HISTORY OF

362

opened; when the boat, being upon a level with the lower water, isable to proceed upon its passage. For ascending, or locking up, the boatbeing in the lock, the lower gates are shut: the lock is then filled bythe upper water, first by the sluices, and finally by the gates beingopened : thus the vessel is on a level with the upper water, and mayimmediately proceed. This operation takes but little time; the wholewill generally be completed in five minutes. As the gates are strainedin proportion to the depth of water supported, when the perpendicularheight of the water is twelve or thirteen feet, more locks than one be-come absolutely necessary. Thus if the fall of water be seventeen feet,two locks are required, each having eight feet six inches fall; and if thefall be twenty-six feet, three locks are necessary, and so in proportion,few locks having less than six feet, or more than ten feet fall.

The side-walls of a lock must be of the greatest strength, whether ofmasonry or timber; and where the natural foundation is bad, especialcare must be taken. Land-ties should be used, in order to resist thegreat pressure of the earth from behind. These may be furtherstrengthened, if of masonry, by making the sides of the lock of a circularform; the walls also should be well backed with stiff clay, as well toprevent the wasting of the water, as the works being secretly under-mined.

Description of Plate 1. which foews the Plan, &c. of a Lock for

a Canal.

A. The upper water of the canal.

B. Lower ditto.

C. Chamber of the lock.

D. The platform on which the upper gates are hung.

E. The lower ditto, shewing the manner of construction.

F. Sluices through which the water passes into the chamber, to raiseit equal with the upper level.

. S

G. Paddles