€■ INTRODUCTION. 



planets which move in the vicinity of the sun. From the rep oils 

 of historians, as well as from the observations of late years, it has 

 been ascertained that more than 450 were seen previous to the year 

 $771 ; and when the attention of astronomers was called to this ob- 

 ject by the expectation of the return of the comet of 1759, no few- 

 er than seven were observed in the course of seven years. From 

 this circumstance, and the probability that all the comets recorded 

 in ancient authors were of considerable apparent magnitude, while 

 the smaller were overlooked, it is reasonable to conclude, that the 

 number of comets considerably exceeds any estimation that might 

 be made from the observations we now possess. But the number 

 of those whose orbits are settled with sufficient accuracy to ascer- 

 tain their identity when they may appear again, is no more than 

 fifty-nine, reckoning as late as the year 1771. The orbits of most 

 of these are inclined to the plane of the ecliptic in large angles, 

 and the greater number of them approached nearer to the sun 

 than to the earth. Their motions in the heavens are not all in the 

 order of the signs, or direct, like those of the planets ; but the 

 number whose motion is retrogade is nearly equal to that, of those 

 whose motion is direct. All which have been observed, however, 

 fiave moved through the aethereal regions and the orbits of the 

 planets, without suffering the least sensible resistance in their mo- 

 tions ; which sufficiently proves that the planets do not move in so- 

 lid orbs. Of all the comets, the periods of three only are known 

 with any degree of certainty, being found to return at intervals of 

 7S y 129, and 575 years; and of these, that which appeared in 1680 

 is the most remarkable. This comet, at its greatest distance, is 

 about 11,200 millions of miles from the sun, while its least distance 

 from the centre of the sun is about 490,000 miles ; being less than 

 one third part of the sun's semi-diameter from his surface. In that 

 part of its orbit which is nearest the sun, it flies with the amazing 

 velocity of 880,000 miles in an hour ; and the sun, as seen from it ? 

 appears 100 degrees in breadth,' consequently 40,000 times as large 

 as he appears to us. The astonishing distance that this comet 

 runs out into empty space naturally suggests to our imagination 

 the vast distance between our sun and the nearest of the fixed stars, 

 ©f whose attractions all the comets must keep clear, to return pe- 

 riodically and go round the sun. 



The fixed stars. ...Having thus briefly surveyed the solar sys- 

 tem, which, though great in itself, is small in comparison with the 

 immensity of the universe, we next proceed to the contemplation 

 of those other vast bodies called the Fixed Stars. These are dis- 

 tinguished by the naked eye from the planets, by being less bright 

 and luminous, and by continually exhibiting that appearance which 

 we call the twinkling of the stars. This arises from their being 

 so extremely small, that the interposition of the least body, of which 

 there are many constantly floating in the air, deprives us of the 

 sight of them ; when the interposed body changes its place, we 

 again see the star; and this succession, being perpetual, occasions 

 the twinkling. But a more remarkable property of the fixed stars, 

 and that from which they have obtained their name, is their never 

 changing their situation with regard to each other, as the planets, 

 from what we have already said, must evidently be always changing 

 theirs. The stars which are nearest to us seem largest, and are 

 therefore called stars, of the first magnitude. Those of the second 



