S^ MR RUSSELL'S RESEARCHES IN HYDRODYNAMICS. 



and in such a case, in order to prevent any irregularity in the application of the 

 force from permitting a wave to pass' on before the vessel, the velocity will re- 

 quire to be maintained at twelve or thirteen miles an hour. Now, when the depth 

 is so much less as to comprise the velocity of the wave within the limits of mo^ 

 derate exertion on the part of the horse, the higher velocities are gained without 

 injury to the animal, and a rate of nine or ten miles an hour is maintained with 

 certainty. 



Two or three years ago, it happened that a large canal in England was 

 closed against general trade by want of water, drought having reduced the depth 

 from twelve to five feet. It was now found that the motion of the light boats 

 was rendered more easy than before ; the cause is obvious. The velocity of the 

 wave was so much reduced by the diminished depth, that instead of remaining 

 behind the wave, the vessels rode on its summit. I am also informed bv Mr 

 Smith of Philadelphia, that he remembers the cu-cumstance of having travel- 

 led on the Pennsylvanian canal in 1833, when one of the levels was not fully sup- 

 plied with water, the works having been recently executed, and not being yet per- 

 fectly finished. This canal was intended for five feet of water, but near Silvers- 

 ford the depth did not exceed two feet, and Mr Smith distinctly recollects having 

 observed to his astonishment, that, on entering this portion, the vessel ceased to 

 ground at the stern, and was drawn along with much greater apparent ease than 

 on the deeper portions of the canal. 



In a canal where the velocity due to the wave is nearly coincident with that 

 velocity of transport which is found to be most desirable for the species of traf- 

 fic, (for example, ten or eleven miles an hour, as has been the case recently on 

 the Forth and Clyde Canal, whose maximum depth is about nine feet), in such a 

 case this velocity is either impracticable or very disadvantageous, giving rise to 

 a constant struggle with the wave. To solve the problem, however, the follow- 

 ing mode has been found efficient : one mile is performed at the rate of eight miles 

 an hour, being so far behind the wave as to suffer little from its accumulation on 

 the prow, and at the end of that mile the boat is brought to the bank where the 

 canal is shallow, and by starting the horses to a gallop of 13 or 14 miles an hour ^ 



for another mile, being in advance of the wave, and this process being continued 

 in alternate miles, a mean velocity of ten and a-half or eleven miles is attained 

 in the transport, at a resistance whose mean is less than the resistance of the 

 mean of the two velocities intermediate. 



In every canal there must be two velocities, at which principally the trans- 

 port is conducted, one sufficiently far behind the wave to render its interference 

 inconsiderable, and another sufficiently in advance to give secmity against its 

 passing in small changes of moving power ; at a velocity one-half of that of the 



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