OF WATER-WHEELS. 107 



The operation of the pile engine may be best understood by refer- LECT 

 ence to a former note in which the doctrine of accelerated motion is IV. 

 examined, and it may be only necessary to add. that the momentum 

 thus acquired is exactly equivalent to so much additional weight fall- 

 ing upon the head of the pile. 



Note 43. The following additional illustration of the preced- 

 ing subjects will be found valuable to the practical artizan, and 

 they have been introduced at the close of the Lecture to prevent 

 an unnecessary interference with our Author's text. 



There are four principal modes of employing the force of water 

 as a prime mover in machinery, and of these the under and 

 over-shot water-wheels, are the most important : of the other two, 

 namely, the bjeast-wheel and Barker's mill, but little need 

 be said, as the latter is but seldom resorted to, and the breast 

 wheel may be considered but as a modification of the two preced- 

 ing. The following diagrams will best illustrate the under and 

 over-shot water-wheels. 



The over-shot wheel it will be seen owes its power to the weight 

 of the water, while the under-shot, on the contrary, depends on its 

 impulse. 



In order to determine the effect of any force employed in 

 machinery, we must consider not only its magnitude, but also the 

 velocity with which it can be brought into action, and we must 

 estimate the ultimate value of the power, by the joint ratio, or 

 the product, of the force and the velocity. Thus, if we had a 

 corn-mill, for example, in which we wished the mill-stone to re- 



