418 SEC. 12. APPLIED MECHANICS. 



cutters, and if the tracer be made to follow the correct line on the drawing 

 (or to follow the edge of the template) the revolving cutters will cut the 

 correct water-line on the model. 



The model is then finished by hand with spokeshaves and scrapers, an 

 operation which takes a man about three hours. 



No. 5. The hauling engine. 



This is the instrument by which the required motion through the water at 

 definite speed is given to the model. The dynamometric truck to which the 

 model is attached is connected by a wire rope with a winding drum, driven by 

 a small stationary double-cylinder steam engine. The engine is regulated by 

 an extremely sensitive governor, acting upon a delicate steam throttle valve, 

 on what is known as the " differential " principle, in which the governor 

 rotates at its own appropriate speed, independently of the engine, the steam 

 valve being opened or closed according as the engine is lagging behind the 

 governor or overtaking it. 



By adjusting the centrifugal weights of the governor, with a right and left- 

 handed screw, and by differently speeding the belt which connects it with 

 the engine, any required speed may be assigned to the engine between the 

 limits of about 150 and 350 revolutions per minute, and by further changing 

 the gear wheels connecting the engine and winding drum, speeds varying 

 from 60 to 1,200 feet per minute may be assigned to the dynamometric 

 truck. 



Nos. 6 and 7. The dynamometric truck with model under it. 



The dynamometric truck runs on a straight and level railway about 200 feet 

 in length, suspended over a waterway 36 feet wide and 10 feet deep. The 

 model floating in the water is as it were " harnessed " to the truck, and travels 

 with it. It is kept from diverging sideways by a knee-jointed frame or 

 " guider " at each end, of such construction as to perfectly prevent the slightest 

 sideways deviation of the model, but in no way to interfere with its rising or 

 falling, or moving in a fore and aft direction with reference to the truck. 

 The towing strain (i.e., the force necessary to make the model accompany the 

 truck in its longitudinal progress) is taken during the experiment by a spiral 

 spring, the extension of which, measuring the towing force, is indicated on 

 a large scale (through the intervention of certain levers) by a pen which 

 makes a line on a recording cylinder covered with a sheet of paper. The 

 recording cylinder is driven by the truck wheels, and thus its circumferential 

 travel indicates distance run ; at the same time another pen, jerked at half 

 second intervals by a clock, records time. Other pens actuated by strings 

 led over pulleys, record the change of level of the ends of the model. Thus 

 the diagrams made furnish an exact measure of the speed, and a continuous 

 record of the resistances and of the change of level of the model throughout the 

 experimental run at steady speed. While starting or stopping, the model 

 is controlled by hand levers to prevent the dynamometric spring being over- 

 strained. 



No. 8. A " designer " template. 



This consists of one of the pile of adjustable templates shown in photo- 

 graph No. 1, and already described. 



No. 9. A segment of a model. 



This specimen segment of a model is partly in a finished condition and 

 partly in the condition in which it is left by the shaping machine, Nos. 3 

 and 4. It thus shows the series of water-line cuts made by the machine, and 

 a part of the original cast surface remaining between the cuts. 



2147aa. Model of the solid of " Least Resistance, 9 ' by 



the late Andrew John Robertson, dated 1861. 



Michael Scott, London. 



