While the grand ridge here in question presents no difficulty to powerful locomotives, the position is altogether different with canals fed by streams of water that will not flow up-hill. In the case of the Birmingham Canal, specially referred to in the extract just given, there are three separate "levels." The lowest is 209 feet, and the highest 511 feet above sea level. Boats doing the cross-country journey, or passing between Birmingham and the coast, would have to overcome such heights as these by means of locks, lifts or inclined planes. Here we have a very different proposition from that which is presented by canals on the flat surfaces of Holland, Belgium and North Germany--with, also, their abundant water supplies, from great rivers or otherwise--whereas the upper levels of the Birmingham Canal are kept filled with water only by means of costly and powerful pumping machinery, supplemented by reservoirs. When the original builders of canals had to cross the grand ridge, or any other elevation over which they required to pass, they sought to economise water consumption and to keep down both cost of construction and working expenses by making the locks on the top levels only just large enough to pass boats of a small size. The dimensions of any boat making a through journey are thus controlled by those of the smallest lock through which it would require to pass. On lower levels where the water problem did not arise--or not to the same degree--the locks could well be made larger, to accommodate larger boats engaged only in local traffic. {303}The material differences in cost of construction and operation between waterways on a low and uniform level and those crossing considerable eminences, by means of locks, were well recognised by Parliament when approving the lists of tolls to be paid on different waterways. On the Aire and Calder the minimum toll, if a boat passed through a lock, was fixed at five shillings. On the Rochdale Canal the minimum toll for a boat crossing the summit level was ten shillings.[49] The reason for this difference is that whereas the Aire and Calder navigation is but little above sea level throughout, the summit of the Rochdale Canal is at a height of 600 feet above sea level, and is crossed by means of ninety-two locks in thirty-two miles. The reader will see, therefore, that the want of a common gauge in the construction of artificial waterways, mainly designed, at the outset, to supply the needs of particular districts, may often have been due to more practical reasons than simply a lack of combination or a difference of view on the part of canal constructors, the problem of gauge on canals built at varying elevations, and all depending on water supply, being entirely different from any question as to the gauge or the running of railways on the same or similar routes.