April 2^, 1888] 



NATURE 



duration is so exceedingly small in case (3) that the 

 momentum communicated is very small. 



II. Support a cylinder with a fly-wheel, as shown in 

 Fig. 2. E E are two pieces of wood, both screwed at the 



\«<^s: 



-<- 



D 



Fig. 



top to another piece of wood, L, of convenient thickness. 

 Each has a slot cut along its centre, in which fits a ball, F, 

 to which is attached a stiff wire, a string, and a weight, 

 as shown in the figure. H is an india-rubber band, which 

 presses ee together with a pressure at least sufficient to 



A 





B 



m 





W 



ioiiliMiiH 



M 



Fig. 2. 



cause the ball f not to slip when the weight M is hung 

 on to its string. Another string is wound round the 

 end A of the cylinder, and a weight attached to it so 

 as to balance the weight of the two pieces of wood, 



613 



E E. The fly-wheel has a friction-brake upon it, and 

 if the retarding force of the brake be constant, the 

 angular displacement of the fly-wheel is proportional to 

 the square of the momentum communicated. 



(i) Lift the weight M a distance of about half an inch, 

 and let it fall. The cylinder goes round through a certain 

 angle, and the ball f is not pulled out of its slot. 



(2) Lift the weight M through 2 or 3 inches, and let it 

 fall. The ball F is pulled out of its slot ; the cylinder 

 goes round, but through a smaller angle than in case (i). 



(3) Let the weight M fall through a height of 4 or 5 feet. 

 The ball f is pulled out of its slot, and the angular 

 displacement of the cylinder is barely perceptible. 



The same explanations are applicable to the results of 

 II. as were made concerning the results of I., provided 

 couple be substituted for force, and moment of inertia for 

 mass. 



III. The following, though somewhat inconvenient as 

 a class experiment, illustrates the same subject. Fix up 



Fig 3. 



a plain deal or other board in the manner of Robins' 

 ballistic pendulum. From a rifle with a small charge of 

 powder, fire a bullet into the board, at right angles to 

 its plane, and as near as possible to its centre of inertia. 

 The bullet lodges in the board, which is deflected through 

 a large angle. Increase the charge of powder, so that 

 the bullet pierces the board. The deflection of the board 

 is now smaller. Put the maximum charge of powder in 

 the rifle, and the deflection of the board on firing the 

 bullet into it is exceedingly small. 



IV. Suspend a light ivory or other ball by a long india- 

 rubber thread several feet long, as shown in Fig. 3. Pull 

 the ball into the position A b', and let it go. Looking at it 

 as seen in the figure, it first begins to describe a curve 

 against the hands of a watch. After two or three periods 

 it begins to go round in a direction with the hands of a 

 watch. 



