46 8 Canftruftion of [Book VII. 



of waves, is founded upon this hypothecs, that all the 

 parts of the water rife and fall in right lines ; yet it 

 muft. be obferved, that both their rife and fall are 

 more frequently made in curved lines than in (Irak 

 ones j fo that the determination given above refpett- 

 ing the fpace which waves roll over in a given time 

 may be regarded only as an approximation to the 

 truth. 



VIII. The motion of wheels acted upon by the fall 

 or force of water, though ftrictly a branch of mecha- 

 nics, is yet intimately connected with the fcience of 

 hydraulics. 



The wheels of fome water-mills are furnifhed with 

 wings, float- boards, or fhelves, at their circumference, 

 with very little or no concavity , others are furnifhed 

 with a kind of ladles or boxes, which will contain a 

 confiderable quantity of water. In the firfl cafe, the 

 water acts upon the wheels principally by its fhock or 

 fall; in the fecond, by its weight. I Ihall firft fpeak 

 of wheels moved by the Ihock of the water. 



It has been proved by experience, that the more nu- 

 merous the wings or float-boards are in proportion to 

 the diameter of the wheel, the fatter it moves. To 

 wheels of twenty feet diameter, there are commonly 

 placed about forty float-boards ; but a greater number, 

 as for example fort\ -eight, would be ftill more advan- 

 tageous. To the wheels of thofe mills which are raifed 

 upon boats or rafts in rivers, there are ulually only 

 eight or ten floats ; but thefe wheels would have more 

 effect if they had fifteen or fixteen. 



When a wheel with wings or float-boards turns m 

 a kind of frame or cafe, fo as to prevent the water 

 from falling immediately into the general current, the 

 impulfe which it receives * from the water is about 



one- 



