CH. V] MECHANICAL RECREATIONS 89 



though the full explanation of most of them also introduces 

 other considerations. Thus it is a common feat to toss on to 

 the top surface of an umbrella a penny so that it alights on its 

 edge, and then, by turning round the stick of the umbrella 

 rapidly, to cause the coin to rotate. By twisting the umbrella 

 at the proper rate, the coin can be made to appear stationary 

 and standing upright, though the umbrella is moving away 

 underneath it, while by diminishing or increasing the angular 

 velocity of the umbrella the penny can be made to run forwards 

 or backwards. This is not a difficult trick to execute : it was 

 introduced by Japanese conjurers. 



The tendency of a body in motion to continue to move 

 in a straight line is sometimes called its centrifugal force. 

 Thus, if a train is running round a curve, it tends to move in 

 a straight line, and is constrained only by the pressure of the 

 rails to move in the required direction. Hence it presses on 

 the outer rail of the curve. This pressure can be diminished 

 to some extent both by raising the outer rail, and by putting 

 a guard rail, parallel and close to the inner rail, against which 

 the wheels on that side also will press. 



An illustration of this fact occurred in a little known inci- 

 dent of the American civil war*. In the spring of 1862 a 

 party of volunteers from the North made their way to the 

 rear of the Southern armies and seized a train, intending to 

 destroy, as they passed along it, the railway which was the 

 main line of communication between various confederate corps 

 and their base of operations. They were however detected 

 and pursued. To save themselves, they stopped on a sharp 

 curve and tore up some rails so as to throw the engine which 

 was following them off the line. Unluckily for themselves 

 they were ignorant of dynamics and tore up the inner rails of 

 the curve, an operation which did not incommode their 

 pursuers, who were travelling at a high speed. 



The second law gives us the means of measuring mass, 

 force, and therefore work. A given agent in a given time can 

 do only a definite amount of work. This is illustrated by the 

 * Capturing a Locomotive by W. Pittenger, London, 1882, p. 104. 



