114 FERGUSON'S LECTURES. 



LECT faces of the water in both are on the same level : winch 

 '^Y^, shews that the water is pressed upward into the small 

 tube by the weight of what is in the great one ; other- 

 wise it could never rise therein, contrary to its natural 

 gravity ; unless the diameter of the bore were so small, 

 that the attraction of the tube would raise the water ; 

 which will never happen, if the tube be as wide as that 

 in a common barometer. And, as the water rises no 

 higher in the small tube than till its surface be on a level 

 with the surface of the water in the great one, this shews 

 that the pressure is not in proportion to the quantity of 

 water in the great tube, but in proportion to its perpen- 

 dicular height therein : for there is much more water in 

 the great tube all round the small one, than what is rais- 

 ed to the same height in the small one, as it stands in 

 the great. 



Take out the small tube, and let the water run out of 

 it ; then it will be filled with air. Stop its upper end 

 with the cork C, and it will be full of air all below the 

 cork : this done, plunge it again to the bottom of the 

 water in the great tube, and you will see the water rise 

 up into the height E ; which shews that the air is a bo- 

 dy, otherwise it could not hinder the water from rising 

 up to the same height as it did before, namely, to A ; 

 and in so doing, it drove the air out at the top ; but 

 now the air is confined by the cork C: and it also 

 shews that the air is a compressible body, for if it were 

 not so, a drop of water could not enter into the tube. 



The pressure of fluids being equal in all directions, 

 it follows that the sides of a vessel are as much pressed 

 by a fluid in it, all around, in any given ring of points, 

 as the fluid below that ring is pressed by the weight of 

 all that stands above it. Hence the pressure upon 

 every point in the sides, immediately above the bottom, 

 is equal to the pressure upon every point of the bottom. 

 To shew this by experiment, let a hole be made at 



