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PHYSICAL anv LITERARY. by 
aay conclude, that their velocities in air 
are equal. Wherefore, if the denfity of 
the fun’s atmofphere, contiguous to his 
furface, be different from the denfity of 
our lower air, as may be fafely prefumed, 
his rays muft have been emitted with more 
or lefs velocity than that of the candle; o- 
therways, they could not have the fame 
velocity afterwards in any common medi- 
um; For, the velocity with which any ray. 
is emitted, is, by the laws of refraction, 
to its velocity in any given med-um, as the 
fine of refraction to the fine of incidence, 
when a ray pafles from the medium of > 
miflion into the given mediym. 
Quer. IV. IF the atmofphere is noe 
much warmed by the paflage of the fun’s 
light thro’ it, but chiefly by its. contact 
with the heated furface of the globe, as 
we fhewed above (No 15.); may we not | 
hence give one very fimple and plaufible 
reafon, why it is coldeft in all climates 
on the tops of very high mountains ; 
namely, becaufe they are removed to the 
greatelt diftance from the general furface 
of 
