148 Remarks on Dr. Enfield’s*Institutes 
-demonstration.—After all, we should have been much _bet- 
ter pleased to sce the proposition egtirely omitted, than any 
attempt made to amend it. The hypothesis that the rays 
which come to the eye at the end of twilight are brought by 
a single reflection, is a very questionable one. ‘The power 
of reflecting light possessed by the atmosphere, must de- 
pend on one or both of two causes: 1. It.may reflect some 
of the rays which pass through it in consequence of a defect 
of transpa rency. 2. It may reflect in the same manner as 
light is. ordinarily. bent back into a denser medium. This» 
last mode of reflection, if it ever takes place without an ab- 
rupt change of density, is evidently more likely to take 
place, in proportion as the variation of density is more ees 
ow whichever of these causes ee to produce twi 
light, it must evidently exist ina far higher degree in thie 
lower, than in the higher =jautinh el atmosphere. Hence 
instead of a single “reflection atthe height of forty-two. 
miles, two or more successive reflections may quite as pro- 
bably transmit to the eye the light with which twilight clo- 
ses.*—But even abthintng the correctness of the assump- 
tion that twilight is produced by a single reflection, it is 
most obvious that no inference can be deduced concerning 
“the height of the atmosphere,” or even the height at 
which it ceases to reflect light. The only legitimate con- 
clusion is, that forty-two miles is the limit beyond which 
light is not reflected in. ———— quantity to affect the or- 
gans of vision. If, instead of this vague proposition, the 
—— aw of atmospheric. — at. - altitudes had ticéii 
in its cein the scene the _ 
pe oe have ve been exhibited in a far more 5 cs 
structive form. 
seats subject ob est moon’s Roeser in 1 propss: 78-82, 
managed with singular infelicity. The introductory pro- 
es should be, that “ the time of the moon’s rotation 
on its axis is equal to the mean time of its revolution round 
the earth,”—instead of beginning with the fact that the 
moon. always has nearly the same side toward the earth,” 
and drawing the strange inference that “af the moon re- 
volves. dust its axis, its periodical time must be equal to 
that of its revolution round the earth.” ‘The librations 
oe oe * See Vince’ Ast. E Ant. 206. 
