8 NATURAL HISTORY 
Air. In other words, vapours are either denfe and heavy, or in a 
rarer ftate ; now fpacious combinations, now fmaller and more con- 
traded ; differently fhap’d, their furfaces either doping or hori- 
zontal, concave at one time and convex at another. 1 hefe 
unevenneffes of the vapourous contents of the Atmofphere muff 
difpofe the fpaces of Air which lie betwixt and on every fide of 
them into equal irregularities ; fometimes into narrow guts and ilraits, 
fometimes into wider and more extended chanels ; now perhaps 
fome hundreds of leagues long, and fometimes not a mile ; here a 
tall column of Air depends, there it is compreffed into a fpreading 
oblate difk, all cauled by the different fizes, fhapes, and fubftance of 
the Clouds and Vapours, and making the Atmofphere lomewhat like 
Earth which it invefts, full, if I may fo exprefs my felt, ot fleeting 
mountains, hills, plains, valleys, ftraits, and expanfes.^ This is the 
general form of the Atmofphere ; and were it poflible for the human 
eye to comprehend this extenlive profped, and to note the fuccef- 
flve alterations made in the Air by repletion and vacuity, by heat 
and cold, we fhould be no more furprifed to fee fuch a multi- 
farious fluid perpetually in motion, reflfted, protruded, con- 
denfed, expanded, retarded, or accelerated, in its different parts, 
than we are to obferve the various eddies, torrents, fwift 
ftreams, and {filler pools of a large river. For inftance, in 
the latter cale, if we fee the current ftrong and fwift, we attri- 
bute the velocity to the narrowing or fhelving chanel, or perhaps 
to the additional influx of extraordinary Rain or Snow ; but if 
we find it impetuous and irrefiftible, we conclude the banks have 
given way, and the Water, ftruggling to defcend, ruflies to that 
place where there is leaft refiftance. As thefe are the caufes of 
direa ftreams and their different velocities, the oppofition of little 
iflands and projecting banks in a river are the caufes of delayed 
and crooked currents. In like manner, the fpaces between the 
floating vapours are fo many chanels through which the Air paffes, 
ever ftruggling to maintain an equilibrium, and tending to any fpace 
which is lefs replete in one part than in another. Wind is nothing 
more than Air in motion ; rarefactions and vacuities in the Atmof- 
phere are the immediate caufcs or this motion, and are produced 
frequently, we may fay continually, by feveral different caufes. The 
Sun, by concurrent circumftances in land, water, and vapour, 
lightens and difperfes the Air from one place, and at one time, 
more than at another. Inflammable exhalations and their explofions 
fhall warm and thin the Air in particular places. A cloud or portion 
of Vapour full of eledrical matter, palling near a cloud or region 
of land more deftitute of eledtrical matter, will filed ftreams oi fire 
upon the lefs eledtric body, and thereby excite violent motions. 
Great 
