l'RKSll)i;X I lAL .\l)l)Ki:SS — SKCTION A. 35 



cHff'iculty that floats must have an imnicnsc area in order to give 

 power of any eonimereial value, while the foree of the waves 

 dining" a storm may wreek any machine which man can devise. 



A certain amount of success has actually been attained in 

 the utilisation ol tidal cuitl;} , antl some important sclunies have 

 been brought forward. The tides i)la\- an imixntant ])an in 

 modern harbour arrangements, and the familiar example of tlie 

 barges floated up and down the jhames is an example of the 

 use of tidal energy. it is a matter of great interest to note 

 that loo years ago the water supply of London was mainly 

 derived from the tidal energy of the Thames. Owing to the 

 obstruction caused by the arches of ( )ld London Bridge, there 

 was a fall of foiu* feet, and this supi)lied the motive ])ower of 

 undershot water-wheels and pum])s placed under some of the 

 arches. 



The usual method suggested is to impound the rising tide 

 in a i3ond or reservoir, and on the ebb of the tide to utilise the 



Fig. 19. — Compressing air into receivers by means of the surf. 



flow to drive water-wheels or turbines. But the great obstacle 

 to success is, that power is only available during the ebb of the 

 tide, and even then the head for continuous out]nit is not more 

 than one-third the range of minimum tides, wdiile the head of 

 water is continually varying ; the falls being low, and the ratio 

 of maximum to minimum head being great, the conditions are 

 very unfavourable for the efficient working of a hydraulic tur- 

 bine. Further, owing to the enormous cost of masonry dams, 

 it is necessary to take advantage of the natural configuration 

 of a coast, to render any scheme worthy of a thought. 



It is a fact not generally known that there .'ire three tidal 

 corn-mills working, or at any rate until recently, in Lngland. 

 One of these, at Walton-on-Naze, is shown in Fig 20. The 

 ebbing tides drives an old-fashioned undershot waiter-wheel, 

 under an average head of 5 feet, for a period of about iive hours 

 per day. Many proposals have been made to utilise both the 

 flowing and ebbing tide; such a one is shown in Fig. 21. One 

 tidal scheme at Havana is said to be successful, and to be still 

 doing useful work. Another w-as actually installed at Cowes, 



