524 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



which was caused to rise and fall to suit the tides, and the power was trans- 

 mitted by either bevel wheel according to which way the water wheel was 

 running, the other bevel pinion being thrown out of gear. By these means the 

 machinery in the mill was always driven in one direction, in spite of the 

 reversal of the water wheel at each turn of the tide. 



The movable frame, with the water wheel and gear, weighed some 20 tons 

 and the bottom of it was extended to form a kind of shutter, which filled 

 up the opening underneath the wheel race, all the water flowing into or out 

 of the pound being thus compelled to pass through the wheel. 



Another type of wheel was devised to overcome the drawback of having 

 to move up and down with the tide. This wheel was fitted with hinged 

 floats, which arranged themselves across the stream at the bottom of the 

 periphery, while they traveled through the water edgeways during the re- 

 mainder of each revolution. With floats of this type the wheel was fixed, 

 and the tide gradually rose over it until in some cases complete immersion 

 took place. 



An arrangement of the sluices was also adopted to compel the water to pass 

 through the wheel in the same direction, whether flowing in or out of the 

 pound, thus doing away with the need for reversing gear between the water 

 wheel and the machinery to be driven. 



These wheels must have been very inefficient, as the loss of power caused 

 by the drag of the upper portion when covered was serious, and the design 

 was soon discarded. 



Following these earlier mills came the more recent examples, many of 

 which are still in existence, while a few of them may be seen in operation. 

 The older mills aimed at using the current of water caused by tidal action 

 and advantage was taken of the flow in either direction. The more modern tide 

 wheel is arranged to operate with considerable fall, and only develops power 

 when the water is flowing out of the pound. 



The undershot wheel with straight radial floats is usually adopted, and the 

 mill is started at half ebb or a little later, work being continued for about 

 five hours, or until the water rises under the wheel and chokes the tail race. 

 These arrangements give only five hours of working during each tide. 



Listing the tidal mills that actually exist, we have a mill at 

 Woodbridge of 10 to 12 horsepower, one at St. Osyth of 20 horse- 

 power, and one at Walton-on-the-Naze of 85 horsepower. These 

 are all small powers, working on a low range of tide and with 

 only a single, small, natural reservoir that allows of only a partial 

 use of the tidal power for a comparatively short period of time, 

 but Mr. Horsnaill shows that if modern turbines were installed at 

 the plant at Walton-on-the-Naze and the power was used to develop 

 electricity, instead of grinding corn, it would show up as a com- 

 mercial development somewhat better than gas power, in spite of 

 the heavy outlay for storage batteries which would be necessary 

 at a plant situated as Walton is. 



The number of proposals for tidal plants is very considerable, 

 and while I think I should not take up your time by discussing all 

 of them, it is worth while to examine a few of the more serious 

 ones. 



