Diftance, Magnitude, &c. of the Fixed Stars, &c. 34. 
ing body being given, the denfity of the central body will be 
given likewife. See Sir Isaac Newron’s Prin. b. HI. pr. viit. 
cor. I. 
3. But the denfity of any central body being given, and the 
velocity any other body would acquire by falling towards it 
from an infinite height, or, which is the fame thing, the velo- 
city of a comet revolving in a parabolic orbit, at its furface, 
being given, the quantity of matter, and confequently the 
real magnitude of the central body, would be given likewife. 
4. Let us now fuppofe the particles of light to be attracted 
in the fame manner as all other bodies with which we are ac- 
quainted; that is, by forces bearing the fame proportion to 
their ws anertie, of which there can be no reafonable doubt, 
gravitation being, as far as we know, or have any reafon to. 
believe, an univerfal law of nature. Upon this fuppofition 
then, if any one of the fixed ftars, whofe denfity was known 
by the above-mentioned means, fhould be large enough fenfibly 
to affect the velocity of the light iffuing from it, we fhould 
have the means of knowing its real magnitude, &c. 
5. It has been demonftrated by Sir Isaac Newron, in the 
39th propofition of the firft book of his Principia, that if a 
right line be drawn, in the direétion of which a body is urged 
by any forces whatfoever, and there be erected at right angles 
to that line perpendiculars every where proportional to the. 
forces at the points, at which they are erected refpectively, the 
velocity acquired by a body beginning to move from reft, in; 
confequence of being fo urged, will always be proportional to 
the fquare root of the area defcribed by the aforefaid perpendi-.- 
culars. And hence, 
6. If fuch a body, inftead of beginning to move from reft,. 
had already fome velocity in the direction of the fame line, . 
when, 
