554

HISTORY OF ENGINEERING.

Book I.

through which this sheet of water is spread, and the obstruction presented by friction to itsdescent, may in some degree account for its inclined position, though on the other hand itappears, that when the floods at Watford affect the wells in that neighbourhood, the sameinfluence extends to those in London a few hours afterwards ; and a steam-engine employed topump water from a well near Watford lowers that in all the wells to a considerable distancearound it.

The Reverend J. C. Clutterbuck has shown that if a line were drawn from a point3 miles south of the Colne at the level of that river, which is 170 feet above Trinity highwater-mark, to the mean tide level in the Thames below London Bridge, the dip would be180 feet in 14 miles, or an average inclination of 13 feet in each mile; the uniformity ofthis inclination is proved by the wells ac Ilendon union workhouse, Crieklewood, and atKilburn, in which 20 years ago the water stood much higher than at present, the exhaustingof the wells in and about London by means of powerful machinery having reduced itslevel considerably. The level at which the water stands in the chalk is subject to periodicalchange; there is also another supply to that portion of the London Basin beneath theplastic clays, which is not fed by infiltration, but probably by means of hydrostatic pressurefrom higher sources.

This periodical change observed in the height of -water in the chalk, is called the oscil-lation of the water level, and is caused by an irruption of rain water, which finds its wayfrom the surface of the London and plastic clays into the chalk through fissures, and atlast arrives at the sand above the plastic clay formation.

The water level line generally inclines about 10 feet in a mile when most depressed, andafter heavy rains, when the clays throw their water from their surface, the irruption of thewater may be seen at the outcrop of the sand of the plastic clay formation; the level willthen be raised in proportion to the quantity of water which passes through the sand intothe chalk beneath it, the elevation of level extending towards the Colne, in a ratio increasingwith the distance from the river ; the fixed summit will remain unaltered, until the levelat the point of irruption has attained an elevation at which the water can flow towards thesouth. After a period of dry weather, the level will decline in the same ratio as it has risenuntil it regains the original level with a regular inclination.

The same effects may be observed in all chalk districts where no streams are found on thesurface; the valleys which lie between the rounded bills seldom exhibit any running water,but wherever wells are sunk and the water-bearing stratum is arrived at, an abundantsupply may be obtained. Taking the level of the water in the wells on the highest ridge, anddrawing from thence a line to the surface of those found in the lowest, or that of the riverinto which they drain, we find that the surface of the water of all the intermediate sinkingscorresponds with the slant line of drainage within the chalk, and if by means of pump-ing or tunneling, we exhaust the water on any part of this line, all the supplies above arcaffected. In whole districts the wells have by this means been rendered dry, and it isonly by sinking them deeper, or out of the influence of such effects, that water can beagain obtained.

Canals .—Where rivers abounded with shoals, wc find it a very early custom to contractthe channel, and thus obtain deep water, or sufficient to float small vessels. Another mode,called flashing, was applied to shallow streams, and consisted in penning up for a time theriver itself within reservoirs, which had openings cut in them, to allow of the passage ofboats.

Wears and sluices were made use of when the rise in the bed of the river was con-siderable; there was an opening in the wear, closed by a flood-gate, which allowed thepassage of vessels r this commonly consisted of two abutments of stone, projecting fromeach bank, in which was a groove to receive a plank, or timber, which was let down anddrawn up at pleasure; or a sill was laid at the bottom, with a groove, into which per-pendicular planks were dropped, and maintained at top by one or two strong horizontaltimbers, resting on the abutments. To open this it was necessary to draw out one plank orpaddle at a time, and then remove the horizontal timbers ; after the boat was hauledthrough, the whole was replaced, and the water in the head or reservoir again permittedto rise.

Inclined planes and rolling bridges were also in use for transferring boats from onepond to another across a wear.

We have already seen that in Italy , France , Holland, and Germany , canals were esta-blished at a very early period, and it is remarkable that we do not find much attention paidto the subject in England, until about the middle of the sixteenth century, when it wasproposed to render the Isis and Avon navigable by means of sasses, and then to unite thetwo streams by a canal of about 3 miles in length ; but nothing of importance was under-taken until James Brindley connected Liverpool with London , Bristol with Hull, andseveral other districts by canals; he was a native of Tunstal in the parish of Wormhill inDerbyshire , and the date of his birth is said to be about the year 1716. He served hisapprenticeship to a millwright at Macclesfield , where he acquired a thorough knowledge