333 
on the Atmosphere of Venus. 
Will also be easily perceived, with respect to the much stronger 
solar light on Venus, that if those last layers of its atmosphere, 
on which even the least vestige of the solar light can be 
traced, be transferred in idea into the atmosphere of our earth, 
admitting a similar density, but a less power of solar light, 
they would certainly exhibit no such perceptible traces. 
And if it be probable, to a degree of evidence, that the twi- 
light on Venus is nearly equal in its extent to that on our 
globe ; if it be mathematically proved that its brighter part, 
which has so much light as to enable us not only to see it, 
but even to measure it across our fainter twilight, illuminates 
a zone 4 0 38' 30" in breadth, we may hence surely infer the 
density and perpendicular height of at least the inferior more 
condensed part of the atmosphere, which is capable of re- 
flecting such a crepuscular light ; and apply those principles 
according to which the height of our own atmosphere, in as 
far, namely, as its lower strata are able to reflect the solar 
rays, and occasion a twilight, has been estimated at 34585 
toises, or nearly nine geographical miles. 
Let akb ej l, fig. 9, Tab. VII. be the surface of Venus ; 
k b e, an arc of it, which, according to the above calculation 
of the extent of the brightest twilight on the planet, is as- 
sumed at 4 0 38' 30" ; b d , the height of that denser part of the 
atmosphere which actually occasions the twilight that is visible 
to us ; and h d, the first ray of the sun, at this time 4 0 38' 30" 
below the horizon, which is reflected by the highest stratum 
of the denser atmosphere d y upon the point e on the surface 
of the planet. We then have, in the triangle d e c, the side 
c e> or the semidiameter of Venus = 834 geogr. miles, the 
right angle e } and the angle c = 2 0 19' 15", or half the angle 
MDCCXCII. Xx 
