396 BIELA'S OR GAMBART'S COMET. [SECT, 



27th of February, 1826, and ten days afterwards it was seen 

 by M. Gambart at Marseilles, who computed its parabolic 

 elements, and found that they agreed with those of the comets 

 which had appeared in the years 1789 and 1795, whence he 

 concluded them to be the same body moving in an ellipse, 

 and accomplishing its revolution in 2460 days. The pertur- 

 bations of this comet were computed by M. Damoiseau, who 

 predicted that it would cross the plane of the ecliptic on the 

 29th of October, 1832, a little before midnight, at a point 

 nearly 18,484 miles within the earth's orbit; and as M. 

 Olbers of Bremen, in 1805, had determined the radius of 

 the comet's head to be about 21,136 miles, it was evident 

 that its nebulosity would envelope a portion of the earth's 

 orbit, a circumstance which caused some alarm in France, 

 .from the notion that, if any disturbing cause had delayed 

 the arrival of the comet for one month, the earth must 

 have passed through its head. M. Arago dispelled these 

 fears by his excellent treatise on comets, in the Annu- 

 aire of 1832, where he proves that, as the earth would 

 never be nearer the comet than 18,000,000 British leagues, 

 there could be no danger of collision. The earth is in more 

 danger from these two small comets than from any other. 

 Encke's crosses the terrestrial orbit sixty times in a century, 

 and may ultimately come into collision, but both are so ex- 

 tremely rare, that little injury is to be apprehended. 



The earth would fall to the sun in 64|- days, if it were 

 struck by a comet with sufficient impetus to destroy its cen- 

 trifugal force. What the earth's primitive velocity may 

 have been, it is impossible to say. Therefore a comet may 

 have given it a shock without changing the axis of rotation, 

 but only destroying part of its tangential velocity, so as to 

 diminish the size of the orbit, a thing by no means impos- 

 sible, though highly improbable. At all events, there is no 

 proof of this having occurred ; and it is manifest that the 

 axis of the earth's rotation has not been changed, because, 

 as the ether offers no sensible resistance to so dense a body 



