104 Mr: J. Aitken on some Experiments on 



the force necessary to keep up the motion is imparted to the 

 chain at one point, which ought not to be, as it produces a 

 tension in the chain which varies in amount at the different 

 parts, being greatest at the part • approaching the driving- 

 pulley. These imperfections must always be borne in mind 

 when experimenting with this apparatus. 



To set the chain in motion, it is first put round the pulley i 

 and then tightened by holding it out by means of one of the 

 movable pulleys, in the form shown at fig. 6, PL VII. After the 

 apparatus is set in motion, the movable pulley may be re- 

 moved, as the centrifugal force tightens the chain. When in 

 motion, we shall find that we can, by means of the movable 

 pulleys, mould the chain into a variety of curves, and that it 

 will retain for some time whatever shape we give it. We may 

 also notice that it resists our efforts to alter its form, and that, 

 after we have succeeded in altering its form, the motion has 

 but little tendency to change the shape of the curves. 



These are, however, only the general results which strike 

 one at first. On more careful experimenting, the imperfec- 

 tions of the apparatus and the effect of the varying rate of 

 rotation of the links become very evident. If we continue 

 the experiment for any length of time, we shall find that 

 the chain slowly changes its shape when put into most 

 forms, and that it is only stable in a very few forms. If, 

 for instance, the centre of suspension is kept over the 

 centre of gravity of the chain, it will keep the circular form, 

 or it will keep the form shown at fig. 7, PL VII., though 

 it will not keep the form shown at fig. 6, PL VII., because 

 the chain tends to continue the bend at a on account of the 

 rotation of the links. It keeps the form shown at fig. 7, 

 PL VII., because the links are prevented from curving any fur- 

 ther at a by the tension produced in the chain by the pulley 

 in keeping up its motion. The chain does not tend to curve 

 inwards at b, partly on account of the pressure of the pulleys 

 i and r on the chain tending to cause the links to leave the 

 pulley i at the point of contact, and partly because there is 

 less tension in the chain at b than at any other part, as the 

 tension produced by keeping up the motion is least at this 

 part. It is well to remember that the half of the chain next 

 the pulley is so influenced by the pulleys and difference in 

 tension, that it is difficult to draw conclusions from the action 

 of the chain at this part. We may show the imperfections of 

 the apparatus in another way. If, for instance, the line drawn 

 vertically through the point of suspension falls to one side of 

 the centre of gravity of the chain, then the chain will keep an 

 oblong shape; but if the point of suspension is gradually brought 



