EXPERIMENTS ON SURFACE-FRICTION. 



119 



f 



nometric governor of very exact action, and of sucli arrangements that any 

 required steady speed between 100 and 1000 feet per minute can be assigned 

 by it to the truck. 



The truck carries the dynamometric apparatus. A skeleton diagram in 

 Plate 11. shows this in full detail, with the special fittings by which it was 

 adapted to the surface-friction experiments ; and as the diagram is fully 

 referenced, the apparatus will be better understood by inspection than by a 

 verbal description here. Its general character is, however, as follows : — 



The plane of which the resistance is to be tested is driven through the 

 water by a suitable frictionless attachment, so arranged that the horizontal 

 force driving it is wholly delivered by a spiral spring, like that of a spring 

 balance, the fixed end of which is held by a strong bracket descending from 

 the frame of the truck. The extensions of this spring under the various 

 forces applied form in each case a measure of the force. The extensions, 

 brought to an enlarged scale by a lengthened index-arm, are self-recorded by 

 a pen which follows the motions of the arm, and traces a line on a sheet of 

 paper carried by a cylinder which receives its motion by a band from a pulley 

 on the hinder axle of the truck, so that the circumferential travel of the 

 paper represents on a small scale the forward motion of the truck. A second 

 pen, actuated by clockwork, marked time on the cylinder as it revolved ; so 

 that in each experiment two lines were marked on the paper, one showing 

 the resistance experienced at each point in the run, the other showing the 

 speed at which each portion of the run was performed. 



The planes were about y\ inch thick, of various lengths, and as finished 

 were uniformly 19 inches broad, .and when under experiment were placed on 

 edge in the water, the upper edge being about 1-^ inch below the surface. 



