IX. NAVAL ARCHITECTURE, ETC. 489 



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 roofed 

 throughout. 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 " guidcr " 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 (z'.e., the force necessary to make the model 

 accompany the truck in its longitudinal progress) is taken during the experi- 

 ment 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 tune 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 dyuamometric spring 

 being overstrained. 



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," by 



the late Andrew John Robertson, dated 1861. Michael Scott. 



2147ab. Model of the Steam Ship Sir John Lawrence," 



embodying to a considerable extent the form of least resistance, 

 designed by Michael Scott, in conjunction with the late Andrew 

 John Robertson. The performance of this ship was excellent. 



Michael Scott. 



2147ac. Three diagrams of a new type of War Ship, 

 designed by Michael Scott in 1869, and published in 1870. 



Michael Scott. 



