ON THE GENERAL THEORY OF THE STEAM ENGINE. 203 



positions that are not altogether in accordance with universally received principles. For 

 instance, in the first section of his researches, printed in vol. xiv. of the Transactions of the 

 Royal Society of Edinburgh, after having stated that the resistance of a small unit of surface 

 to a fluid, when either the fluid is in motion, or the surface itself is equal to the statical 

 pressure of a column of fluid having for its height the height due by gravity to that velocity, 

 he asserts that " this statical quantity being the measure of the pressure of the fluid upon the 

 " anterior surface of the immersed solid, will also be the measure of the quaquaversus pressure 

 " of the fluid in every direction, and therefore will measure the pressure of the water upon 

 " the vessel causing its emersion." This assumed hypothesis is obviously inconsistent with 

 the mode in which the action and reaction takes place between the vessel and the fluid ; and 

 its random nature, independently of other misapplications of mathematical principles, to 

 which it is not necessary to advert, is amply sufficient to account for the startling nature of 

 the conclusion to which it leads, viz., that the vessel rises with every increase of velocity, and 

 that "at 43'8 miles an hour, the floating body emerges wholly from the fluid and skims the 

 " surface." By what mysterious action of forces the vessel is thus elevated or suspended in 

 the air, is, of course, left for the reader to find out, but we trust it will not long exercise his 

 ingenuity. 



The principal feature of Mr. Russell's observations is the propagation of the great solitary 

 wave of displacement, which appears to act an important part in the modification of the 

 resistance to the vessel ; and Mr. Russell's beautiful illustrations of its mechanism and 

 properties are very interesting and important. It is remarkable that the velocity of the 

 generating body does not in any way affect the velocity of this wave ; a wave, for example, 

 of 8 miles an hour being produced alike from bodies moved at the rate of 2, 5, 6, and 12 miles 

 an hour. The velocity appears to depend chiefly on the depth of the fluid, and Mr. Russell, 

 in his experiments on canals, has found that in each case it does not differ sensibly from that 

 which is acquired by a heavy body in falling freely by gravity through a space equal to the 

 depth of the centre of gravity of a cross section of the fluid, below the surface. It is not 

 difficult to conceive how this wave exercises its influence and the changes that must take 

 place in the resistance, accordingly as the vessel falls behind the wave, mounts upon it, or 

 passes over it. 



The only rational explanation of the phenomenon of the vessel rising out of the fluid, 

 appears to depend on the form of its construction. Thus, the action of the fluid, excepting 

 the slight influence of friction, must, at each point, always be directed perpendicularly to the 

 surface of the vessel ; and hence, when in motion, the increase of head pressure must tend to 

 elevate the head of the vessel, whilst the loss of stern pressure will cause the stern to lose a 

 portion of its statical support, and so sink deeper into the fluid. And if we simply consider 

 that the elevated position of the keel of the vessel, so acquired, has a direct tendency to 

 increase the elevating action of the fluid at the head of the vessel, and to retrieve the loss of 

 stern support, the partial rising out of the fluid becomes an immediate consequence of the 

 resolved forces ; and it is evident that the vessel cannot, under any circumstances, emerge 

 wholly out of the fluid. The truth or error of this explanation might easily be tested by 

 employing a floating body of a rectangular form : if it be correct, the tendency ought to be 

 to bury itself in the fluid instead of rising out of it. 



Mr. Mornay (page 123) appears to be of the opinion that in the motion of a body through 



