SECT. XXXV. EXCKE'S COMET. 365 



The influence of the ethereal medium on the motions of Halley's 

 comet will be known after another revolution, and future as-r 

 tronomers will learn, by the accuracy of its returns, whether it 

 has met with any unknown cause of disturbance in its distant 

 journey. Undiscovered planets, beyond the visible boundary of 

 our system, may change its path and the period of its revolution, 

 and thus may indirectly reveal to us their existence, and even 

 their physical nature and orbit. The 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 maybe 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 enor- 

 mous distance of 15,500 millions of miles, to recall the comet 

 to its perihelion. The periods of some comets are said to be of 

 many thousand years, and even the average time of the revolu- 

 tion of comets generally is about a thousand years ; which proves 

 that the sun's gravitating force extends very far. La Place 

 estimates that the solar attraction is felt throughout a sphere 

 whose radius is a hundred millions of times greater than the dis- 

 tance 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 evi- 

 dence 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 an- 

 cient of those whose orbits have been traced, their elements were 

 computed from Chinese observations. 



Besides Halley's and Lexel's comets, ten or twelve others are 

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

 return to the sun at stated periods. Six of them have periods of 

 less than eight years. 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, 



