bottom—­equal in durability to any work, ancient or modern.  It is about half way cut through the solid granite rock, which in that part furnishes a natural wall.  My friend had watched its progress, and gave me many interesting details of the engineering processes employed:  among others, the tremendous application of steam and gunpowder.  An engine bored holes in the rock fifteen feet deep and twelve inches in diameter; and these were so placed, and in such numbers, that at a single blast 170 tons of granite were blown into the air—­an operation hardly conceivable.  This canal leaves the town in a westerly direction—­being, at its outset, about a quarter of a mile from the Merrimac, but gradually approximating for a quarter of a mile, until it touches and unites with that river.  Between the two, is one of the prettiest of public walks, ten feet wide, having rows of trees on each side, and terminating in a point; being the end of a splendid granite wall, at its base thirty feet thick, and tapering to half the thickness, dividing the natural from the artificial stream.  Here we come to a point of great interest:  on the right is an artificial dam across the river, with two sharp lines at an angle of sixty-seven degrees, the point meeting the stream, thus stopping the waters, and insuring a supply for the reservoir, while it forms a cascade of about twenty feet.

My friend gave me a very graphic description of the opening of the works.  The whole was built in a cofferdam, quite dry, and the opening was a holiday.  Every spot within sight was covered with spectators, for whom the engineer had contrived a surprise.  The works used in keeping the water out of the reservoir, and protecting the new dam, were undermined, and charged with gunpowder.  At a given signal, the train was fired, and in an instant the whole blew up; and when the smoke cleared away, the fragments were floating down the Merrimac, and the canal full of water.

On the left from the point, the egress of water is regulated by flood-gates of a superior construction.  The building crosses the canal, and contains seven huge gates, which are raised or dropped into their places by beautiful machinery.  To each gate is attached an immense screw, which stands perpendicularly, twenty feet long and ten inches in diameter.  At its upper end, it passes through a matrix-worm in the centre of a large cog-wheel, lying horizontally The whole is set in motion by the slightest turning of a handle; and here I saw the application of the Turpin Wheel I spoke of before—­no engine or complication, but a wheel fifteen feet in diameter, fixed horizontally, submerged in the stream, receiving the falling waters, and thus rapidly revolving, and by a gear, giving motion to the machinery for raising or lowering the immense gates, stopped or set going by merely turning a stop-cock, and requiring no more force than an ordinary water-cistern.