PRODUCTION OF MECHANICAL EFFECT. 439 



convenient and non-wasteful the accumulator 

 whether Faure's electric accumulator, or other 

 accumulators of energy hitherto invented or to be 

 invented which might be used to store up the 

 energy yielded by the tide-mill during its short 

 harvests about the times of high and low water, 

 and to give it out when wanted at other times 

 of six hours. There may, however, be a dozen 

 places possible in the world where it could be ad- 

 vantageous to build a sea-wall across the mouth 

 of a natural basin or estuary, and to utilise the 

 tidal energy of filling it and emptying it by means 

 of sluices and water-wheels. But if so much could 

 be done, it would in many cases take only a 

 little more to keep the water out altogether, and 

 make fertile land of the whole basin. Thus we 

 are led up to the interesting economical question, 

 whether is forty acres (the British agricultural 

 measure for the area of 162,000 square metres) 

 or 100 horse-power more valuable. The annual 

 cost of 100 horse-power night and day, for 365 

 days of the year, obtained through steam from 

 coals, may be about ten times the rental of forty 



