22 SHAPE OF THE EARTH. 



This may be illustrated by experiment. If upon a 

 rapidly revolving disc we place a ball of dust, it will be 

 almost immediately spread out, and its particles will 

 arrange themselves in a series of regular curves, varying 



ceases to be luminous. " The falling stars and other fiery meteors 

 which are frequently seen at a considerable height in the atmos- 

 phere, and which have received different names according to the 

 variety of their figure and size, arise from the fermentation of the 

 effluvia of acid and alkaline bodies which float in the atmosphere. 

 When the more subtile parts of the effluvia are burned away, the 

 viscous and earthy parts become too heavy for the air to support, 

 and by their gravity fall to the earth." Keith's Use of the Globes. 

 According to Sir Humphry Davy, in the Philosophical Transac- 

 tions for 1847, " the luminous appearances of shooting stars and 

 meteors cannot be owing to any inflammation of elastic fluids, but 

 must depend upon the ignition of solid bodies. 3 ' 



3. The body shines by the reflected light of the sun, and ceases 

 to be visible by its passing into the earth's shadow, or, in other 

 words, is eclipsed. Upon the two former suppositions the fact of 

 the star's disappearance conveys to us no knowledge of its position, 

 or of its distance from the earth; and all that can be said is, that 

 if it be a satellite of the earth, the great rapidity of its motion in- 

 volves the necessity of its being at no great distance from the earth's 

 surface much nearer than the moon ; while the resistance it 

 would encounter in traversing the air would be so great that it is 

 probably without the limits of our atmosphere. Sir J. W. Lub- 

 bock leans to the third hypothesis. Sir J. "W. Lubbock, On Shoot- 

 in ff Stars : Phil. Mag. No. 213, p. 81. 



Sir J. Lubbock also published a supplementary paper on the 

 same subject, in No. 21 -i, p. 170. 



^Ir. J. P. Joule entertains an hypothesis with respect to Shooting 

 Stars similar to that advocated by Chladni to account for meteoric 

 stones, and he reckons the ignition of these miniature planetary 

 bodies by their violent collision with our atmosphere, to be a re- 

 jnarkable illustration of the doctrine of the equivalency of heat to- 

 mechanical power, or vis viva. 



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 -^^ of its density at the earth's surface, 

 the resistance offered to the motion of the stone will in this case 

 be at least 51,600 Ibs. ; and if the stone traverse twenty miles with 

 this amount of resistance, sufficient heat will thereby be developed 

 to give 1 Fahrenheit to 0,907,980 Ibs. of water. Of course by far 



