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The rotatory forces mentioned in the lad note 
if they were infinitely greater than they really are, 
would not, in the lead degree, didurb the diurnal 
rotation ; becaufe every one of them is dedroyed, by 
an equal one, in a contrary direction, on the other 
fide of, and at an equal diftance from, the perpendi- 
cular ray. 
I have enquired, what may be the utmod 
droke, which the retina of a common eye fuf- 
tains, when the eye, in a bright day, is turned up 
directly to the fun. This force will evidently be at 
its maximum, if the emidion be reckoned at its 
maximum. The number of particles which enter 
an eye, looking up diredtly at the fun, are to the 
number out of each emidion, which are directed to- 
wards 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 druck, is to the 
fum of the forces of all the particles which drike 
the earth, in the fame proportion. If, therefore, the 
diameter of the pupil, when the eye is expofed to 
the diredt impulfe of the fun’s rays, be reckoned^. th 
of an inch, which I apprehend mud rather exceed 
fphere, of which p AB q is the projection. The number of 
particles impinging on the zone pqon , being as gf x T f or 
as x x a — x, if each impinged perpendicularly, and its whole 
force were effective, the fum of the effective forces imprcfl'ed 
upon the whole, would be as gf x TCx TB% or x xa J — u z x t 
And this would be the fluxion ot the progreflive force of motion 
of the globe excited by the particles impinging upon the feg- 
rnent of which p AB q is the projection, if all impinged in 
directions perpendicular to the furface, and the whole of their 
forces were effective. The fluent o f x x a — x X lax — x z is 
5 X 2 ux- .rT. And the fluent of k X a 1 — a z x is a'x — \a L x‘ l . 
than 
