1 16 The Drainage of Fens and Low Lands. 



by the water passing down the drain, instead of being less than that 

 due to the theoretical discharge as measured by the wheel, was 

 about 21 per cent, greater. Messrs. Watt observe as regards this: — 

 " We took every care possible in getting at the true velocity. The 

 drain — mean width 28 feet 9 inches by 4 feet 9 inches deep — was 

 new, and entirely cleared out and free from weeds or obstructions. 

 We put three floats, which were regulated by tubes to sink from the 

 surface to within a few inches of the bottom, one in the centre, and 

 one on each side; they just projected out of the water and had a 

 feather put into the top so that there was the least possible obstruc- 

 tion from wind. All the floats practically went down the stream at 

 the same pace, but if there was any difference the mean of the 

 advance was taken. We have therefore every reasonable confidence 

 that the number of cubic feet that passed down the drain is not far 

 from being correct. As regards, the allowance of 10 per cent, 

 usually allowed for leakage, this is altogether in excess. The breadth 

 of the ladles in the wheel is 40 inches, the clearance on each side is 

 \ inch ; the area therefore for leakage is only 2 J per cent, upon the 

 width of the ladle. Then the head which would cause any leakage 

 is very small, being only that between the level of the water in one 

 space between the ladles and the level in the next succeeding space ; 

 moreover, these ladles act something like a pocketed piston having 

 grooves m. it, and the general motion of the wheel, and the upward 

 current of the whole mass reduces this leakage to a minimum. In a 

 broad wheel it is practically imperceptible. As regards the dip, we 

 found by very careful inspection that the ladles were more full by at 

 least I foot than was represented by the mere dip ; the fact being that 

 the velocity imparted to the water by the outer diameter of the ladles, 

 combined with the gradual contraction of the drain to a line passing 

 through the centre of the wheel increases the velocity of the water, 

 the result being that the cavities between the ladles are filled almost 

 to the inner lining, so that the quantity lifted in the 100 Foot 

 wheel would be represented by 5 feet depth in the ladles instead of 

 4 feet. At Podehole, where the feeding sluice or shuttle is depressed 

 owing to the great depth of water in the feeding channel, it allows 

 enough water to pass with the velocity due to the head, so as actually 

 to fill the ladles quite full. The calculated discharge of the wheel, 

 with 5 'feet dip, and the other figures as given, would then work out 

 at 6089*23 cubic feet, or equal to 76 per cent, of efificiency, as 

 against 6288*11 cubic feet and efficiency of 78*62 per cent as 



