ASTRONOMY. 



therefore be considered as members of the solar 

 system. The others are only casual visitors ; 

 unless they meet with something to alter their 

 orbits, they can never return, but must run off 

 into the immensity of space. 



The most remarkable of the comets ascertained 

 to return is one usually denominated H alley's 

 Comet, from the astronomer who first calculated 

 its period. It revolves round the sun in about 

 seventy-seven years, its last appearance being at 

 the close of 1835. The annexed cut represents 



some of the various appearances it presented on 

 that occasion in different parts of its orbit a, 6, 

 c, in approaching the sun ; d, e, in retreating. 



Another, called Encke's Comet, from Professor 

 Encke of Berlin, has been found to revolve once 

 | n 3i years ; but in this case the revolving body 

 is found, at each successive approach to the sun, 

 to be a little earlier than on the previous occasion, 

 owing probably to a cause to be afterwards de- 

 scribed. A third, named Biela's Comet, revolves 

 round the sun in 6 years. It is very small, and 



has no tail. During its visit in 1846, this comet 

 was seen to separate into two distinct comets, 

 which kept moving side by side, till they disap- 

 peared. On the return of the comet in the autumn 

 of 1852, the distance between the two nuclei had 

 much increased. In 1770, a comet got entangled 

 amidst the satellites of Jupiter, and was thereby 

 thrown out of its usual course, while the motions 

 of the satellites were not in the least affected by 

 its proximity. This proves the extreme lightness 

 of the matter composing comets. 



The comet now called H alley's, at its appear- 

 ance in 1456, covered a sixth part of the visible 

 extent of the heavens, and was likened to a Turkish 

 scimitar. That of 1680, which was observed by 

 Sir Isaac Newton, had a tail calculated to be 

 60,000,000 miles in length a space two-thirds of 

 the distance of the earth from the sun. There 

 was a comet in 1744 which had six tails, spread 

 out like a fan across a large space in the heavens. 



PHYSICAL ASTRONOMY. 



Not long after Kepler had made those remark- 

 able discoveries which completed the view of the 

 regular courses and periods of the celestial 

 motions, the causes of the motions, or the forces 

 whereby they are sustained, were also discovered 

 by Sir Isaac Newton. He was the first to shew 

 that the vast planetary balls whirl about their own 

 centres, and fly through the celestial spaces, on 

 exactly the same principles as a cannon-ball or 

 a stone moves when thrown into the air. The 

 principles of celestial mechanics, therefore, are 

 the principles of motion discovered from the 

 observation of bodies on the earth ; they are 

 the three Laws of Motion and the doctrines of 

 the Composition of Forces, as illustrated under 

 MATTER AND MOTION, and the doctrine of 

 Universal Gravitation. 



The fall of unsupported bodies to the earth is 

 the most familiar action in nature ; but it is not 

 two centuries since Newton discovered that this 

 action extends to the moon, the sun, and the 

 planets. He proved that the moon is constantly 



falling towards the earth, and would fall into it, 

 but for another motion she has, which is always 

 carrying her off, and would of itself shoot her far 

 away into space in a straight course ; so that her 

 actual circuit is the balance of two forces one her 

 weight, or gravity, towards the earth (centripetal 

 force), the other an undying impulse to fly off at 

 a tangent, like a whirled stone when the sling is 

 let go (centrifugal force). 



It was through the application partly of the laws 

 of motion, and partly of Kepler's laws, that Newton 

 established the universal prevalence of gravity. It 

 being once found that the planetary motions could 

 be kept up by a combination of forces ; that is, by 

 a force that projected the body once for all into 

 free space with a great velocity, which, by the first 

 law of motion, would be always kept up, whether 

 it went off straight through space, or were com- 

 pelled to go round a circle and some second 

 force to hold it in that circle ; the great question 

 arose : What is the cen- 

 tral force what power 

 is it that causes the 

 moon to fall towards 

 the earth, instead of 



running off; and in like \ ^ / \A 



manner obliges the 

 planets, with their im- 

 mense speed, to keep 

 constantly falling to- 

 wards the sun? A planet P, if not held in by 

 some tie, would fly off along PA, instead of being 

 always carried round the sun S. 



Newton proved by mathematical reasoning, from 

 the first law of Kepler, that the deflecting force 

 points exactly to the sun, and is therefore likely 

 to be lodged in his body. From Kepler's second 

 and third laws, he proved that the force is in- 

 versely as the square of the distance ; and, finally, 

 he shewed that the deflection of the moon from a 

 straight line is exactly equal to the fall of a stone, 

 if it were at the distance of the moon. The moon 

 being sixty times farther off from the earth's 

 centre than we are, gravity is there 3600 times 

 weaker ; so that the speed acquired by a stone 

 falling one second near the earth's surface, would 



