The drop at Rowley Bottom was approximately 135ft. This would conventionally have needed 22 locks of about 6ft drop each. Because this canal was particularly short of water along the top level, the only way to ensure there would be sufficient water to operate the locks would have been to pump water from the bottom of the flight back to the top. This would have been expensive in machinery and fuel, so a cheaper option was sought.
Robert Weldon's Hydrostatick Caisson Lock seemed like the ideal answer because it lost no significant amounts of water during its operation. The greatest height through which one of these machines could operate, given the strengths of the materials of the day, was about 45ft.

The committee of the S.C.C. decided to use 3 caisson locks instead of 22 conventional ones, but the possibility of the caisson proving to be failure was, no doubt, a major consideration in the planning. It was probably the reason behind the decision to place these caissons at Rowley Bottom so that if a lock flight should have to be built later, it would be easy to back-pump because the top and bottom locks would be close together.
How the Caisson Lock worked