0 / Pneumatics. *7 3

sustain this pressure without suffering any change* n their figure-, and so do the most brittle bodieswithout being broke.

The air is rarefied, or made to swell withheat; and of this property, wind is a necessary The causeconsequence. For, when any part of the air is ofw/«A.seated by the sun, or otherwise, it will swell,an d thereby affect the adjacent air : and so, byvarious degrees of heat in different places, thereVnll arise various winds.

"When the air is much heated, it will ascendtowards the upper part of the atmosphere, andadjacent air will rush in to supply its place;an d therefore, there will be a stream or current°f air from all parts towards the place wherethe heat is. And hence we fee the reason whythe air rushes with such force into a glass-house,ot towards any place where a great fire is made.

And also, why smoke is carried up a chimney,and why the air rushes in at the key-hole of thedoor, or any small chink, when there is a firein the room. So we may take it in general, thatthe air will press towards that part of the worldw here it is most heated.

Upon this principle, we can easily account for Thethe trade-winds , which blow constantly from eastto West about the equator. For, when the funnines perpendicularly on any part of the earth,lc will heat the air very much in that part, whichai f wid therefore rife upward, and when the funWithdraws, the adjacent air will rush in to fill itsP bee; and consequently will cause a stream orcurrent of air f rom all parts towards that whichis most heated by the fun. But as the fun, withtespect to the earth, moves from east to ■west,the common course of the air w;ll be that waytoo; continually pressing after the fun : and

therefore,