532 LECTURE xxviir. 



valent to somewhat more than 11 pounds raised through lOfcet in a second, 

 instead of 10 pounds, which is a fair estimate of the usual force of a man, 

 without any deduction for friction. (Plate XXIII. Fig. 305.) 



It is obvious that if tlie plungers were so well fitted to the cavity as to 

 prevent the escape of any water between them, the ascending pipe might 

 convey the water to any required height ; the machine would then become a 

 forcing pump, and the plungers might be shortened at pleasure, so as to 

 assume the form of a piston sliding within a barrel. The piston might also 

 be situated above the level of the reservoir, and in this case the water would 

 be forced up after it by the pressure of the atmosphere to the height of about 

 30 feet, but not much further: and even this height would be somewhat too 

 great for practice, because the water might sometimes follow the piston in its 

 ascent too slowly. Such a pump, partaking of the nature of a forcing and a 

 sucking pump, is sometimes called a mixed pump. In Delahire's pump, the 

 same piston is made to serve a double purpose, the rod working in a collar 

 of leathers, and the water being admitted and expelled in a similar manner, 

 above and below the piston, by means of a double apparatus of valves and 

 pipes. (Plate XXIII. Fig. 306.) 



For forcing pumps of all kinds, the common piston, with a collar of loose 

 and elastic leather, is preferable to those of a more complicated structure : 

 the pressure of the water on the inside of the leather makes it sufficiently 

 tight, and the friction is inconsiderable. In some pumps the leather is 

 omitted, for the sake of simplicity, the loss of water being compensated by 

 the greater durability of the pump; and this loss will be the smaller in propor- 

 tion as the motion of the piston is more rapid. (Plate XXIII. Fig. 307.) 



Mr. Bramah has very ingeniously applied a forcing puYnp, by means of the 

 well known properties of hydrostatic pressure, to the construction ot a con- 

 venient and powerful press. The water is forced, by a small pump, into, a 

 barrel in which it acts on a much larger piston ; consequently this piston is 

 urged by a force as much greater than that which acts on the first pump rod, 

 as its surface is greater than that of the small one. (Plate XXIII. Fig. 

 308.) 



In the common sucking pump, the valve through which the water escapes 



