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MicrocrapnHia, 
enough to be imagin’d by any one that atteritively confiders it; Efhall ras 
ther omit it, and proceed to fhew, that the mafsof Air neer the furface of 
theEarth,confifts,or is made up,of parcels, which do very much differ from 
4 one another in point of denfity and rarity 3 and confequently the R 
_ light that pafs through them will be varioufly illntied bere owed ES 
there another,according as they pafs fo or fo throu gh thofe differing parts; 
__ and thofe parts being always in motion, either upwards or downwards,or 
e to the right or left, or in fome way compounded of thefe, they do by this — 
_ their motion inflect the Rays, now this way, and prefently that way. 
_ This irregular,unequal and unconftant inflection of the Rays of light, 
is the reafon why the limb of the sux, Atoon, 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 fometimes ma- 
gnify:d,and fometimes diminifhed ; fometimes elevated, otherwhiles de- 
___ prefsd 3 now thrown to the right hand, and then to the left. 
And that thereis fuch* property or unequal diftribution of parts, is 
_ manifeft from the various degrees of heat and cold that are found in the 
Air; from whence will follow a differing denfity and rarity, both as to 
quantity and refraction ; and likewife from the vapours that are inter- 
os, (which,by the way, Iimagine,as to refraction or inflection; to do 
e<the fame thing, asif they were rarify'd Air; and that thofe vapours that 
__aicend,are both lighter, and lefs denfe, then the ambient Air which boys 
themup; and that thofe which defcend, are heavier and more denfe) 
The firft of thefe may be found true, if you take a good thick piece of 
Glafs,and heating it pretty hot in the fire, lay it upon fuch another piece’ 
of Glafs, or hang it in the open Air by~a piece of Wire, then looking 
upon fome far diftant Object (fuch as a Steeple or Tree) fo as the Rays 
from that Object pafs directly over the Glafs before they enter your eyes 
you fhall find fuch a tremulation and wavering of the remote Obje«t, as 
will very much offend your eye: The like tremulous motion you may 
obferve to be caus'd by the afcending fteams of Water, and the like. 
Now, from the firft of, thef it is manifeft, that from the rarifaétion of the 
parts of the Air,by heat,there is caus'‘d a differing refraétion,and from the 
afcenfion of the more rarify‘d parts of the Air, which are thruft up by the 
colder, and therefore more condens‘d and heavie, is caus'd an undula- 
tion or wavering of the Object; for I think, that there are very few, 
will grant, that Glafs, by as gentle a heat as may be endur'd by ones 
hand, fhould fend forth any of its parts in {teams or vapours, which does 
not feem to be much watfted by that violent fire of the green Glafs-houfe 5 
but, if yet it be doubted, let Experiment be further made with that bo- 
dy that is accounted, by Chymifts and others, the moft ponderous and. 
fix'd in the world ; forby heating of a piece of Gold, and proceeding in 
the fame manner, you may find the fame effects. Hern 
This trembling and fhaking of the Rays, is more fenfibly caus d by an 
actual flame, or quick fire, or any thing elfe heated glowing hot 5 as by 
a Candle, live Coal, red-hot Iron, or a piece of Silver, and the like : the 
fame alfo appears very confpicuous, if you look at pbs hae” 
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