which lieth neerest to the Earth, has,
for the most part, its constituent parcels variously agitated, either by
heat or winds, by the first of which, some of them are made more rare, and
so suffer a less refraction; others are interwoven, either with ascending
or descending vapours; the former of which being more light, and so more
rarify'd, have likewise a less refraction; the latter being more heavie,
and consequently more dense, have a greater.

Now, because that heat and cold are equally diffus'd every way; and that
the further it is spread, the weaker it grows; hence it will follow, that
the most part of the under Region of the Air will be made up of several
kinds of _lentes_, some whereof will have the properties of _Convex_,
others of _Concave_ _glasses_, which, that I may the more intelligibly make
out, we will suppose in the eighth _Figure_ of the 37. _Scheme_, that A
represents an ascending vapour, which, by reason of its being somewhat
_Heterogeneous_ to the ambient Air, is thereby thrust into a kind of
Globular form, not any where terminated, but gradually finished, that is,
it is most rarify'd in the middle about A. somewhat more condens'd about
BB, more then that about CC; yet further, about DD, almost of the same
density with the ambient Air about EE;, and lastly, inclosed with the more
dense Air FF, so that from A, to FF, there is a continual increase of
density. The reason of which will be manifest, if we consider the rising
vapour to be much warmer then the ambient heavie Air; for by the coldness
of the ambient Air, the shell EE will be more refrigerated then DD, and
that then CC, which will be yet more then BB, and that more then A; so that
from F to A, there is a continual increase of heat, and consequently of
rarity; from whence it will necessarily follow, that the Rays of light will
be inflected or refracted in it, in the same manner as they would be in a
_Concave-glase_; for the Rays _GKI_, _GKI_ will be inflected by _GKH_,
_GKH_, which will easily follow from what I before explained concerning the
inflection of the _Atmosphere_.

On the other side, a descending vapour, or any part of the air included by
an ascending vapour, will exhibit the same effects with a _Convex lens_;
for, if we suppose, in the former Figure, the quite contrary constitution
to that last describ'd; that is, the ambient Air FF being hotter then any
part of that matter within any circle, therefore the coldest part must
neces