'336 LECTURE XXVI r I. 



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 th.at of the water in the pipe, or when the force em- 

 ployed 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 w^eight- of water 

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

 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, but it is best known in Germany 

 under the name of HoU's machine, and it has been introduced into this 

 country by Mr. Westgarth and Mr. Trevithick. A cliain 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 Schemnitz. 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 of the reservoir or 

 air vessel, on the water in a second reservoir, at any distance either below or 



