be selected for the line which offers the fewest possible inequalities, and those the smallest in amount.
Canals possess advantages over all other roads, in being able to support an almost unlimited amount of load. The pressure on the wheels of carriages on a railroad is limited by the strength of the rail, and is seldom more than about three tons upon each wheel. The pressure on the wheels of carriages on a turnpike road is limited by the strength of the crust of the road. On the broad wheels of the heaviest waggons the pressure never exceeds two tons; but the weight capable of being sustained by a canal is only limited by the magnitude of the boats which the breadth of the canal allows to float upon it. In fact, the weight of the boat and its cargo is equal to the weight of the water which is displaced by the part of the boat immersed in the canal.
In considering the power of traction or impulsion necessary to move a body, whether on a canal or on a road. I must carefully distinguish that force which is requisite to put the body from a state of rest into a state of motion, from that which is requisite to sustain the motion when once imparted to it. If a body were sustained by a surface perfectly level and perfectly smooth, so as to oppose no resistance whatever to motion upon it, without friction, the body once put in motion by an impulse would continue to move for a considerable time, without the application of any further impulsion or traction. But such a surface as is here supposed has no practical existence: although, as already explained, it is the object of roads of every kind to approach as near to this imaginary limit as possible.
The continual power of traction necessary to sustain the motion of a body, therefore, arises from the re-
