hew, that the mass of Air neer the surface of the
Earth, consists, or is made up, of parcels, which do very much differ from
one another in point of density and rarity; and consequently the Rays of
light that pass through them will be variously inflected, here one way, and
there another, according as they pass so or so through those differing
parts; and those parts being always in motion, either upwards or downwards,
or to the right or left, or in some way compounded of these, they do by
this their motion inflect the Rays, now this way, and presently that way.

This irregular, unequal and unconstant inflection of the Rays of light, is
the reason why the limb of the _Sun_, _Moon_, _Jupiter_, _Saturn_, _Mars_,
and _Venus_, appear to wave or dance; and why the body of the Starrs appear
to tremulate or twinkle, their bodies, by this means, being sometimes
magnify'd, and sometimes diminished; sometimes elevated, otherwhiles
depress'd; now thrown to the right hand, and then to the left.

And that there is such a property or unequal distribution of parts, is
manifest from the various degrees of heat and cold that are found in the
Air; from whence will follow a differing density and rarity, both as to
quantity and refraction; and likewise from the vapours that are interpos'd,
(which, by the way, I imagine, as to refraction or inflection, to do the
same thing, as if they were rarify'd Air; and that those vapours that
ascend, are both lighter, and less dense, then the ambient Air which boys
them up; and that those which descend, are heavier and more dense) The
first of these may be found true, if you take a good thick piece of Glass,
and heating it pretty hot in the fire, lay it upon such another piece of
Glass, or hang it in the open Air by a piece of Wire, then looking upon
some far distant Object (such as a Steeple or Tree) so as the Rays from
that Object pass directly over the Glass before they enter your eye, you
shall find such a tremulation and wavering of the remote Object, as will
very much offend your eye: The like tremulous motion you may observe to be
caus'd by the ascending steams of Water, and the like. Now, from the first
of these it is manifest, that from the rarifaction of the parts of the Air,
by heat, there is caus'd a differing refraction, and from the ascension of
the more rarify'd parts of the Air, which are thrust up by the colder, and
therefore more condens'd and heavie, is caus'd an undulation or wavering o