118 LECTURES TO SCIENCE TEACHERS. 



speed of about eighteen knots in the curve of resistance 

 shown in Fig. 18. 



We thus see that the speed at which the rapid growth of 

 resistance will commence, is a speed somewhat less than 

 that appropriate to the length of the wave which the ship 

 tends to form. Now, the greater the length of a wave is, 

 the higher is the speed appropriate to it ; therefore the 

 greater the length of the waves which the ship tends to 

 form, the higher will be the speed at which the wave- 

 making resistance begins to become formidable. We may 

 therefore accept it as an approximate principle, that the 

 longer are the features of a ship which tend to make waves, 

 the longer will be the waves which tend to be made, the 

 higher will be the speed she will be able to go before she 

 begins to experience great wave-making resistance, and 

 the less still will be her wave-making resistance at any 

 given speed. 



This principle is the explanation of the extreme import- 

 ance of having at least a certain length of form in a ship 

 intended to attain a certain speed; for it is necessary, 

 in order to avoid great wave-making resistance, that the 

 " wave features," as we may term them, should be long in 

 comparison with the length of the wave which would 

 naturally travel at the speed intended for the ship. 



Time will not admit of my describing to you in detail 

 how the principles I have been explaining affect the prac- 

 tical question of how to shape ships. I must leave you to 

 imagine for yourselves, if you feel interested in following 

 up the question, how the desirability of length of " wave 

 features," for lessening wave-resistance, is to a greater or 

 less extent counteracted by the desirability of shortness of 

 ship for lessening surface-friction ; and how in many other 

 ways a certain variation of form, while it is a gain in one 

 way is a loss in another, so that in every case the form of 

 least resistance is a compromise between conflicting methods 

 of improvement. 



My principal object has been to combat the old fallacy 

 of "head-resistance," as it has been sometimes called, due 

 to the inertia of the water acting against the area of the 

 ship's way. I hope I have made it clear to you, that the 

 inertia of a frictionless fluid could offer no opposing force 

 to a submerged body of any shape moving through it, for 



