Page:Colymbia (1873).djvu/67

Rh velocity. The tides in this region of the ocean only raise the water a few feet, but that is amply sufficient to provide the necessary power to move the huge caissons and their levers.

As the height of the tides differs every day, increasing towards the period of spring-tides, and again declining towards the period of neap-tides, there are contrivances for meeting these variations. A small machine, separately connected with the water and moving up and down with the tides, acts on machinery so contrived that when the tide rises high it shortens the perpendicular shaft of the large caisson to which the lever is attached, and so counteracts the effect of the greater height of water, and lengthens this shaft when the caisson sinks lower, than the average: or the same effect is produced by a similar machinery that lengthens the horizontal arm of the lever of the larger caisson as the tide rises higher. When this plan is adopted, the basin in which the large caisson floats is necessarily longer than when the first plan is followed, in order to admit of the to-and-fro movement of the caisson. Another plan is to supply both of the basins in which the caissons float not directly from the sea, but at second-hand, through reservoirs in immediate connexion with the tide, whereby both the height of the water in the basins and the time of its entrance can be regulated. The engines are not necessarily close to the sea. In many instances they are some miles inland, the tidal water being conveyed to them by canals. The Colymbians avail themselves of this simple motive power to work all their machinery, and they use machinery for all their manufactures to a much greater extent than even we do.

The irresistible power of the ocean's tides, as regular