THE LA WS OF FLUID RESISTANCE. 117 



calculated in all cases from general experimental data, 

 neither theory nor general experiment have as yet supplied 

 means of calculation applicable to the wave-making resist- 

 ance. In the absence of this knowledge we have to rely on 

 direct experiments with different forms of vessels, and to 

 supply these is one of the objects of the experiments upon 

 the resistances of models of various forms which I am now 

 conducting for the Admiralty. 



By these experiments I hope not only to obtain a great 

 many comparisons, showing at once the superiorities of some 

 forms over others ; but to deduce general laws by which 

 the influence of variation of form upon wave-making resist- 

 ance may be predicted. Already, indeed, some most 

 instructive propositions concerning the operations of this 

 cause of resistance have shaped themselves ; but it would 

 take far too long to describe them in this discourse. I 

 will merely refer to one broad principle which underlies 

 most of the important peculiarities of the wave-making 

 element of resistance. 



We have seen that the waves originate in the local dif- 

 ferences of pressure caused in the surrounding water by the 

 vessel passing through it ; let us suppose, then, that the 

 features of a particular form are such that these differences 

 of pressure tend to produce a variation in the water-level 

 shaped just like a natural wave, or like portions of a natural 

 wave of a certain length. 



Now an ocean wave of a certain length has a certain 

 appropriate speed, at which only it naturally travels, just 

 as a pendulum of a certain length has a certain appropriate 

 period of swing natural to it. And just as a small force 

 recurring at intervals corresponding to the natural period 

 of swing of a pendulum will sustain a very large oscillation, 

 so, when a ship is travelling at the speed naturally appro- 

 priate to the waves which its features tend to form, the 

 stream-line forces will sustain a very large wave^ The 

 result of this phenomenon is, that as a ship approaches this 

 speed the waves become of exaggerated size, and run away 

 with a proportionately exaggerated amount of power, 

 causing corresponding resistance. This is the cause of 

 that very disproportionate increase of resistance experi- 

 enced with a small increase of speed when once a certain 

 speed is reached, an instance of which is exhibited at a 



