HYDRAULICS. 



29 



that of the wheel, and in almost all 

 cases superior to its weight, the effect 

 will be to produce a constant tendency 

 to raise the wheel out of its bearings ; 

 while, if the power had been derived 

 from the point A on the top of the cog- 

 wheel, the water would be driving the 

 wheel forward in the direction of the 

 current, while the derived power would 

 act in a directly opposite direction with 

 nearly equal force ; and as the one acts 

 below while the other acts above, it 

 follows that they must nearly balance 

 each other, and thus produce no impe- 

 diment to the steady revolution of the 

 wheel. What has been stated with re- 

 spect of the undershot-wheel equally 

 applies to all others ; for in the breast- 

 wheel, the power should be derived from 

 the point opposite to that on which the 

 water is acting, and so of the overshot- 

 wheel, where the power should be taken 

 behind the wheel nearly in its horizon- 

 tal diameter as at A in (figs. 1 9 and 20.) 



The varieties of water-wheels above 

 described comprehend all those that are 

 generally used in Great Britain ; but in 

 America and some parts of Europe, 

 horizontal water-wheels, or wheels with 

 oblique floats, acting on the principles 

 of the smoke-jack by oblique impulse, 

 are very common and very simple in 

 their construction ; but as they are less 

 efficient in mechanical power and ad- 

 vantage, it is needless to describe them 

 here: the reader is therefore referred 

 to page 46 of the fourth volume of the 

 Quarterly Journal of Science and the 

 Arts, edited at the Royal Institution, 

 where a short but comprehensive ac- 

 count is given by Mr. Adamson of all 

 the various contrivances, on this prin- 

 ciple, which have from time to time been 

 used, together with a particular descrip- 

 tion of one of the best forms of this kind 

 of water-mill. 



The best mode of obtaining the utmost 

 power out of a small stream of water, 

 when it happens to be in an elevated 

 situation, and there is an opportunity of 

 discharging the waste water in an inter- 

 mediate position between that which is 

 to be raised and that employed to pro- 

 duce the effect, as in the Hungarian 

 machine, is what is called the Water- 

 pressure Engine, being, in fact, a steam- 

 engine, worked by water instead of 

 steam, and possessing the powers of 

 Bramah's hydrostatic press. This kind 

 of engine is particularly applicable to 

 pumping, or any other purpose, in mines 

 which have the advantage of an adit 



level, and in which there also happens 

 to be a small stream of running w r ater 

 at the surface. Such a stream is most 

 frequently applied to turning a water- 

 wheel and then runs to waste ; but, by 

 the application of this machine, it may 

 be converted to highly useful purposes. 

 The form and operation of the water- 

 pressure engine will be understood by 

 referring to fig. 21, in which a b is a 

 metal cylinder, truly bored, closed at its 



two ends, and having an air-tight piston 

 and piston-rod working through a stuf- 

 fing-box, precisely in the same way as 

 if prepared and made for a steam-en- 

 gine. The cylinder has, likewise, the 

 same nozzles and side pipes as the steam- 

 engine, which are united in a common 

 four-way (or rather double passage) 

 cock at e. The operation of this kind 

 of cock is too well known to need par- 

 ticular description. It presents four 

 external openings, and has the effect of 

 permitting two opposite currents to pass 

 through it at the same time, the direc- 

 tion of which may be reversed by turn- 

 ing the cock a quarter round, although 

 the supply is constantly delivered in at 

 the same orifice. Thus, as the cock is 

 shown in the figure, any water that may 

 be conveyed from the elevated reservoir 

 h through the pipe g, which is connected 

 with one of the orifices in the cock at e y 

 would find an immediate passage up the 



