116 



FERGUSON'S LECTURES. 



LECT. 

 V. 



- 







paradox, which we shall first shew by an experiment, 

 and then account for it upon the principle above-men- 

 tioned, namely, that the pressure of fluids is directly as 

 their perpendicular height, without any regard to their 

 quantity. 



Let a small glass tube DCG, open 

 throughout, and bended at B, be 

 joined to the end of a great one A I 

 at cd, where the great one is also 

 open ; so that these tubes in their 

 openings may freely communicate 

 with each other. Then pour water 

 through a small necked funnel into 

 the small tube at H; this water will 

 run through the joining of the tubes 

 at cd, and rise up into the great 

 tube ; and if you continue pouring 

 until the surface of the water comes to any part, as A 

 in the great tube, and then leave off, you will see that 

 the surface of the water in the small tube will be just as 

 high, at D; so that the perpendicular height of the 

 water will be the same in both tubes, however small the 

 one be in proportion to the other. This shews, that 

 the small column DCG balances and supports the great 

 column Acd; which it could not do if their pressures 

 were not equal against one another in the recurved bot- 

 tom at B. If the small tube be made longer, and in- 

 clined in the situation G E F, the surface of the water 

 in it will stand at F, on the same level with the surface 

 A in the great tube ; that is, the water will have the 

 same perpendicular height in both tubes, although the 

 column in the small tube is longer than that in the great 

 one ; the former being oblique, and the latter perpen- 

 dicular. 



Since, then, the pressure of fluids is directly as their 

 perpendicular heights, without any regard to their quan- 

 tities, it appears that whatever the figure or size of *he 



