 bably to be noticed in their fall to the ground. Hence it is, 

 that although multitudes of shooting stars appear every night, 

 few meteoric stones have been found, those few corroborating 

 the truth of our hypothesis by the marks of intense heat which 

 they bear on their surfaces*." 



The likelihood of the above hypothesis will be rendered 

 evident, if we suppose a meteoric stone, of the size of a six- 

 inch cube, to enter our atmosphere at the rate of eighteen 

 miles per second of time, the atmosphere being ji^dth of its 

 densily at the earth's surface. The resistance offered to the 

 motion of the stone will in this case be at least 51,600 lbs.; 

 and if the stone traverse twenty miles with this amount of 

 resistance, sufficient heat will thereby be developed to give 1° 

 Fahrenheit to 6,967,980 lbs. of water. Of course by far the 

 largest portion of this heat will be given to the displaced air, 

 every particle of which will sustain the shock, whilst only the 

 surface of the stone wUl be in violent collision with the atmo- 

 sphere. Hence the stone may be considered as placed in a 

 blast of intensely healed air, the heat being communicated 

 from the surface to die centre by conduction. Only a small 

 portion of the heat evolved will therefore be received by the 

 stone; but if we estimate it at only yi^jdth, it will still be 

 ecjual to 1° Fahrenheit per 69,679 lbs. of water, a quantity 

 quite equal to the melting and dissipation of any materials of 

 which it may be composed. 



The dissolution of the stone will also be accelerated in most 

 cases by its breaking into pieces, in consequence of the un- 

 equal resistance experienced by different parts of its surface, 

 especially after its cohesion has been partially overcome by 

 heat. 



It appears to me that the varied phaenomena of meteoric 

 stones and shooting stars may all be explained in the above 

 * Mancliester Courier newspaper, May 12, 1847. 



