CANAL NAVIGATION. 77 



a similar announcement, the waves moving with a velocity of 50 or 60 miles an 

 hour, breaking in a heavy ground-swell upon a remote beach. 



The effect of the fonnation of waves with a greater velocity than that of the 

 vessel in forming an anterior accumulation, a posterior depression, and a stem 

 surge, as shewn in Sects. IV. — VI., furnishes a satisfactory explanation of the 

 phenomena of shallow navigation. 



It is well known that it is extremely difficult to row or sail weU in shallow 

 water ; this is the consequence of increased section of resistance from being be- 

 hind the wave. But if by strong impulse the vessel were placed on the wave, it 

 would then become easier than in deeper water at the same velocity. In water 

 two feet deep, it is very difficult to row 4 and 5 miles an hour, and compara- 

 tively easy at six, the one less, the other greater, than the velocity of the wave. 

 It is also found that in shallow water the stem of the vessel invariably takes the 

 ground while the bow remains free, although drawing at least equal depth of 

 water ; this is the direct result of the depression between the anterior wave and 

 the stern surge, as in Sects. IV. & V. 



The difference of the immersion of a vessel below the surface of the fluid 

 when on the summit of its wave, or in the depression behind it. Sect. IV., ac- 

 counts satisfactorily for a long list of phenomena otherwise inexplicable. It has 

 long been observed, that a vessel in motion wUl take the ground in water that is 

 more than sufficient to float her when at rest, and it is equally well kno^vn that 

 there are circimistances in which a vessel when in motion, will pass over a shal- 

 low without touching the ground, while it is not covered with the depth of water 

 necessary for her statical floatation. Now, it is obvious, in the first instance, that 

 if the vessel move with a velocity less than that of the wave, the prow being on 

 the anterior wave, and the stem in the succeeding depression, the vessel will take 

 the ground, and probably carry away her helm ; while, if the vessel were poised 

 on the summit of the wave in a horizontal position of equilibrium, and with the 

 diminished immersion due to the velocity, much less depth of water would be 

 sufficient, than with the slower motion, or even than is required to float the vessel 

 in a state of rest. I have seen a vessel in five feet water, and drawing only two 

 feet, take the ground in the hoUow of a wave, having a velocity of about 8 miles an 

 hour, whereas at 9 miles an hour, the keel was not within four feet of the 

 bottom. 



A highly scientific friend of mine, Mr Smith of Philadelphia, member of the 

 Franklin Institute, has frequently observed in the Dutch canals, boats carrying 

 passengers, kept in floatation by communicating to them rapid motion in shallow 

 parts of canals, where they would otherwise have taken the ground, thus taking 

 the advantage of moving on the summit of the wave. 



I have also been informed, on the best authority, of the following fact, ob- 



