254 LECTURE XXVIII. 



every minute, to a height of 11^ feet, for ten hours a day, without fatigue ; 

 this is the greatest effect produced by a labourer that has ever been correctly 

 stated by any author ; it is equivalent to somewhat more than 1 1 pounds 

 raised through 10 feet 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 the 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 some- 

 what 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 ap- 

 paratus 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 struc- 

 ture : the pressure of the water on the inside of the leather makes it suffi- 

 ciently 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 proportion as the motion of the piston is more rapid. (Plate XXIII. 

 Fig. 307.) 



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

 the well known properties of hydrostatic pressure, to the construction of a 

 convenient 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 is placed within the piston itself, so that the same barrel serves for 

 the ascent of the water, which rises in one continued line while the piston 

 is raised, and rests on the fixed valve while it is depressed. The velocity of 

 the stroke ought never to be less than 4 inches in a second, nor greater than 

 two or three feet ; the stroke should also be as long as possible, in order to 

 avoid unnecessary loss of water during the descent of the valves. The di- 

 ameter of the pipe through which the water rises to the barrel, ought not to 

 be less than two thirds of the diameter of the barrel itself. (Plate XXIII. 

 Fig. 309.) 



A bag of leather has also been employed for connecting the piston of a 



* Hist, et Mem. de 1'Acad. 1716, p. 322. 



