344 HALLEY'S COMET. SECT. XXXVI. 



which he had witnessed in this comet at its approach to 

 the sun in the year 1682, and something of the kind 

 seems to have been noticed in the comet of 1744. Pos- 

 sibly the second tail of the comet of 1724, which was 

 directed toward the sun, may have been of this nature. 



The influence of the ethereal medium on the motions 

 of Halley's comet, will be known after another revolu- 

 tion, and future astronomers 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 genera- 

 tions 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 perihelion. 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 ex- 

 tends very far. La Place estimates that the solar at- 

 traction 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 be- 

 yond the year 1456, yet it may be traced, with some 

 degree of probability, even to a period preceding the 

 Christian em. 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 



