120 FERGUSON'S LECTURES. 



LECT. S orne water be poured into the pipe at c, which will run 

 -^vx., into the bellows, arid separate the boards a little. Then 

 lay three weights b, c, d, each weighing 100 pounds, 

 upon the upper board ; arid pour more water into the 

 pipe B, which will run into the bellows, and raise up 

 the board with all the weights upon it ; and if the pipe 

 be kept full, until the weights are raised as high as the 

 leather which covers the bellows will allow them, the 

 water will remain in the pipe, and support all the 

 weights, even though it should weigh no more than a 

 quarter of a pound, and they 300 pounds : nor will all 

 their force be able to cause them to descend and force 

 the water out at the top of the pipe. 



The reason of this will be made evident by consider- 

 ing what has been already said of the result of the 

 pressure of fluids of equal heights, without any regard 

 to their quantities. For, if a hole be made in the upper 

 board, and a tube be put into it, the water will rise in 

 the tube to the same height that it does in the pipe ; and 

 would rise as high (by supplying the pipe) in as many 

 tubes as the board could contain holes. Now, suppose 

 only one hole to be made in any part of the board, of 

 an equal diameter with the bore of the pipe B : and 

 that the pipe holds just a quarter of a pound of water ; 

 if a person claps his finger upon the hole, and the pipe 

 be filled with water, he will find his finger to be pressed 

 upward with a force equal to a quarter of a pound. 

 And as the same pressure is equal upon all equal parts 

 of the board, each part, whose area is equal to the area 

 of the hole, will be pressed upward with a force equal 

 to that of a quarter of a pound : the sum of all which 

 pressures against the under side of an oval board 16 

 inches broad, and 18 inches long, will amount to 300 

 pounds ; and therefore so much weight will be raised 

 up and supported by a quarter of a pound of wate*- in 

 the pipe. 



Hence, if a man stands upn the upper board, and 



