INLAND NAVIGATION.

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G. Paddles in the gates, to reduce the water to the lower level.There is a cbain-bar, run with lead in a course of stone, set at waterlevel.

A Tunnel is a perforation, for the passage of boats through a hillfor some or all of the following reasons: To save the purchase of landin going a long way round, or to save the labour, in cutting a canal thelength required in going that round, or when, in some instances, a levelcannot be found.

The manner of cutting a tunnel through a hill or mountain variesaccording to the quality of the soil; whether stone, loose stones, earth,ores of different kinds, chalk, &c. In either cafe the progress is veryflow and expensive. From three to seven guineas per yard, forward,have been paid for labour only.

The tunnel at Sapperton is rock-stone, consequently it was obliged tobe blown up, piece-meal, by gunpowder. It was intended to haveformed an arch, over head, out of the solid rock, and to have avoidedturning one by hand; but this not being practicable, it became thereforenecessary to work an arch of stone on centres, to prevent the loose stonefalling in. The length of this tunnel is about two miles and threequarters: it has air-holes, or vents, at various places, of different mag-nitudes ; some of the openings being left, which were first made to takeup the refuse, or fragment stone.

The celebrated tunnel through Harecastle-hill, Staffordshire, was cutunder the direction of the ingenious Mr. Brindley. As the soH waschiefly loose stone, he caused the bed of the canal to be rammed withwell-tempered stiff clay, a foot thick, and turned an arch above withbricks; taking care to have the spandrells worked close every yard, asthe arch proceeded; which was also pinned up to the top at the samedistances. These precautions have had the desired effect, no accident

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