3 ip SEC TIO N M EC IT A NICS. 



been held by it would arrange itself into hills and hollows. These hills 

 and hollows represent, not precisely the stream line pressures, but the 

 effect of those pressures. The pressures are in fact modified by the 

 deformations of surface, and those hills and hollows, when once formed, 

 assume the form of and behave as, waves, and become ihus to a cenain 

 extent independent of the parent that created them. Thus, they travel 

 off into the surrounding ocean, taking with them more or less, indeed 

 at high speed very much, of the energy which was put into them in their 

 creation, and this is one great cause of resistance to a ship moving at 

 high speed at the surface. Thus when the ship is moving only at 

 moderate speed so moderate that the waves cannot raise themselves 

 but immediately subside in their track, they do not travel away into 

 the surrounding v/atcr, and then the ship's resistance consists, I may 

 say solely, of surface friction. There is indeed, besides, a small amount 

 of head resistance, due to the distortion of the stream lines, which results 

 from the frictio i of the streams against each ether, in addition to that 

 due to their friction against the sides of the ship, and'to the slight loss of 

 absolute pressure which thus ensues ; but if the ship, has rinp lines this 

 is inconsiderable, and at moderate speed the ship's resistance is, prac- 

 tically, simply that of surface friction. With our models, for instance, 

 whenever we go at very low speed say fifty feet per minute, wi.h a 

 model ten or twelve feet long we find by actual experiment the resis- 

 tance is just exactly that due to surface friction. I state that as a 

 fact, but I must tell you that in order to be able to state it with 

 certainty, it was necessary to ascertain the measures of the surface 

 frictional forces, and we instituted a very expensive scries of experi- 

 ments to determine that. We tried plane surfaces nineteen inches 

 wide across the line of motion, varying in length in the line of motion, 

 from three and four inches up to fifty feet, and scarcely more than 

 one-eighth of an inch in thickness, and by a dynamometer we ascer- 

 tained exactly the resistance at all speeds of all portions of these 

 planes. Several curious facts are connected with the law of such 

 resistance. One is this. The resistance is more ardent, if I may 

 use the expression, at the anterior end of the plane than at the 

 posterior. When one begins to think of the surrounding conditions, 

 it seems wonderful that the difference is not greater than it actually is. 

 Inch by inch along the length of the plane there must be growing into 



