'^24 ASTRONOMY. 



rays of which it is composed ; which spissitude, supposing 

 the rays to issue in right lines on all sides from a point, will 

 be reciprocally as the square of the distance.* The mathe- 

 matics of this solution we do not call in question ; the 

 question with us is, whether there be any sufficient reason 

 to believe, that attraction is produced by an emanation. 

 For my part, I am totally at a loss to comprehend, how 

 particles streaming from a centre, should draw a body to- 

 wards it. The impulse, if impulse it be, is all the other 

 way. Nor shall we find less difficulty in conceiving, a 

 conflux of particles, incessantly flowing to a centre, and 

 carrying down all bodies along with it, that centre also it- 

 self being in a state of rapid motion through absolute 

 space ; for, by what source is the stream fed, or what be- 

 comes of the accumulation ? Add to which, that it seems 

 to imply a contrariety of properties, to suppose an ethereal 

 fluid to act, but not to resist ; powerful enough to carry 

 down bodies with great force towards a centre, yet, in- 

 consistently with the nature of inert matter, powerless and 

 perfectly yielding with respect to the motions which result 

 from the projectile impulse. By calculations drawn from 

 ancient notices of eclipses of the moon, we can prove, that 

 if such a fluid exist at all, its resistance has had no sensi- 

 ble effect upon the moon's motion for two thousand five 

 hundred years. The truth is, except this one circumstance 

 of the variation of the attracting force at different distances 

 agreeing with the variation of the spissitude, there is no 

 reason vvhatever to support the hypothesis of an emanation ; 

 and, as it seems to me, almost insuperable reasons against 

 it. 



* Let the liajht of a candle fall upon a square object like A B C D, 

 Fig. 4, Plate XXXIX, and if a screen be placed parallel to the object, 

 and at double the distance, the shadow E F G H, received upon it, will 

 be four times the size of the object itself. For the rays passing in 

 straight lines by the angles A, B, C, D, the sides E F, F G, G H, H E, 

 must be each double of A B, B C, CD, DA: therefore, the shadow 

 may be divided into four squares, each equal in size to the object. At 

 three times the distance from the candle, the sides of the shadow- 

 would each be three times as large as the sides of the object, and its 

 area would, therefore, contain nine times the space. For the same 

 reason if the distance be increased four, five, or six times, the area of 

 the shadow will contain sixteen, twenty-five, or thirty-six squares, 

 each equal to the object. Now the quantity of light which falls upon 

 the object would, if it had not been intercepted, have spread over that 

 part of the screen which is occupied by the shadow ; and as the sur- 

 face is increased, over which a certain quantity of rays is spread, in the 

 same ratio their spissitude or density will be diminished ; consequently 

 this spissitude will be reciprocally as the squares of the distances. 



Paxton. 



