90 Mr. J. Aitken on some Experiments on 



them, as the subject is one of great difficulty, and one of which 

 we have a very limited knowledge. A rigid body is one which 

 is generally supposed to be capable of resisting a certain amount 

 of force without being permanently put out of shape. The 

 force required to put it permanently out of shape may be small ; 

 yet it is a perfectly definite amount. Now the chain, when in 

 motion, may have its shape altered by any force, however small, 

 the greater force only making the alteration take place more 

 quickly. This being the case, does not this new property in 

 the chain correspond more to plasticity or viscosity than to 

 rigidity ? Does it not conduct itself more like a piece of wax 

 or a mass of treacle or tar than like a piece of lead ? Of course 

 I am here using the word rigidity in its extreme sense, a sense 

 in which perhaps no solid can be accurately said to possess it. 



Rigidity and viscosity as applied to matter can perhaps 

 scarcely be called different things, but may be more properly 

 called degrees in the same scale, the scale beginning in perfect 

 fluidity, passing through viscosity and plasticity to perfect 

 rigidity ; but no substance with which we are acquainted has 

 properties corresponding with either extreme end of the scale. 



III. In all the experiments, gravitation acts on the chains 

 while in motion. The next experiments are to illustrate the 

 manner in which gravitation acts on the moving chain and 

 changes its form. When the chain is hanging from the 

 driving-pulley, gravitation is balanced by the tension in the 

 chain, and there is equilibrium ; but suppose now that the lower 

 end of the loop is raised to one side of the driving-pulley, as 

 shown in figs. 4 and 5, PI. III. The chain can easily be put 

 into this position by means of the movable pulley E, fig. 3, 

 PL IV. When the movable pulley is removed, and the chain 

 is only supported by the driving-pulley A, then the tension no 

 longer balances the gravitation, and the form of the loop is 

 changed. The chain does not keep its form, and swing as 

 a solid body or as a chain not in motion would do, so as to 

 bring its centre of gravity under the driving-pulley, but the 

 chain in falling to a position of equilibrium passes through a 

 beautiful series of curves. The forms which the chain passes 

 through depend on the direction of motion of the chain rela- 

 tively to the driving or supporting pulley. If, for instance, 

 the upper part of the chain is moving towards A the driving- 

 pulley, then the chain in falling to its position of equilibrium 

 passes through a series of forms, a few of the intermediate 

 shapes of which are represented in fig. 4, PI. III., and finally 

 arrives at its position and form of equilibrium similar to that 

 shown in fig. 3, PI. III. If, however, the direction of motion is 

 the opposite of this, and the upper part of the chain is moving 



