118 FERGUSON'S LECTURES. 



T 'EC T - the weight E ; and then part of the water will run out 

 <*-v~x^ at d. Mark the height at which the surface H of the 

 water stood in the vessel, when the bottom began to 

 give way at d ; and then, holding up the other vessel 

 A B (the first figure in the preceding page) in the same 

 manner, cause water to be poured into it at H; and 

 you will see that when the water rises to A in this ves- 

 sel, just as high as it did in the former, its bottom wili 

 also give way at d, and it will lose part of the water. 



The natural reason of this surprising phenomenon is, 

 that since all parts of a fluid at equal depths below the 

 surface are equally pressed in all manner of directions, 

 the water immediately below the fixed part Bf (in the 

 first figure) will be pressed as much upward against its 

 lower surface within the vessel, by the action of the 

 column A g, as it would be by a column of the same 

 height, and of any diameter whatever ; (as was evident 

 by the experiment with the tube, p. 113.) and therefore, 

 since action and reaction are equal and contrary to each 

 other, the water immediately below the surface Bf will 

 be pressed as much downward by it, as if it was imme- 

 diately touched and pressed by a column of the height 

 gA, and of the diameter Bf: and therefore, the water 

 in the cavity B Ddf will be pressed as much down- 

 ward upon its bottom C C, as the bottom of the other 

 vessel (the second figure) is pressed by all the water 

 above it. 



To illustrate this a little farther, let a hole be made 

 &if in the fixed top Bf (as in the first figure of the pre- 

 ceding page) and let a tube G be put into it ; then, if 

 water be poured into the tube A, it will (after filling 

 the cavity B d) rise up into the tube G, until it comes 

 to a level with that in the tube A ; which is manifestly 

 owing to the pressure of the water in the tube A, upon 

 that in the cavity of the vessel below it. Consequently 

 that part of the top Bf, in which the hole is now made, 

 would, if corked up, be pressed upward with a force 



