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the earth is always loaded with more or less, as we find in rarifying the medium by an air-pump, on which occasion it never fails to let fall a cloud of vapours.  It is impossible that light should enter Air thus impregnated without being refracted: and that the refraction of the Atmosphere is thus to be accounted for is plain from hence, that refraction is always greatest in hazy weather, when the air contains most water in it, as the French Academy observed long ago.  This will give a reason why refraction is so much less near the Equator. for the air being rarer with Heat in that Climate, will not sustain such a body of water as with us, but lets it fall as soon as the cold of the evening comes, whence their excessive Dews.  Again, the Air in the Polar reigons being more dense, will sustain a much greater quantity, so that their Atmosphere after the frost of a whole winter will be almost converted into a rarer sort of Ice, whence the excessive Refraction in those Parts.  We are to assert then, that water and terrestrial Exhalations have a determinate height in the Atmosphere; but as for the Element of Air, for any thing that refraction proves to the contrary, it may be extended to the Moon, of the fixed stars themselves. Jones

Upon the summit of an high mountain, the Atmosphere is rarer than at the bottom, therefore the refraction from the top of an Hill is greater than the Horizontal refraction, where the density of the Atmosphere, between the object and the observer, is nearly the same.

The Principles we are to establish are few and plain.  If they agree perfectly with what we observe in Nature, they must be true and worthy of reception: if they should be contradicted by common appearances that are before us, we have mistaken our Evidence, and they ought to be rejected to make way for some better System, if time and experience should discover any such to the world; but I am fully persuaded we are safe enough.  The Principles are these,
First, That all the actions of nature are & must be mechanical; in other words,   that the System of nature created, disposed, & set in^to motion by the Finger of God, acts as a [[/underline]] machine [[\underline]] does, by one portion of matter pressing upon another, with which it is in contact: for if in any part you interrupt the contact of a machine you necessarily destroy its motion.  That therefore
Secondly, The System of the World is a [[/underline]] plenum [[\underline]] or Space filled with matter, so that evry part throughout the whole contains matter resting upon other matter, without any such void as may destroy its Mechanical continuity.
Thirdly, That this plenum is constituted or composed of the fluid Etherial matter of the heavens, the whole of which is one and the same substance, but capable of three difference conditions, under which it is observed to subsist: which conditions are understood by the names of Fire, Light and Air.  These are so many gradations of the etherial fluid.  [[/underline]] Fire [[\underline]] is this matter in most refined state, subsisting in the smallest [[/strikethrough]] state [[\strikethrough]] and most subtile parts to which it is capable of being reduced. [[/underline]] Light [[\underline]] is the immediate state of this matter between fire and air.  it is the emanation of fire, disseminated throughout the world; not always visible, but rendred so by some certain motions impressed upon it. [[/underline]] Air [[\underline]] is the etherial fluid in its grossest state; and as fire is air comminuted & broken into its smallest parts, so air is fire or light congealed again into masses or grains.
Fourthly, That this fluid in any one of its conditions can receive no support or increase but from the same fluid in another of its conditions; so that if the density of [[/strikethrough]] the [[\strikethrough]] Air be increased, it is owing to the [[/underline]] adherence [[\underline]] of congealation of more fire into that state; if light be increased there must either be a fresh supply from a focus of fire, or air must be reduced by motion and friction into that state.  If fire is quickened & strengthened it must be from the influx of [[/underline]] air [[\underline]], for light cannot sustain its action.
Fifthly, and lastly, That this fluid, subsisting in the conditions above mentioned, is the cause of all motion in the natural creation.  other matter, as Water, Earth, and the various sorts of fluids and solids being wholly passive and