386 EARTH NOT AFFECTED BY COMETS. [SECT, xxxvi. 



thermometer of a structure so delicate that it would have 

 made the hundredth part of a degree evident. In all pro- 

 bability, the tails of comets may have passed over the earth 

 without its inhabitants being conscious of their presence; 

 and there is reason to believe that the tail of the great 

 comet of 1843 did so. 



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, 

 that the time has not been long enough to admit of a suffi- 

 cient accumulation of impetus to produce a perceptible action. 

 Indeed, M. Dusejour has proved that, under the most favour- 

 able 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 augmented the period of its revolution by 

 more than two days ; and, if comets had any perceptible dis- 

 turbing energy, the reaction of the comet ought to have in- 

 creased the length of our year. Had the mass of that comet 

 been equal to the mass of the earth, its disturbing 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 in- 

 creased by the fraction of a second, its mass could not have 

 been equal to the ^th part of that of the earth. This ac- 

 counts 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 



