84 The Drainage of Fens and Low Lands. 



according to the height of the water in the outfall, so as to 

 adapt the power of the engine to the varying lift. 



The work done by a scoop wheel is measured by the 

 quantity of water lifted in a given time. To ascertain this it 

 is necessary to know the head and dip of the scoops. The 

 " dip " is the depth the scoops are immersed in the water 

 when vertical in the trough. The " head " is the difference in 

 level of the surface of the water on the inner and outer side 

 of the wheel. The cubical quantity of water lifted each 

 revolution is ascertained by multiplying the mean circum- 

 ference of that portion of the wheel which is immersed by the 

 width of the scoops and by the length of the immersed part. 

 From this must be deducted the space occupied by the start 

 posts and scoops. Mr. Wilfrid Airy, who paid considerable 

 attention to the performances of scoop wheels, and published 

 a pamphlet,* giving the result of his investigations into their 

 working some years ago, considered that a deduction should 

 also be made for leakage of the water which falls from the 

 wheel in the " clearance," or space left between its sides and 

 bottom and the masonry. He estimated this as equal to an 

 amount due to the area of such clearance multiplied by the 

 velocity due to the head or height the water was lifted. 

 Thus, taking a wheel 30 feet diameter, with scoops 4 feet 

 wide, and having a clearance of J inch, the lift being 5 feet 

 and the dip 5 feet, the calculation for loss from this source 

 would be as follows : — The area between the scoops and the 

 masoniy would be 5*0 4- 5 'O 4- 4*0 X i '^^^ = 'S^S feet 

 The velocity due to the head 8 \/5 = 17*89 feet per second. 

 The area multiplied by this velocity gives 625*7 cubic feet a 

 minute, or nearly 1 1 per cent, of the whole quantity lifted. 

 This is greater than is found to be the case in practice. If the 

 theoretical velocity as found above be reduced 30 per cent., 

 to allow for the friction of the water against the sides of the 



* * Remarks on the Construction of the Course, and Design for a New and Im- 

 proved Scoop Wheel,* by W. Airy, 1870. 



