108 ON THE STEAM BOATS OF 



the ' North America,' before the further improvements we shall mention, were performed in 

 lOlirs. 48min., after deducting stoppages. 



The circumstances of the tide in the river caused the curious result, that among the passages 

 whence the above average was deduced, those which were performed in the shortest time were 

 upwards, or in opposition to the fall of the stream. The Hudson is affected by the tide 

 beyond Albany, but in the higher parts the flood is rarely attended by a strong current, and 

 at times no other effect is produced by it than a variation in the velocity. But the wave 

 which causes the tide reaches Albany in about eighteen hours ; and thus, a vessel leaving 

 New York soon after low water carries the flood tide with it, and if it perform the passage in 

 ten or twelve hours, feels its full influence for the whole of the way. In descending, the vessel 

 meets at least two successive waves, and thus has the tide alternately favourable and un- 

 favourable. 



At New York the water of the Hudson at high water is usually nearly as salt as that of the 

 ocean ; but there have been two or three instances within the memory of man, when it was so 

 fresh at low water that outward-bound ships have rilled their supply of water from it. At a 

 distance of seventy-five miles from New York the water is always perfectly fresh, and is rarely 

 perceptibly brackish above the highlands, which are fifty miles from New York. For the latter 

 distance the channel for the largest ships is never less than 1000 yards in width, and is in many 

 places seventy feet in depth. Ships drawing fifteen feet water have a good beating channel 

 at all times of tide as high as the city of Hudson, 120 miles from New York : the remaining 

 twenty-four miles are comparatively shallow and narrow. 



The writer made in the ' New Philadelphia' one of the most remarkable passages ever 

 performed. Leaving New York at five o'clock P.M. with the first of the flood, he landed 

 at Catskill, distant 111 miles, a quarter of an hour before midnight. As passengers were 

 landed and taken in at seven intermediate points, the rate at which the passage was per- 

 formed was not less than eighteen English miles per hour. Now, as the current in no case 

 exceeds four miles per hour, the absolute velocity through the water must have been at least 

 fourteen miles. 



It may be here remarked, that the demonstration which attempts to prove that the absolute 

 velocity of a vessel propelled by steam in a current differs when the direction of the motion 

 is with the stream, from that with which it may be moved against the stream, is at variance 

 with the facts. Upon examination, this demonstration will be found to rest upon false premises : 

 the conditions laid down are not those which actually exist. In order to view the subject 

 in a proper light, let us suppose that a steam boat is abandoned to the current : in this case 

 it must speedily acquire the velocity of the stream, and be at rest in relation to the water on 

 which it floats. 1 When the machinery begins to act, no difference of circumstances can arise 

 from the direction in which the prow of the boat is turned, and all the motions in reference 

 to the mass of fluid will be performed exactly as if that mass were not in motion. In 

 moving with the current, then, the rate of progress by the land will be the sum of the ordinary 

 rate of the boat's motion, and the velocity of the stream ; in moving against the current, the 

 rate of progress will be the difference between these two velocities. 



In obtaining these velocities of thirteen miles and upwards per hour, it does not appear 



1 Here the particles of the fluid are supposed to move in parallel straight lines, and with the same velocity at all depths. ED. 



