66 PHILOSOPHICAL TRANSACTIONS. [aNNO 1770. 



of the results, which I shall here briefly state, as data for the discussion of Dr. 

 Franklin's questions. 



I suppose the particles of light to be equal spherules. This perhaps is not the 

 case. Each color has probably its own size; but there will be a mean size, 

 which is sufficient for my purpose. This mean size I suppose to be so small, 

 thai the diameter of each spherule does not exceed one millionth of one mil- 

 lionth of an inch. I shall show hereafter, that there is much reason to suppose, 

 that the particles of light are in fact much less than spherules of this diameter. 

 I suppose the density of each particle 3 times that of iron.* The number of 

 such spherules that contain as much matter as an iron ball of one yard diameter, 

 I have found to be 15552 xxxvi ;•!-:}: consequently 576 xxxvi such spherules con- 

 tain as much matter as an iron ball of 1 foot diameter. Let such a ball be sup- 

 posed to move uniformly with a velocity that should carry it 1000 yards in I". 

 The light of the sun traverses the semidiameter of the orbis magnus in 7"" nearly. 



In the ensuing calculations, I shall reckon the sun's horizontal parallax Q". 

 According to this hypothesis, the semidiameter of the orbis magnus will contain 

 22919 semidiameters of the earth. In V of time, light, according to the velo- 

 city assigned to it, traverses ^-j-g- of this space, or 54.57 semidiameters of the 

 earth, or 381092323 London yards. Hence the velocity of each particle of 

 light, will be to that of the iron ball moving 1000 yards in 1% as 3810g2 to 1, 

 very nearly. And the ball being 1 foot diameter, it has been shown that the 

 matter in each particle is to the matter in the ball, as I to 576 xxxvi. Hence 

 the force of the motion in each particle of light, is to the force of motion in the 

 ball, as 38 1092 to 576 xxxvi; that is, as 1 to 1511444 xxvii, or, it is equal to 



the force of motion in an iron ball of 1 foot diameter moving rrm-r, ^ of 1000 



° 1511444 xxvu 



yards in 1% or to that of an iron ball of'l inch diameter moving ,. ^^. of 1 



yard in 1% or to that of an iron ball of 4- of an inch diameter moving 



,.^^ — : of 1 yard in 1'; that is, moving less than — — — r of 1 foot in 1*; that 



loOOO XXI 4?0DO XXI 



is, moving less than a foot in 4555xxi seconds, or in more than one hundred 

 forty-four thousand millions of millions of Egyptian years ; or the force of mo- 

 tion in each particle of light coming from the sun, is less than that in an iron 

 ball of ^ of an inch diameter, moving at the rate of less than an inch in 12 

 thousand millions of millions of Egyptian years. 



Dr. Franklin's first question is answered. A particle of matter, which is pro- 

 bably larger than any particle of light, moving with the velocity of light, has a 



• Instantaneous product, &c. p. 30. 

 + Ibid. 



J The Roman numerals placed after the Arabic characters, denote the number of zeros that must 

 be added to the Arabic iigvires, to complete the expression of the number intei^ded. — Orig. 



