40 Experimental Philosophy. [Lecture 4. 



that the pressure there must be equal at every 

 part. The vessel fig. 1 1, however, is of a differ- 

 ent shape, and will hold more than three times 

 the quantity of water ; yet the pressure at the 

 base is still the same as in the former instance, 

 because the bottom F, G, supports only the 

 column of water I, F, G, K, which is the same as 

 that contained in the vessel fig. 6. All this may 

 be easily comprehended ; but the great difficulty 

 lies in understanding how the very small tube in 

 fig. 12. can exert a pressure at the bottom or 

 base of the vessel equal to that in the preceding. 

 Here it will be necessary to remember the maxim 

 that was laid down, That the pressure of fluids is 

 in proportion to their height, and not to their 

 quantity. Thus we may observe the column of 

 water in fig. 12. is equal in height to the columns 

 in fig. 11. and 13; and if we advert to what was 

 said, when speaking of fig. 9, we shall perceive 

 that the small column L, M, P, Q. displaces a 

 quantity of water contained in the lower part of 

 the vessel M, P, N, O, and forces it to rise to the 

 top of the vessel at s, for instance, which, if 

 strong enough, will cause a re-action equal to the 

 pressure of a column of water M, P, r, s. The 

 same will take place at the other side, and at 

 every part of the vessel which is covered, so that 

 in effect the pressure at the bottom N, O, will be 

 the same as if the column of water were equal in 

 size from the bottom N, O, to the top of the 

 tube, as shown by the dotted lines. All this may 



