23G Professor W. E. Daily [Jan. 30, 



This may be illiistr; ted by the following simple experiments : 



Experiment 1. — Stretching a tyro under the tension induced by 

 rapid rotation. 



Each part of the tyre is compelled to move in a circle. There- 

 fore a force must act towards the centre. The equal and opposite 

 aspect of this force can only appear as a tension in the tyre. You 

 notice as the speed is increased how the tension lengthens the tyro 

 until it is hanging quite loose. Reducing the speed it gradually 

 contracts and finally grips the wheel tightly. 



Experiment 2. — A mass of 1 lb. attached to a truly turned wheel 

 and rapidly revolved caused great vibration. 



The question now arises, How are these forces to be dealt with in 

 order to stop the vibration? The answer is, Apply another mass to the 

 wheel in such a way that the force its rotation produces on the frame 

 is exactly equal and opposite to the force from the first mass. Apply- 

 ing such a mass you see the wheel runs quietly at a much higher 

 speed than I dared to run it before. 



These experiments sufficiently indicate the nature of the problems 

 engineers have to deal with. 



A model of a locomotive was freely supported on rollers and 

 driven, the parts being entirely unbalanced. The consequent vibra- 

 tion indicated the unbalanced forces. 



Balance weights were then applied and the model stood still on 

 the rollers. 



Since revolving balance weights are added the part of the recip- 

 rocating masses which they balance horizontally appears as an un- 

 balanced vertical force, but the weight of the model masks this force 

 entirely. Relieving the weight the force soon makes itself evident 

 by the vertical oscillation it produces. This vertical force appears as 

 a variation of pressure on the rail in an actual case, the weight on the 

 wheel in ordinary practice being more than sufficient to mask the 

 force. The extent to which the pressure is varied is shown by 

 the slide in the screen. The curves are drawn for an actual case of 

 an express locomotive running at 60 miles per hour. 



To obtain balance in the longitudinal direction and without the 

 *' hammer blow," as this variation of vertical force is called, Mr. Webb 

 has arranged the cranks of his recent compound locomotives in the 

 way shown in the model of the engine on the table. The model has 

 kindly been sent from Crewe by Mr. Webb for this occasion. There 

 is still a tendency to twist about a vertical axis, but this cannot be 

 eliminated without setting the crank at different angles, and the 

 arrangement shown is preferred on account of the mechanical sim- 

 plicity which results. 



This four-crank model of a marine engine shows the arrangement 

 which must be adopted if both unbalanced force and unbalanced twist 

 or torque is to be eliminated. 



The conditions which must be fulfilled are : — 



1. The forces acting on the frame shall have no resultant. 



