… and the perils thereof:
Having reached the highest level of the great table-land, we traversed a space of fifteen miles without a lock; and here a curious phenomenon, illustrating the incompressibility of water, arrested our attention. About every twenty or thirty minutes, the horses are obliged to stop for five or six minutes, to take breath, the cause of which was this: — The velocity of the boat impelled the water in the canal with such force that it gradually rose so as to approach the summits of the banks, when it began to recoil, so as actually to form a back-water or stream, when the horses were unable to make head, and therefore stopped till the equilibrium of the canal was restored.
James Johnson MD A Tour in Ireland with Meditations and Reflections S Highley, London 1844
Irish waterways history 