ASTRONOMY. 245 



they would not do much harm if they were to impinge ; and even then 

 the mischief would probably be local, and the equilibrium would be soon 

 restored, provided the nucleus were gaseous, or very small. It is, however, 

 more probable that the earth would only be deflected a little from its course 

 by the approach of a comet, without beingtouched by it. The comets that 

 have come nearest to the earth were that of the year 837, which remained 

 four days within less than one million two hundred and forty thousand 

 leagues from our orbit ; that of 1770, which approached within about 

 six times the distance of the moon. The celebrated comet of 1680 also 

 came very near to us ; and the comet whose period is six years and three 

 quarters, was ten times nearer the earth in 1805 than in i832, when it 

 caused so much alarm. Comets in or near their perihelion move with 

 prodigious velocity. That of 1 680 appears to have gone half round the 

 sun in ten hours and a half, moving at the rate of eight hundred and 

 eighty thousand miles an hour. If its enormous centrifugal force had 

 ceased when passing its perihelion, it would have fallen to the sun in 

 about three minutes, as it was then only one hundred and forty-seven 

 thousand miles from his surface. A body of such tenuity as the comet, 

 moving with such velocity, must have met with great resistance from the 

 dense atmosphere of the sun, while passing so near his surface at its 

 perihelion. The centrifugal force must consequently have been 

 diminished, and the sun's attraction proportionally augmented, so that 

 it must have come nearer to the sun in 1680 than in its preceding 

 revolution, and would subsequently describe a smaller orbit. As this 

 diminution of its orbit will be repeated at each revolution, the comet 

 will infallibly end by falling on the surface of the sun, unless its course 

 be changed by the disturbing influence of some large body in the 

 unknown expanse of creation. Our ignorance of the actual density of 

 the sun's atmosphere, of the density of the comet, and of the period of its 

 revolution, renders it impossible to form any idea of the number of 

 centuries which must elapse before this event takes place. But this is 

 not the only comet threatened with such a catastrophe ; Encke's, and that 

 discovered by M. Biela, are both slowly tending to the same fate. By 

 the resistance of the other, they will perform each revolution nearer and 

 nearer to the sun, till at last they will be precipitated on his surface. 

 The same cause may effect the motion of the planets, and ultimately be 

 the means of destroying the solar system. But, as Sir John Herschel 

 observes, they could hardly all revolve in the same direction round the 

 sun for so many ages without impressing a corresponding motion on the 

 etherial fluid, which may preserve them from the accumulated effects of 



