76 PHILOSOPHICAL TRANSACTIONS. [aNNO 1770. 



of an inch in IOC. This is the utmost effect of the force impressed on the earth 

 by each emission. If the emissions were incessant, this might be considered as 

 a central force, counteracting the sun's attraction ; for its tendency is to push 

 the earth directly from the sun. I need not say, that it is infinitely too small, 

 in comparison of the sun's attraction, to produce any sensible effect. 



The rotatory forces mentioned in the last note, if they were infinitely greater 

 than they really are, would not in the least degree disturb the diurnal rotation; 

 because every one of them is destroyed, by an equal one, in a contrary direction, 

 on the other side of, and at an equal distance from, the perpendicular ray. 



I have inquired, what may be the utmost stroke, which the retina of a com- 

 mon eye sustains, when the eye, in a bright day, is turned up directly to the sun. 

 This force will evidently be at its maximum, if the emission be reckoned at its 

 maximum. The number of particles which enter an eye, looking up directly at 

 the sun, are to the number out of each emission which are directed towards the 

 earth, in the duplicate proportion of the diameter of the pupil to the diameter of 

 the earth. And the force with which the eye is struck, is to the sum of the 

 forces of all the particles which strike the earth, in the same proportion. If 

 therefore the diameter of the pupil, when the eye is exposed to the direct im- 

 pulse of the sun's rays, be reckoned tV of an inch, which I apprehend must 

 rather exceed than fall short of its real magnitude, in those circumstances, it will 

 be found that every stroke which it receives from them, exceeds not that which 

 an iron shot, -^^ of an inch diameter, would give, moving only at the rate of 16.16 



TB to of. Therefore the whole force of the particle impinging at a, is to its effective part, as tb* to 

 of^. That is, the effective part is as of'. The number of particles impinging on the zone has been 

 shown to be as gf x rf. The progressive force of motion excited in the earth's centre, by all the 

 particles impinging on the infinitesimal zone pqon, must be as the number of the particles and the 

 effective part of each jointly; that is, as gf' x if X of*, or writing a for tb, and x for b/", as ;r x 

 (a — x) X (2<7x — x') And this is the fluxion of the progressive force of motion excited in the 

 globe, by the particles impinging on that segment of the sphere, of which fA'&q is the projection. 

 The number of particles impinging on the zone pqon, being as gf x t/, or as *■ x (a — x), if each 

 impinged perpendicularly, and its whole force were effective, the sum of the effective forces im- 

 pressed on the whole, would be as gf x t/"x tb% or x x {t^ — a'x). And this would be the fluxion 

 of the progressive force of motion of the globe, excited by the particles impinging on the segment of 

 which pA Bf is the projection, if all impinged in directions perpendicular to the surface, and the 

 whole of their forces were effective. The fluent of ;c x (a — x) x (2flx — x») is 4 X (2flx — x'). 

 And the fluent of x x (a* — a'x) isa'x — Ja'x*. When x = a, the first of these two fluents is 

 the sum of the progressive forces actually impressed on the whole hemisphere acb, and the latter is 

 the sum of the forces which would be so impressed, if all the impinging particles impinged perjien- 

 dicularly, and the whole force of each were effective. But when x = a the first fluent becomes Ja*. 

 And the latter becomes Ja*. Whence it is manifest, that the progressive motion communicated to 

 the globe of the earth, by the particles of light, is to the force which they would communicate, if 

 the whole force of each were effective, in the proportion before assigned, of 1 to 2. — Orig. 



