338 EARTH NOT AFFECTED BY COMETS. SECT.XXXVI. 



the seasons ever been influenced by these bodies. The 

 light of the comet of the year 1811, which was so bril- 

 liant, did not impart any heat even when condensed on 

 the bulb of a thermometer, of a structure so delicate 

 that it would have made the hundredth part of a degree 

 evident. In all probability, the tails of comets may have 

 passed over the earth without its inhabitants being con- 

 scious of their presence ; and there is reason to believe 

 that the tail of the great comet of J1843 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 vapor, is so rare, and their transit 

 so rapid, that the time has not been long enough to ad- 

 mit of a sufficient accumulation of impetus to produce a 

 perceptible action. Indeed M. Dusejour has proved, 

 that under the most favorable 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 revolu- 

 tion by more than two days ; and if comets had any per- 

 ceptible 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 disturbing action would have increased the 

 length of the sidereal year by 2 1 ' 53 ; but as Delainbre'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 ^ l (T ^th part of that of the earth. 

 This accounts for the same comet having twice swept 

 through the system of Jupiter's satellites without de- 

 ranging the motion of these moons. M. Dusejour has 

 computed that a comet, equal in mass to the earth, pass- 

 ing at the distance of 12,150 leagues from our planet, 

 would increase the length of the year to 367 lt 16 h 5' n , 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 



