Note 179 — Chapter XIV · DOCK AND PIER WORKS. A.D. 1831—1859. ÆTATIS 26—54.
Reading edition
The advantage of forming an air-chamber in the lower part of the gate, and allowing the water to enter above it, is that when the size of the air-chamber is properly adjusted to the weight of the gate, there need not come on the wheels, while moving, more than a trifling amount of weight.