or inflection of the Atmosphere; and this will be manifest, if we consider the Atmosphere as a transparent Globe, or at least a transparent shell, encompassing an opacous Globe, which, being more dense then the medium encompassing it, refracts or inflects all the entring parallel Rays into a point or focus, so that wheresoever the Observator is plac’d within the Atmosphere, between the focus and the luminous body, the lateral Rays must necessarily be more converg’d towards his eye by the refraction or inflection, then they would have been without it; and therefore the Horizontal Diameter of the luminous body must necessarily be augmented.

This might be more plainly manifest to the eye by the sixth Figure; but because it would be somwhat tedious, and the thing being obvious enough to be imagin’d by any one that attentively considers it, I shall rather omit it, and proceed to shew, 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