r" 

 402 PERIODIC COMETS. 



bring it to the earth. Hence it will be understood how a person is thrown 

 forward when his horse, in falling down, suddenly stops ; in what manner 

 travellers seated on the imperial of a steam-carriage, moving with great ve- 

 locity over an iron railroad, are launched into the air like so many pro- 

 jectiles when an accident instantaneously stops the motion of these ingenious 

 contrivances. But is our earth anything else than a carriage, which, in its 

 progress through regions of space, requires neither wheels nor railways ? All 

 we said, therefore, is directly applicable to it. 



Our velocity round the sun is about twenty miles per second. If a comet 

 of a sufficient mass in meeting the globe should, by a single shock, instanta- 

 neously stop its motion, the bodies placed upon its surface, such as animated 

 beings, our carriages, furniture, utensils, all objects in short not implanted di- 

 rectly or indirectly in the soil, would fly off to the point of the earth shocked 

 by the comet with the velocity with which they were in common originally 

 endued a velocity of twenty miles per second. The effects of such an event 

 may be better conceived if I here remark that a twenty-four-pound shot has 

 not even on its discharge from the gun a velocity of more than twelve hun- 

 dred feet per second. All animated nature would certainly be destroyed in an 

 instant. 



As for the waters of the ocean since they are moveable as nothing fastens 

 them to the solid portion of the earth they would also be projected in mass 

 toward the point of percussion. This terrific liquid mass would in its im- 

 petuous course overthrow every obstacle in its way. It would pass the 

 summits of the highest mountains, and in its reflux would produce ravages 

 scarcely less tremendous. The disorder which is occasionally observed in 

 the strata of the different sorts of earth forming the crust of the globe is, it may 

 be said, but a microscopic accident compared with the frightful chaos that 

 would inevitably occur on a shock of a comet sufficiently powerful to stop the 

 earth. 



It is only necessary to diminish in some degree these prodigious effects to 

 find what results would be experienced from the shock of a comet, which, 

 without stopping our globe, should sensibly decrease its velocity. Certain it 

 is, however, that the globe has never been stopped completely ; for in such 

 case, the central force not being counterbalanced, it must have fallen in a di- 

 rect line toward the sun, where it would have arrived sixty-four and a half 

 days after the shock. 



The velocity of the earth and the magnitude of its orbit are so nearly con- < 

 nected, that one cannot change without at the same time producing a variation ; 

 in the other. It is unknown whether the dimensions of the orbit have remained ( 

 constant ; nothing, then, proves that the velocity of the globe in the course of \ 

 ages has not been more or less altered by a cometary concussion. At all \ 

 events, it is incontestable that the inundations which would be produced by 

 such an event do not explain the effects which the variations of the earth has 

 undergone, now so well described by geologists. 



A few words, again, before quitting this subject, on the consequences of ( 

 cometary shock as respects its influence on the rotary movement of the earth. > 



The earth turns upon itself in twenty-hours from the west to the east. The < 

 axis of rotation is called the axis of the world ; its extremities, the poles ; and | 

 the circle, equally distant from the two poles, the equator. The circle of the 

 equator is about 25,000 miles in circumference. 



Twenty-five thousand miles are in consequence the space through which a 

 point on the equatorial region, solid or fluid, passes every twenty-four hours by ^ 

 the rotation of the globe. An observer situated above the earth and its atmo- : 

 sphere, would not be drawn into this movement, but would see all the parts of , 



