24 



HYDRAULICS. 



course it is the same thing whether the 

 power of the moving water be applied 

 to the float-boards of a wheel which re- 

 volves in a fixed building, or whether 

 any extraneous force be applied to the 

 axis of a wheel to cause it to move in 

 still water : in the first case the power 

 of the water will be transferred to the 

 axle of the wheel, and is applicable to 

 the driving or moving of machinery ; 

 while, in the second case, the power ap- 

 plied to the axle will be resisted by the 

 quiescent water, and will be converted 

 into so much power for moving the 

 building or boat in which the wheel is 

 placed ; and upon this principle depends 

 the action of those steam-boats which 

 are impelled through the water by 

 means of water-wheels driven round by 

 the power of steam-engines applied to 

 their axles, instead of permitting the 

 water to move the float-boards and 

 transfer its power to the axis. 



Whenever the weight and motion of 

 water can be made use of, as well as its 

 momentum, much greater effects can be 

 produced than the last described ma- 

 chine is capable of, and with a much 

 less lavish expenditure of the fluid, for 

 then its utmost powers of action are 

 brought into play at once ; and accord- 

 ingly those water-wheels that are dis- 

 tinguished by the names of breast- 

 wheels, and over-shot wheels, will pro- 



duce much greater power with a much 

 less supply of water than the under- shot 

 wheel already described. Both these 

 wheels however require a considerable 

 fall in the stream upon which they are 

 placed, and consequently destroy it for 

 the purposes of navigation, unless that 

 ingenious Hydraulic contrivance the 

 Canal Lock be resorted to, by means of 

 which barges or vessels of any magni- 

 tude may be transported from one level 

 to another without difficulty, and with 

 very little loss of time. The Over-shot 

 Water- Wheel, which of all others gives 

 the greatest power with the least expense 

 of water, requires a fall in the stream 

 equal to rather more than its own dia- 

 meter, therefore it is customary to give 

 this description of wheel a greater 

 length in proportion to its height than is 

 given to any other, by which an equal- 

 ity of power is obtained. In the con- 

 struction of the over- shot wheel a hol- 

 low cylinder or drum that is impervious 

 to water is first prepared, and hung 

 upon a proper central axis. A number 

 of narrow troughs, or cells, generally 

 formed of thin plates of metal, extending 

 from one end of the drum to the other, 

 are next fixed round the outside of the 

 wheel so as to give a transverse section 

 through the middle of the wheel, the 

 appearance shown at fig. 19. The 

 w r ater is conducted by a level trough of 



fig- 19 



the same width as the wheel over its 

 top, as at h i, and is discharged into the 

 buckets or cells placed round the wheel 

 to receive it, as at k I ; from the parti- 

 cular form of these buckets they retain 

 the water thus thrown into them, until 

 by their motion they descend towards 

 the point I, when their mouths being 

 turned downwards they discharge their 



contents into the tail- stream m, where 

 the water runs to waste. The buckets 

 on the opposite side n of the wheel de- 

 scend with their mouths downwards, and 

 thus remain empty, until they arrive 

 under the end h of the water-trough 

 to be refilled ; at h there is a penstock 

 or sluice for regulating the quantity of 

 water and preventing waste, since, if the 



