CANAL NAVIGATION. 81 



vessel, so that an additional force will only increase the magnitude of the wave, 

 and thus adding to its velocity, prevent the vessel from penetrating through or 

 rising upon it. What I have stated accords perfectly with the experiments I 

 have given, and with the experience of practical men. In these experiments it 

 will be seen, that immediately behind the wave large increments of force are not 

 accompanied with similar increase of velocity, while at the instant of passing the 

 wave, the velocity makes with a given force a sudden transition to a higher ve- 

 locity. And so in experience it is found very difficult, or quite impracticable, to 

 pass the wave with a slowly accelerating motion, A sudden impulse from a low 

 to a high velocity is found to be the easiest mode of effecting the change, and the 

 method used is not to make the change immediately from a very high velocity 

 behind the wave, to a very high velocity before it ; but when it is intended to start 

 a vessel over the wave, the speed must first be allowed to diminish, until it become 

 nearly half of that of the wave, by which means the anterior wave is allowed to 

 pass away with its proper velocity from before the bow of the boat, the stern 

 surge is permitted to overtake it, and fill up the cavity behind the wave, and the 

 surface of the water is reduced more nearly to a plane ; and if now, in these cir- 

 cumstances, a sudden impulse be communicated to the vessel, it will easily attain 

 a velocity greater than that of the wave. 



A change in the depth of a canal produces a very marked change in the re- 

 sistance in the vicinity of the wave. Certain portions of the Glasgow and Ar- 

 drossan Canal have their depth suddenly increased, and when a vessel that has 

 been moving on the summit of the wave reaches these points, it finds its velocity 

 diminished, in consequence of the wave having acquired a greater rapidity due to 

 the increased depth. 



The wind acting on the surface of a long canal has a force sufficient to send 

 away so much of the fluid from one of its extremities, and accumulate it towards 

 the other, that in a canal running about twenty-five miles in a direction east and 

 west, a strong westerly wind will occasion a difference in depth of two feet, 

 being at the east end one foot more, and at the west end one foot less than five 

 feet, the average depth of the canal. It is observed in this case, that to maintain 

 the vessel over the wave, a greater force is required at the deeper end, and a 

 lessened force at the other. 



In canals where the power of horses is applied to vessels navigating at 

 high velocities, much inconvenience will be experienced, and much loss incurred, 

 by giving to the water a depth which wiU produce a wave of so high a velocity, 

 as to approach the limit of the available speed of horses. When the depth ex- 

 ceeds seven or eight feet, the struggle to conquer the wave wiU take place at or 

 above nine miles an hour, being a velocity at which horses cannot advantageous- 

 ly exert much force above what is required for the transport of their own bodies ; 



VOL. XIV. PART I. L 



