SECT. XXXV. PATHS OF COMETS. 359 



comet was eclipsed as it passed over a star of the 7th magnitude, 

 whence M. Babinet computed that the light of the comet must 

 have been sixty times less than that of the star, and that 

 matter so attenuated could not penetrate the earth's atmosphere, 

 but the constitution of these bodies is still a matter of conjecture. 

 The passage of comets has never sensibly disturbed the stability 

 of the solar system ; their nucleus, being in general only a mass of 

 vapour, is so rare, and their transit so rapid, even when they had 

 a solid part, that the time has not been long enough to admit of a 

 sufficient accumulation of impetus to produce a perceptible action. 

 Indeed, M. Dusejour has shown that, under the most favourable 

 circumstances, a comet cannot remain longer than two hours and 

 a half at a less distance from the earth than 10,500 leagues. The 

 comet of 1770 passed within about six times the distance of the 

 moon from the earth, without even affecting our tides. According 

 to La Place, the action of the earth on the comet of 1770 aug- 

 mented the period of its revolution by more than two days ; and, 

 if comets had any perceptible disturbing energy, the reaction of 

 the comet ought to have increased the length of our year. Had the 

 mass of that comet been equal to the mass of the earth, its disturb- 

 ing action would have increased the length of the sidereal year by 

 2 h 53 m ; but, as Delambre's computations from the Greenwich 

 observations of the sun show that the length of the year has not- 

 been increased by the fraction of a second, its mass could not 

 have been equal to the 5Moth part of that of the earth. This 

 accounts for the same comet having twice swept through the 

 system of Jupiter's satellites without deranging the motion of 

 these moons. M. Dusejour has computed that a comet, equal in 

 mass to the earth, passing at the distance of 12,150 leagues from 

 our planet, would increase the length of the year to 367 d 16 h 5 m , 

 and the obliquity of the ecliptic as much as 2. So the principal 

 action of comets would be to alter the calendar, even if they were 

 dense enough to affect the earth. 



Comets traverse all parts of the heavens ; their paths have 

 every possible inclination to the plane of the ecliptic, and, unlike 

 the planets, the motion of more than half of those that have 

 appeared has been retrograde, that is, from east to west. They 

 are only visible when near their perihelia ; then their velocity is 

 such, that its square is twice as great as that of a body moving 

 in: a circle at the same distance : they consequently remain but a 



