123 



sion, and their power would increase in proportion to the 

 area of their immersed surface. A powerful undershot 

 wheel requires not only to be of large diameter, but also of 

 great breadth, to give sufficient area to the acting floats. 

 These, on the contrary, may be made exceedingly narrow, 

 and there would be no advantage in their breadth exceeding 

 one-eighth of their diameter. 



These are among the useful applications of such formed 

 wheels, when impelled by fluids. We have now to consider 

 them as applied to give motion to fluids when impelled by 

 some other source of power ; and as in the first instance the 

 rotation of the wheel upon its axis was the quickest when the 

 angle of the vanes with the plane of rotation was the smallest, 

 so I have found that the motion of the propelled fluid is 

 greater in proportion as the angle of the vane with the plane 

 of rotation is greater, within certain limits. For if the wind 

 acting in the direction a b against the c 

 body c D, placed obliquely to its course, 

 would impel the body towards e, so I 

 conceive will the body c d, striking the 

 air in the direction a b, have an equal 

 tendency to impel it towards e. Thismay 

 be illustrated by a circular vessel filled 

 with water, in which draw any flat body 

 held as in the position a frequently to- 

 wards B, and the water will soon assume 

 a rotary motion in the direction of the 

 arrow, more or less quick as the plane 

 of the body makes a greater or less 

 angle to the dotted line a b. 



This appears to me nothing more or less than the action 

 of sculling, by which the oblique action of the oar causes a 

 quantity of water to recede from the stern of the boat. 



