116 LECTURES TO SCIENCE TEACHERS. 



speeds at which the wave-resistance is more than some forty 

 per cent, of the whole. 



It is a point worth noticing here, what an exceedingly 

 small force after all is the resistance of a ship, compared 

 with the apparent magnitude of the phenomena involved. 

 Scarcely any one, I imagine, seeing for instance the new 

 frigate Shah steaming at full speed, would be inclined at 

 first sight to credit, what is nevertheless the fact, that the 

 whole propulsive force necessary to produce that apparently 

 tremendous effect is only 27 tons, in fact less than one two- 

 hundredth part of the weight of the vessel. And of this 

 small propulsive force, at least 15 tons, or more than one- 

 half, is employed in overcoming surface-friction simply. 



Thus, although the vessel carries at her bow a wave seven 

 feet high, the forces which produce this are so far neutra- 

 lised by other similar forces that the whole of her resistance, 

 exclusive of surface-friction, might be represented by the 

 sternward pressure on her bow which would be due to a 

 single wave fourteen inches high. Indeed, a wave thirty 

 inches high would represent a sternward pressure equal to 

 the whole resistance of the ship. 



The truth is, that the forces which are at work, namely 

 the stream-line pressures due to the inertia of the fluid, are 

 indeed very great ; what we have to deal with, in the shape 

 of eddy-making or wave-making resistance, is nothing but 

 a minute difference or defective balance between these great 

 forces, and fortunate it is that they balance as well as they 

 do. With a well-shaped ship at moderate speed we have 

 scarcely any resistance but skin friction, for the balance of 

 stream-line pressures is almost perfect; but nevertheless 

 they are all the while in full operation, a forward force 

 counteracting a backward force, each equal to perhaps five 

 times the existing total resistance of the ship. We can 

 easily imagine, then, that when we once begin to tamper 

 with this balance, we may produce unexpectedly great 

 resistance ; and thus when we are dealing with speeds at 

 which the wave-making resistance comes into play, a small 

 variation in form may cause a comparatively large variation 

 in the wave-making resistance. It is this fact which gives 

 the wave-making resistance such a vital importance in con- 

 nection with the designing of ships; but unfortunately, 

 although the surface-friction element of resistance is easily 



