TIDAL POWER. 
through suitable sluice-gates. In one of these basins, called the upper, 
the water-level is never allowed to fall below one-third of the tidal range, 
while in the lower basin the level is not allowed to rise above one-third 
of the tidal range. The working head then varies from 0.53 H to 
0.80 H, and operation is continuous, as indicated in Fig. 4, which shows 
the cycle of operations. The upper basin is filled from the sea through 
the appropriate sluice-gates from A to B, and the lower basin discharges 
into the sea from OC to D. For a given total basin area and a given | 
tidal range the output is only about one-half that obtained in system 
(a), and one-third that obtained in systems (b) and (c), so that, except 
where the physical configuration of the site is particularly favorable, 
the cost per horse-power is likely to prove very high. 
(e) Two tidal basins of approximately equal size are used. Turbines 
are installed in the walls dividing the sea from each basin. Fig. 5 
shows the cycle of operations. From A to B the upper basin discharges 
through its turbines into the sea. From B to E the sea enters the lower 
basin through its turbines. The upper basin is filled from the sea 
through its sluice-gates between C and D, and the lower basin is emptied 
through its sluice-gates from F to G. The head varies from 0.25 H to 
0.62 H, and the output is some 25 per cent. greater than in system (d), 
but the number of turbines required is much greater than in (d). * 
It is possible, at the expense of additional complication, to arrange 
in each of these systems that the head shall be maintained constant 
during any one working period, but since this means that the working 
head must then be the minimum obtaining during the period, a loss of 
energy is involved, with a great additional cost of construction and 
‘complication in manipulation, and with little compensating advantage. 
The great difficulty in developing a tidal scheme as compared with 
an orthodox low head water-power scheme arises from the relatively 
great fluctuations in head. In any scheme in which the working head 
is a definite fraction of the tidal range, the working head at spring tides 
