£00 HYDRAULICS. 



to cover the hole through which the water rises. 

 The greatest difficulty in the construction of a 

 piston is to give a sufficient passage through it for 

 the water, and yet allow a firm support for the 

 valve and fixture for the piston-rod. It occasions 

 a considerable expence of the moving power to 

 force a piston with a narrow perforation through 

 the water lodged in the working-barrel. There 

 can be no doubt, therefore, that metal pistons are 

 preferable, because their greater strength allows 

 much wider apertures. For common purposes, 

 however, they are made of wood, as elm or beech. 



There are many ingenious contrivances to avoid 

 the friction of the piston in the pumps ; but this is 

 of little importance in great works, because the 

 friction, which is completely sufficient to prevent 

 all escape of water in a well-constructed pump, is 

 but a trifling part of the whole force. 



In the great pumps which are used in mines, 

 and are worked by a steam-engine, it is very usual 

 to make the pistons and valves without any leather 

 whatever. The working-barrel is bored truly cy- 

 lindrical, and the piston is made of metal, of a 

 size that will just pass along it without sticking. 

 When this is drawn up with a velocity competent 

 to a properly loaded machine, the quantity of water 

 which escapes round the piston is insignificant. 

 The piston is made without leathers, not to avoid 

 friction, which is also insignificant in such works, 

 but to avoid the frequent necessity of drawing it 

 up for repairs through such a length of pipes. 



If a pump absolutely without friction be wanted, 

 the following seems preferable, for simplicity and 

 performance, to any we have seen, when made use 

 of in proper situations. Let N O (Fig. 10.) be 

 the surface of the water in the pit, and K the place 



