98 Mr. J. Aitken on some Experiments on 



move very slowly, on account of the motion of the chain ; and 

 if we attempt to make the experiments with a chain not in 

 motion, the waves travel so quickly that the eye cannot 

 follow them. If we could reduce the tension in the chain due 

 to its weight, then the waves would travel slowly, even in a 

 chain not in motion. The simplest method of doing this is 

 to lay a length of chain on a horizontal polished surface ; in 

 the absence of a better, a wooden floor will do. One end of 

 the chain is fixed ; the other end is then, by means of the hand, 

 caused to take any of these wave-forms. When this is pro- 

 perly done the wave travels along a great length of chain. 

 In making this experiment the most important point to be 

 attended to is not to bring the hand to rest after the wave is 

 started. The hand with the end of the chain must be gradually 

 and steadily withdrawn ? so as to keep up a slight tension in the 

 chain ; otherwise the wave will travel but a short distance. In 

 making the experiments in this way, as the tension in the 

 chain is small, the waves move so slowly that they can easily 

 be followed with the eye. Beside the wave-forms mentioned, 

 the chain may be made to take up any others that may be 

 desired ; and the waves may be put in motion either in a hori- 

 zontal or vertical plane. 



VII. All these experiments only illustrate the balance of 

 the centrifugal force and the tension when the motion is con- 

 fined to one plane. The next experiment is to show that these 

 forces are also in equilibrium when the motion is not confined 

 to any plane, but is constantly changing from one plane to 

 another. In order to illustrate this we can take either the 

 short or long chain used in the previous experiments, and, by 

 means of the movable pulleys, bend the lower part of it, while 

 in motion, into a plane at right angles or at any angle to the 

 plane of motion of the upper part, when we shall find that, 

 though gravitation slowly unbends the chain, the centrifugal 

 force has no tendency to alter its shape. This point, however, 

 may be illustrated in another and better way, if we take a 

 circular disk of paper, or any other flexible material (A, fig. 1, 

 PI. VII.), and mount it on the apparatus fig. 1, PL IV., so that 

 it can be rotated round its centre ; this is easily done by cut- 

 ting a hole in the centre of the disk and fitting it to the 

 arrangement c at the end of the spindle a. 



Let us first see what the result is if we first tend the disk 

 and then put it in motion. The results may be summed up 

 under three heads : 1st, the bent part rotates with the disk ; 

 2nd, the centrifugal force tends to unbend the disk; and, 3rd, 

 the elasticity of the disk also tends to restore it to its original 

 shape. If now we first put the disk in motion and then bend 



