74 .PHILOSOPHICAL TRANSACTIONS. [aNNO 1770. 



very nearly. Therefore the utmost probable amount of one emission is ^jVft of 

 the maximum, be the figure of the corpuscles what it will. And the sun may 

 lose the quantity of a whole emission 2084 times in a second without any sen- 

 sible consequences. 



I cannot apprehend, from any quarter, so unphilosophical an objection, as that 

 the extreme minuteness of the particles of light, which I have shown to be ne- 

 cessary, if light be really matter, is an argument against their existence. Size is 

 a mere accident, and no part of the essence of any being. Great and small, ap- 

 plied to finite things, are purely terms of comparison. One Being only is abso- 

 lutely Great: he whose substance pervades and fills the whole and every part of 

 absolute space; because, in respect of him, all things that are are little. 



Notwithstanding the maximum of moving force, in each particle of light, is 

 so inconsiderable as I have shown it to be, yet the number of particles, out of 

 each emission, which are directed towards the earth, and fall upon its enlight- 

 ened hemisphere, being exceedingly great, it may perhaps be imagined, that the 

 force impressed by them all upon the earth, if they all actually strike its surface, 

 may amount to something worth attending to. I have taken the pains to satisfy 

 myself on this question, and shall briefly mention the result of my computations. 



Reckoning every emission at its maximum, I find that the number of the 

 particles out of each, which should fall on the earth's enlightened hemisphere, is 

 492023XXXI, or that of which the logarithm is 36.6919854. The moving forces 

 of this whole number, amount to as much as the force of an iron ball, of one 

 yard diameter, flying 68 miles and 887 yards in one second. But the progressive 

 force, which they might communicate to the earth, does not exceed that of an 

 iron ball of the same size, flying 34 miles and 443 yards in one second.* And, 

 if this whole force be transferred from the iron ball to the globe of the earth, 



• Only one particle out of each emission from any very small given part of the sun's surface, can 

 strike the earth's surface perpendicularly. The force of every particle, which impinges obliquely, is 

 resolvable into two, one perpendicular to the surface, and the other parallel to it. That part of the 

 force, which is perpendicular to the earth's surface, produces a progressive motion of the whole 

 globe. The other, which is parallel to the surface, contributes nothing to the progressive motion, 

 but tends to produce a rotation of the globe on an axis. Hence the progressive force of motion, com- 

 municated to the globe, is less than it would be, if all the particles struck the surface perpendicu- 

 larly. It is further lessened on another account, namely, that the whole of the perpendicular force 

 is not effective in moving the earth's centre. I find by computation, that the diminution on the 

 whole is J. For the ease of the mathematical reader, I shall briefly state the principles on which 

 this computation of the force with which the earth may be struck by light, has been framed. 



The number of particles, which are directed to the earth out of each emission, is to half the num- 

 ber of the whole emission, in the duplicate proportion of the chord of the sun's horizontal parallax, 

 to the chord of 90°, by the known doctrine of Archimedes. Hence the number of the whole emis- 

 sion being determined, the number of those which tend towards the earth, is given. And the force 

 ©f each single particle being given, the sura of the forces of that given number is given; and this w 



