1 82 THE HAMBURG ACCOUNT OF WRIGHT'S THEORY. 



at the same time, that many of the modern discoveries 

 [of Sir Isaac Newton], "such as relate particularly to 

 the Planetary System, are but as so many confirmations 

 of the conjectures and imaginations" of the ancients, 

 who, as they were not able to demonstrate these things 

 mathematically, employed merely an analogical way of 

 judging, and they left the establishment of their views to 

 posterity. That the reader may be able to form an idea 

 of the astronomy of the ancients, he shows how far they 

 carried their speculations concerning the visible creation, 

 and with this he concludes the second letter. 



THE THIRD LETTER treats : " Concerning the nature, 

 magnitude, and motion of the planetary bodies round the 

 Sun, etc" In this letter Mr. Wright gives a brief sketch 

 of the astronomy of Copernicus, and explains the magni- 

 tudes, distances, motions, and revolutions of the planets 

 and comets. Concerning the latter he appears to have 

 made a very happy remark, for, in so far as it should be 

 found true, it would contribute very much to complete the 

 theory of the comets. He says, " I am strongly of opinion 

 that the comets in general, through all their respective 

 orbits, describe one common area, that is to say, all their 

 orbits, with regard to the magnitude of their proper planes, 

 are mathematically equal to one another ; which, if it once 

 could be proved, and confirmed by observation, the theories 

 of all the comets that have been justly observed, might 

 easily be perfected, and their periods at once determined, 

 which now we can only guess at, or may wait whole ages 

 for more certainty of. What leads me to believe that this 

 may prove to be really the case is this. I find by calcu- 

 lation that the orbits of the two last comets, whose elements 

 have been most corrected by Sir Isaac Newton and Dr. 

 Halley, are to one another according to their numbers 

 nearly as 13: 17, notwithstanding one of them is one of 

 the most erratic that ever came under our observation ; 

 and the other one of the most neighbouring to the sun. 

 But it is well known to all mathematicians that the 

 first of these comets moved in so eccentric a trajectory, 

 that the least error in its almost incredible proximity to the 

 sun will produce a very sensible difference in the area of 

 the orbit. And accordingly, if we moderate the Perihelion 

 distance of this comet by making it but 1000 instead of 612, 



