ON HYDRAULIC MACHINES. 257 



effect at all. Mr. Cole has improved the construction of the chain pump, 

 so as materially to increase the quantity of water raised by it.* (Plate 

 XXIII. Fig. 318.) 



It is frequently necessary to procure alternate motion in pumps by 

 means of wheel-work, and for this purpose the application of a crank is the 

 most usual and perhaps the best method. Provided that the bar by which 

 it acts be sufficiently long, very little will be lost by the obliquity of its 

 situation, and it is easy, by means of rollers, or of a compound frame, to 

 confine the head of the pump rod to a rectilinear motion. When any 

 other mode is employed, it must be remembered that the motion of the 

 pump rod ought always to be slower at the beginning of each alternation, 

 since a considerable part of the force is consumed in setting the water in 

 motion, especially where the pipe is long, and the velocity considerable. 

 But it may happen that, from the nature of hydraulic pressure under 

 other circumstances, the resistance may be nearly equal throughout the 

 stroke : for example, when the motion of the piston is slow in comparison 

 of that of the water in the pipe, or when the force employed in producing 

 velocity is inconsiderable, in comparison with that which is required for 

 counteracting the pressure. In such cases it may sometimes be eligible to 

 employ inclined surfaces of such forms as are best adapted to communicate 

 the most advantageous velocity to the pump rod by their pressure on a 

 roller, which may be confined to its proper direction by the same means as 

 when a crank is used. (Plate XIV. Fig. 184... 187.) 



The Chinese work their cellular pumps, or bead pumps, by walking on 

 bars which project from the axis of the wheel or drum that drives them, 

 and whatever objection may be made to the choice of the machine, the 

 mode of communicating motion to it must be allowed to be advantageous. 



Pumps have sometimes been worked by means of the weight of water 

 acting within a barrel, which resembles a second pump placed in an in- 

 verted position. The only objection to the machine appears to be the 

 magnitude of the friction, and even this inconvenience may perhaps be 

 inconsiderable. The invention is by no means modern,t but it is best 

 known in Germany under the name of HolTs machine,^ and it has been 

 introduced into this country by Mr. Westgarth and Mr. Trevithick.|| 

 A chain pump, or a series of buckets, may also be applied, in a manner 

 nearly similar, to the working of machinery of any kind. (Plate XXIII. 

 Fig. 319.) 



The mediation of a portion of air is employed for raising water, not 

 only in the spiral pump, but also in the air vessels of Schernnitz.[ A 

 column of water, descending through a pipe into a closed reservoir full of 

 air, obliges the air to act, by means of a pipe leading from the upper part 



* London Magazine for 1768, p. 499. 



t It is figured in Fludd's Naturae Simia, Oppenheim, 1618, p. 467. 

 J Hist. etMem. 1760, H. 160. 



Bailey's Machines, ii. 52. Smeaton, Transactions of the Society of Arts, 

 vol. v. 

 || Nich. Jour. 8vo, i. 161. 



^ Wolfe's Description of Hero's Fountain at Schemnitz, Ph. Tr. 1762, p. 547. 

 Poda's do. Prag. 1771. Nich. Jour. iv. 8, 117. 



s 



