42 Experimental Philosophy. [Lecture 4. 



valve at its lower end, and is forced through at o 

 into the cylinder. This forms a pressure at m, by 

 the action of one man working at s, which 

 squeezes cotton bags, hay, or other packages, 

 into twenty times less compass than they would 

 otherwise occupy. The effect would be the same 

 if c\ instead of a pump, were a slender tube, pro- 

 vided it was long in proportion to the pressure 

 which was required. 



From all these experiments it is easy to con- 

 ceive why the banks of ponds, rivers, and canals 

 blow up, as it is called. If water can insinuate 

 itself under a bank or dam, even to the thick- 

 ness of a shilling, the pressure of the water in 

 the canal will force it up. In fig. 1 8, a is the sec- 

 tion of a river or canal, and c is a drain running 

 under one of its banks. Now it is evident 

 that if the bank g is not heavier than the co- 

 lumn of water de, that part of the bank must 

 infallibly give way. This eifect is prevented 

 in artificial canals, by making the sides very 

 tight with clay heavily rammed down, or by cut- 

 ting a trench, n, from two feet to eighteen 

 inches wide along the bank of the river or canal, 

 and a little deeper, which being filled up with 

 earth or clay well moistened with water, forms a 

 kind of wall when dry, through which the water 

 cannot penetrate. 



Another maxim in hydrostatics, of equal im- 

 portance with the former, is, that every body 

 lighter than water, or, in other words, which 



