OF PNEUMATICS. 199 



the pressure on the surface of the water in the well, that LECT. 

 causes the water to ascend in the pump, and follow the ,^v~x^ 

 piston or bucket, when the air above it is lifted up ; it 

 is evident, that a column of water, 33 feet high, is equal 

 in weight to a column of quicksilver of the same dia- 

 meter 291 inches high ; and to as thick a column of air, 

 reaching from the earth's surface to the top of the at- 

 mosphere. 



In serene calm weather, the air has weight enough to The 

 support a column of quicksilver 31 inches high ; but, ' MrorMtef 

 in tempestuous stormy weather, not above 28 inches. 

 The quicksilver, thus supported in a glass tube, is 

 found to be a nice counterbalance to the weight or 

 pressure of the air, and to shew its alterations at dif- 

 ferent times. And being now generally used to denote 

 the changes in the weight of the air, and of the weather 

 consequent upon them, it is called the barometer, or 

 weather-glass. 



The pressure of the air being equal on all sides of a 

 body exposed to it, the softest bodies sustain this 

 pressure without suffering any change in their figure ; 

 and so do the most brittle bodies without being 

 broken. 



The air is rarefied, or made to swell with heat ; and The cause 

 c , . . j . -n of winds. 



ot this property, wind is a necessary consequence, r or, 



when any part of the air is heated by the sun, or other- 

 wise, it will swell, and thereby affect the adjacent air : 

 and so, by various degrees of heat in different places, 

 there will arise various winds. 



When the air is much heated, it will ascend towards 

 the upper part of the atmosphere, and the adjacent air 

 v, ill rush in to supply its place ; and therefore, there will 

 Ue a stream or current of air from all parts towards the 

 place where the heat is. And hence we see the reason whj 

 the air rushes with such force into a glass-house, or towards 

 any place where a great fire is made. And also, why 

 smoke is carried up a chimney, and why the air rushes 



