Greenock is situated at the bottom of a sloping hill or ridge, the top of which is upwards of 500 ft. above the level of the Clyde. To the top of this ridge Mr. Thom has conducted, along the summits of other ridges, from a distance of six miles, a copious supply of water, not only for all the ordinary purposes of the town, but for driving machinery. The water is collected into reservoirs, at the distance of several miles from Greenweck; and each of these reservoirs is capable of containing a supply for the consumption of the inhabitants, for more thansix months "so thatnot only the surplus waters of one wet season may be retained for supplying the dry season of the same year, but the surplus of several wet years may be stored up to supply a drought of several years' duration, should such ever occur." The water is brought to the summit of the hill in such a quantity as to supply 1200 cubic feet per minute, which Mr. Thom estimates, on a fall of 30 ft., as equal to a Bolton and Watt's steam-engine of fifty horse power. In what manner the water is applied in succession to the mills, will be easily understood by the inspection of fig. 68., which is an imaginary section from the summit of the hill (a), through all the water-wheels, to high watermark on the Clyde (b). The line of wheels dotted, shows that there are two series of situations for mills. The same aqueduct supplies water for the inhabitants at the rate of 2 cubic feet per head per day. The filteration of the water is effected in filters, invented by Mr. Thom, which are then formed. "Each filter is 50 ft. long, 12 ft. wide, and 8 ft. deep. The water is made to percolate through them, either upwards or downwards, at pleasure. When it percolates downwards, and the supply of filtered water becomes sensibly less (which, after some time, must happen to every filter, by the lodgement of sediment), then, by shutting one sluice and opening another, the water, in made to pass upwards with considerable force, and, carrying the sediment along with it, to fall into a waste drain made for that purpose. When the lodged sediment is thus removed, and the water begins to run clear, the direction of the sluices is again changed, and the filter operates as before." Among the most ingenious arrangements connected with these works are, the self-acting sluices to the different reservoirs, by which both the reception and the delivery of water are regulated to the greatest nicety, with a view to the most rigid economy of the fluid; but it would be deviating too far from the direct objects of our work to describe them (We have sent the pamphlet (A brief Account of Shaw's Water Scheme, &c., Greenock, 8vo, pp. 88., 1829), in which these works are described, to the editor of the Mechanic's Magazine, in whose most valuable and widely circulated work they will probably be recorded for the benefit of engineers.). What has been effected by Mr. Thom should encourage gardeners, and others connected with the improvement of landed property, never to think any end, which is at all desirable and possible, too difficult of attainment.
