Rigidity produced by Centrifugal Force. 93 



and was no longer free to overcome the gravitation , and the 

 chain began to fall. At this point its fall may be stopped, or 

 it may be caused to rise again, by destroying the tension at the 

 lower part of the chain. This we can do in two ways. We 

 may either cause the chain to strike the platform as nearly as 

 possible at right angles, as shown at fig. 4 ; the motion of 

 the links will thereby be partially destroyed, and the tension 

 at the lower part thereby reduced, and the chain will again 

 rise : or, if when the chain is meeting the platform at an acute 

 angle, and the upper part of the chain is falling, we turn the 

 platform so as to cause the chain to meet it at a less acute 

 angle, then the chain will again rise ; or we may reverse the 

 experiment. Suppose that while the chain is meeting the 

 platform at right angles, as shown at fig. 4, and is keeping its 

 elevated position, we bend the platform, as shown at B/, so as 

 to cause the chain to meet the platform at an acute angle ; 

 then the chain at once begins to fall in the manner shown by 

 the dotted lines, fig. 4. But so long as it keeps the form 

 shown in full lines, tig. 4, it will keep balanced in its elevated 

 position, for a long time standing on the platform B and only 

 touching the pulley A at the point q, — the reason for this 

 being that, if we partially stop the motion of the links by 

 causing them to strike the platform, or if we partially alter 

 the direction of their motion by causing them to strike the 

 platform at such an angle as partly to change the direction of 

 their motion, then there will be less tension in the lower part 

 of the chain than in the upper, as the tension in the lower part 

 will be only that due to partially changing the direction of the 

 motion of the links. 



In fig. 3, PI. V., the chain meets the platform at a very 

 acute angle, and the change in the direction of the motion of 

 the links is almost entirely effected by the tension in the chain, 

 and only to a very small extent by the platform ; there is 

 therefore not sufficient unbalanced centrifugal force at the 

 upper part of the chain to keep it in its elevated position. 

 But the case is different in fig. 4. In this case the platform 

 assists in altering the direction of the motion of the links at 

 the lower part, and the difference between the tensions in the 

 upper and lower parts is sufficient to keep the chain in its 

 elevated position against the force of gravitation. 



A chain forming a loop of 4 or 5 feet may easily be kept in 

 the air for any length of time, as it tends to strike the plat- 

 form nearly at right angles, as shown at fig. 5, PLY. If the velo- 

 city of the chain is not sufficient to raise all the chain from the 

 platform, then the apparatus under these circumstances is 

 working something like a fountain, in which the driving- 



