OF WATER-WHEELS. Ill 



number of float-boards. This,, on being exposed to the action of 

 a running stream, was afterwards employed to give motion to a ^^ 

 variety of mills, and is at the present time employed in almost 

 even," species of machinery. 



Among the most celebrated hydraulic machines, we may 

 enumerate the machine of Marly. This, when first constructed, 

 appears to have produced one-eighth of the power expended, so 

 that seven-eighths of its power were usually lost. This misap- 

 plied power has been injurious to the engine ; and the wear it 

 has occasioned, has reduced the mechanical effect very mate- 

 rially. But this may be considered as an extreme case, and we 

 select it merely as an instance of that total ignorance of the 

 first principles of mechanics, which characterized some foreign 

 engineers of the last century. 



It may, however, be advisable to examine the ratio of power 

 expended hi comparison with that of the effect produced in some 

 of the most simple hydraulic machines ; and by this calculation, 

 the amount of friction, &c. may be accurately ascertained. 



Potter Effect 



Under-shot water-wheel 9~3 



Over-shot do 10 = 8 



Hydraulic Ram. (This machine will make from 20 to 



100 strokes per minute.) 10 == 6 



Large machine at Chremnitz, (each stroke occupying 



about three minutes.) .... 8 = 3 



But the water-mill, which is the usual machine employed, 

 even in its most improved form, is far from being beneficial 

 either to the agriculturist or the manufacturer. The former is 

 injured by the laws which prohibit the draining of mill-streams 

 for the purposes of irrigation, by which much improvement is 

 kept back that would otherwise take place ; while the health 01 

 the latter, in the immediate neighbourhood of manufacturing 

 districts, is much injured by the stagnant condition of the water 

 which is thus unnecessarily dammed up. 



W ind. which we may consider as the next substitute for ani- 

 mal power, appears to have been first employed to give motion 

 to machinery in the beginning of the sixth century. The use of 

 this species of mechanic force, is however principally limited to 

 the grinding of corn, the pressing of seed., and other simple ma- 

 . nipulations ; the great irregularity of this element precluding its 

 application to those processes which require a continued motion. 

 A wind -mill with four sails, measuring seventy feet from the 



