an inch in ico" . This is the utmoft effect of the 
force imprefied upon the earth by each emiflion. If 
the emiffions were inceflant, this might be confidered 
as a central force, counteracting the lun’s attraction : 
of the earth, perpendicular to that which feparates the enlightened 
and the dark hemifphere, and which {ball be called the termi-» 
nator. Upon the plane of this femkircle, fuppofe the termi- 
nator, and its parallels in the enlightened hemifphere, to be pro- 
jected, into right lines AB, kb, Im , no, &c. which are diameters 
of the circles refpeClively, of which they are the projections. 
The fun’s diftance may be confidered as infinite ; and therefore, 
the rays of light, i. e. the directions of the particles, when they 
reach the earth’s furface, are to be confidered as parallel to each 
other, and all of them perpendicular to the plane cf the termi- 
nator. Now imagine the whole enlightened hemifphere to be 
divided into innumerable little zones A B k h, hk ml, ml no, 
&c. by fmall circles parallel to the terminator. Let the breadths 
of thefe little zones, meafured upon a great circle palling 
through the poles of the terminator, that is, let the infinitefimal 
arcs B k, km, no, &c. be fo proportioned to each other, that per- 
pendiculars kd, me, of, &c. being drawn from the extremities of 
thefe arcs, to the right line AB, which is the common inter- 
fetion of the great circle ACB, and the plane of the termi- 
nator, the infinitefimal fegments of that line B d,de,ef, &c. 
may be equal. Now imagine the particles of light which fall 
upon any one of thefe little zones, for inftance, noqp , to meet 
with no refiftance from the earth’s furface, but to penetrate the 
globe, and to pafs on without refraction or inflection, in the di- 
rection perpendicular to the terminator, till they arrive at the 
plane of the terminator, and there fuppofe them to flop, and 
each to lye till, in the place on which it falls. It is evident that 
the particles of light that fall upon, and have been fuppofed to 
pafs through, the fpherical zone p q on, will, with their proper 
interftices, cover that annular fpace upon the plane of the ter- 
minator,- which is the orthographical projection of the zone 
p q o n, upon that plane, and is comprifed between the circum- 
ferences of circles, of which the right lines T g and T f are the 
radii. Hence the number of the particles of light, which fall upon 
the evanefeent zone pq on, are as that evanefcent annular fpace 
•for 
