SECT, xxxvi.] ENCKE'S COMET. 393 



secrets of the yet more distant heavens may be disclosed to 

 future generations by comets which penetrate still farther 

 into space, such as that of 1763, which, if any faith may be 

 placed in the computation, goes nearly forty-three times 

 farther from the sun than Halley's does, and shows that the 

 sun's attraction is powerful enough, at the enormous distance 

 of 15,500 millions of miles, to recall the comet to its peri- 

 helion. The periods of some comets are said to be of many 

 thousand years, and even the average time of the revolution 

 of comets generally is about a thousand years ; which proves 

 that the sun's gravitating force extends very f^r. La Place 

 estimates that the solar attraction is felt throughout a sphere 

 whose radius is a hundred millions of times greater than the 

 distance of the earth from the sun. 



Authentic records of Halley's comet do not extend beyond 

 the year 1456, yet it may be traced, with some degree of 

 probability, even to a period preceding the Christian era. 

 But as the evidence only rests upon coincidences of its 

 periodic time, which may vary as much as eighteen months 

 from the disturbing action of the planets, its identity with 

 comets of such remote times must be regarded as extremely 

 doubtful. 



This is the first comet whose periodicity has been 

 established. It is also the first whose elements have been 

 determined from observations made in Europe ; for, although 

 the comets which appeared in the years 240, 539, 565, and 

 837, are the most ancient of those whose orbits have been 

 traced, their elements were computed from Chinese observa- 

 tions. 



Besides Halley's and Lexel's comets, ten others are now 

 known to form part of the solar system ; that is to say, they 

 return to the sun at stated periods. That generally called 

 Encke's comet, or the comet of the short period, was first 

 seen by MM. Messier and Mechain, in 1786, again by Miss 

 Herschel in 1805, and its returns, in the years 1805 and 

 1819, were observed by other astronomers, under the im- 



