achieve neutral buoyancy, so it was never possible in ordinary operation to lift it to water level to allow a descending boat to float in. Instead, a masonry chamber ("cistern") was built with walls higher than the water level in the top pound and itself filled completely with water, so that even at its upper position the box remained below the surface. Abandonment The May 1799 test, above, occurred when a party of investors was aboard the vessel and they nearly suffocated before they could be freed. Work on the second lock was suspended (the third lock had not been started) and early in the following year an inclined plane, to carry boats’ cargoes in wheeled tubs, was built instead. Eventually a flight of nineteen locks on a longer alignment up the slope was constructed, with a Boulton & Watt Steam Pumping Station, capable of lifting 5,000 tons of water in 12 hours, used to recirculate the water. Other installations In April 1815 the Regents Canal Company built a double caisson lock (or "hydro-pneumatic lock") at the site of the present-day Hampstead Road Lock, north London. The designer was military engineer William Congreve. Here the motivation was, principally, water supply problems but also to effect a quicker passage of vessels, as those going in opposite directions could pass in the lock. The caissons—always submerged, as at Combe Hay—were without bottoms and were arranged so that their sides dropped into deep underwater channels formed by "minor walls" inside the main walls. The vertical movement of the two caissons was effected by a balance pipe ("the channel of communication") passing under the lock floor between the two caisson chambers (but with a slight upturn to reach above the water levels), so that causing an increase of water level in one caisson ("the water of compression") displaced air through the pipe, thus forcing a corresponding decrease in the water level in the other. This increased the buoyancy of the latter caisson, which accordingly rose as the first sank. A connecting underwater chain passing through rollers, while not in itself doing any work, controlled the relative positions. Boats were admitted through double gates; the inside ones on the caisson "adjusted to fit close to those of the outer". Congreve's patent then envisaged that having achieved "absolute equipoise" (neutral buoyancy) a rack-and-pinion, removable weights or a small winch could overcome inertia and move the caissons.