466 Scientific Intelligence. [June 



do consistently with the laws of nature. The orbits of some of them are 

 extremely elliptical, of others almost circular, so that they follow no 

 constant law. They have no preferable plane in which they move, 

 some moving at right angles, others not. In short, they are a kind 

 of physical vagabonds. Some of them move direct, or in the direc- 

 tion of the planets, others retrogade ; 68 move one way, 69 the 

 other. They possess no characters such as the planets possess, as the 

 satellites of Jupiter, the belts of Saturn, &c, by which we can iden- 

 tify them, for they are surrounded by a mass of vapour, and are 

 therefore, seen by us indistinctly ; sometimes their tails grow longer, 

 sometimes shorter. The comets can only be seen at the focus of their 

 orbits or at that point where they are not too far from the earth, and 

 not too near the sun. But as this position is the very point where 

 curves of an ellipse, parabola or hyperbola, correspond, we must have 

 recourse to some other method than a single observation, to determine 

 their orbit. This is done by their periodicity. If they return perio- 

 dically, we are sure that their orbit is an ellipse ; if it is a parabola 

 or hyperbola, they will shoot off into space and never appear again. 

 Now, it is easy to calculate the degree of ellipticity of the orbit, 

 because Newton has demonstrated in his Principia, that the squares 

 of the periodic times are to each other as the cubes of the distance of 

 the respective planets from the sun. Knowing, therefore, the pe- 

 riodic times, the ellipticity and size of the orbit can readily be deduced. 

 To this point, therefore, Newton brought the question — He said, 

 <( the orbit of comets is an ellipse, but I have not time to determine 

 the axis : I leave this to succeeding astronomers." Halley took it up 

 in 1 700, where Newton left off. Before his time, 425 comets had 

 appeared, but 24 only had been observed, the rest were only seen 

 From the observations which had been made upon these 24 comets, 

 he calculated their courses. He found the elements of one which 

 appeared in 1607, to agree with one he had observed in 1682, and 

 on examining other observations, he found, that the following table 

 could be formed in reference to one comet. 



He ascribed the differences in the periodic times to the attraction 

 of Jupiter and Saturn. He guessed this as if by instinct, for he 

 really had not at the time the philosophic means of determining it. 

 In 1757, Lalande proposed to Clairaut, the calculation of Halley's 

 comet which was expected to return speedily. They were assisted 

 by a French lady, the wife of a chronometer maker. The calcula- 

 tion was enormous, because the ^orbit must be divided into degrees, 

 and each degree requires as great a calculation as the whole orbit. 

 They tell us, that they were employed from morning to night, not 

 excepting meal hours, incessantly for six months in this computation. 



