SECT. XXXVI. BIELA*S OR GAMBARTS COMET. 347 



comet, which revolves nearly between the orbits of fhe 

 earth and Jupiter, is only accelerated one day at each 

 revolution, while Encke's, revolving nearly between the 

 orbits of Mercury and Pallas, is accelerated two, the 

 ethereal medium must increase in density toward the 

 sun. The comet in question was discovered by M. 

 Biela at Johannisberg on the 27th of February, 1826, 

 and ten days afterward 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 perturbations 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 envelop a portion of the earth's orbit, a circum- 

 stance 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 An- 

 nuaire 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 ter- 

 restrial orbit sixty times in a century, and may ulti- 

 mately come into collision: but both are so extremely 

 rare, that little injury is to be apprehended. 



The earth would fall to the sun in 64i days, if it 

 were struck by a comet with sufficient impetus to de- 

 stroy its centrifugal force. What the earth's primitive 

 velocity may have been, it is impossible to say. There- 

 fore a comet may have given it a shock without changing 

 the axis of rotation, but only destroying part of its tan- 

 gential velocity, so as to diminish the size of the orbit a 

 thing by no means impossible, though highly improbable. 



