304 SECTION MECHANICS. 



The diagram we obtain then is an exact record of the resistance of 

 the model experimented on, when moving at the recorded speed. 

 The dynamometrical carriage is drawn by a wire rope, moved by a 

 steam engine, fitted with a delicate adjustible governor, which enables 

 us to administer such speed as is required. We can send the truck at 

 any required speed from 50 to 400 feet per minute, nay, even to 1200 

 were we to desire it. But besides that, we have the definite record of 

 the speed, and any small inexactness in the speed supplied by the 

 governor, is corrected by the record which I have described. When 

 we have thus got an exact transcript of the resistance of the model at 

 each speed, the analysed results constitute that curve of resistance 

 which I have described to you. 



I should have mentioned that there is another phenomenon which we 

 also record in connection with the motion of every model, namely, its 

 rise or fall, that is to say the change of level it exhibits at either end. 

 It is a new fact, which, I believe, we were the first to show, that every 

 form moving along the surface of the water like a ship at reasonable 

 speed, subsides in the water ; some forms subside more at the head, some 

 more at the stern, but there is always a bodily subsidence produced 

 at the ordinary speeds of propulson. To record this, a little apparatus 

 connected with the ends of the model makes a second diagram on a 

 revolving piece of paper, one line for each end, showing any change 

 of level exhibited by the model in the course of the run. The model 

 immediately settles into condition, and the condition is maintained 

 throughout, and we have a diagram of it. 



I have now described the manufacture of the models, the method 

 of testing them, and the result, viz., that for each model we obtain a 

 curve of resistance such as I have sketched. I will next endeavour to 

 explain the comparison between the curve of resistance of a ship and 

 that of her model, and in order to do that, I must travel a little into 

 theoretical view of the matter. The old notion of a ship's resistance 

 was, that you began with a midship-section, and that as you gave the 

 ship finer and finer ends you reduced the resistance to some fractional 

 multiple of tr^e resistance which would be experienced on moving the 

 midship-section itself through the water, as a plane moving at right 

 angles to itself; and various formula?, mathematical or emperical, were 

 supposed to define the appropiate multiplier. Now, mathematicians 



