14 PHILOSOPHICAL TRANSACTIONS. ^ANNO 1703. 



the moon, which if used in practice will determine her place nicely enough, 

 even in the quadratures, as has been found by comparing it with Mr. Flamsteed's 

 observations; where when the error is greatest it exceeds not two minutes, but 

 for the most part it is so small, that it may be justly enough imputed to the 

 uncertainty of observations. After this he treats more particularly of the 

 eclipses of both sun and moon, and the manner of calculating them ; he gives 

 an account of the moon's motion round her axis, and shows how all those vari- 

 ous nutations and librations of the moon, that have hitherto so much puzzled 

 the philosophers, do only arise from her menstruous uniform rotations round 

 her axis. From thence he passes to the ways of determining the magnitudes, 

 densities, and quantities of matter in the secondary planets, and the change of 

 figure, both in the primaries and them; which must necessarily arise, if they 

 are fluid, from their mutual gravitation towards each other ; from which he ex- 

 plains the tides and the motion of the seas. He ends the book with a descrip- 

 tion of the ring of Saturn, and accounts for all its various appearances seen out 

 of the earth. 



In the 5th book the author treats of comets; where, after having given the 

 several opinions of philosophers about their place, duration, origin, orbits and 

 tails; he tells us, that certainly they are bodies that move round the sun, and 

 differ from the other planets in that their orbits are very oblong ellipses, where- 

 as the planets describe ellipses which come very near to circles; and from this 

 hypothesis it is easy to give an account of the most of their phaenomena; parti- 

 culary he shows, that when a comet comes down from its aphelion, or when it 

 ascends to it, its trajectory will seem to be a right line, and may be assumed as 

 such, when its motions are to be determined without great niceness ; and on 

 this account it is, that Kepler, and several others of the astronomers, did really 

 suppose that they went on always in straight lines, without ever returning. 

 Their tails he takes to be vapours, which being heated by the sun, arise copi- 

 ously from their bodies when they descend towards him, and are carried up- 

 wards by the air in which they swim; where he takes notice, that if this matter 

 of the tail (which is spread at a great distance from the comet itself) should 

 touch and mix with our atmosphere, it may infect it with qualities pernicious 

 to the temperament of our vegatables and animals; and from thence may 

 arise all those effects which, by long observation and common consent of 

 mankind, have been attributed to comets, notwithstanding their being ri- 

 diculed by some modern philosophers. From this he passes to show the 

 method by which the orbits of the comets are to be determined. The ways 

 he takes are not only excellent in that case, but likewise of great use in 

 iolving other intricate and difficult problems in natural philosophy. And then 



