210 FERGUSON'S LECTURES. 



LECT. silver in the vessel N is forced up into the tube by the 

 ^\\^ pressure of the atmosphere. And if the receiver could 

 be perfectly exhausted of air, the quicksilver would 

 stand as high in the tube as it does at that time in the 

 barometer : for it is supported by the same power or 

 weight of the atmosphere in both. 



The quantity of air exhausted out of the receiver on 

 each turn of the handle, is always proportionable to 

 the ascent of the quicksilver, on that turn,; and the 

 quantity of air remaining in the receiver is proportiona- 

 ble to the defect of the height of the quicksilver in the 

 gage, from what it is at that time in the barometer. 



I shall now give an account of the experiments made 

 with the air-pump in my Lectures ; shewing the resist- 

 ance, weight, and elasticity of the air. 



1. To shew the Resistance of the Air. 



I. There is a little machine consisting of two mills, 

 a and b, which are of equal weights, in- 

 dependent of each other, and turn equally 

 free on their axes in the frame. Each 

 mill has four thin arms or sails, fixed into 

 the axis : those of the mill a have their 

 planes at right angles to its axis, and 

 those of b have their planes parallel to it, 

 Therefore, as the mill a turns round in common air, it 

 is but little resisted thereby, because its sails cut the 

 air with their thin edges : but the mill b is much resist- 

 ed, because the broad sides of its sails move against 

 the air when it turns round. In each axle is a pin near 

 the middle of the frame, which goes quite through the 

 axle, and stands out a little on each side of it : upon 

 these pins, the slider d may be made to bear, and so 

 hinder the mills from going, when the strong spring 

 c is set on bend against the opposite ends of the pins. 



Having set this machine upon the pump-plate L L 

 (page 207) draw up th'e slider d to the pins on one side 



