82 Mr. J. Aitken on some Experiments' on 



general use. He has, however, some time ago suggested, and 

 now generally uses, the word centreward in place of centri- 

 petal. It is much to be hoped that in his reforms he will not 

 spare the word centrifugal, but will replace it by some word 

 more etymologically correct. While objecting to the name 

 centrifugal force, we are under the necessity of retaining it 

 till some better term has been introduced. It will, however, 

 be necessary for us clearly to understand what we mean by 

 centrifugal force. According to the First Law of Motion, any 

 body when in motion tends to move at a uniform velocity 

 and in a straight line. If we wish the body to move in a 

 circular or any curved path, then we must cause some force to 

 act on it to compel it to deviate from its free path ; and the re- 

 sistance ivhich the body offers to this deviation is what we call 

 centrifugal force. Or, more simply, centrifugal force is the re- 

 sistance w r hich a body offers when in motion to change of 

 direction of motion. If we remove the deviating force, then 

 there is no centrifugal force, and the body simply tends to 

 continue moving in a straight line*. 



Suppose now that, instead of one body revolving round a 

 centre, we have a number of bodies of the same mass, all 

 moving at the same velocity, all placed at equal distances from 

 each other and at equal distances from the centre, then we 

 may cause this series of bodies to revolve round the centre by 

 tying them to the centre, when they will exert a radial tension ; 

 or we may cause them to revolve round the centre by linking 

 all the bodies together like a chain. When such a series of 

 bodies are in motion round a centre, they exert a pressure at 

 right angles to the direction of their motion, the result of 

 which is, a tension is produced in the system tending to burst 

 the links, in the same manner as the tension is produced in the 

 shell of a cylindrical boiler by the pressure of the steam. 

 According to the dynamical theory of gaseous pressure, these 

 two tensions are produced in a very similar manner — in the 

 first by the resistance to change of direction of motion of the 

 links, and in the second by the resistance to change of direc- 

 tion of motion of the molecules of the steam. 



The tension produced in a series of bodies revolving round 

 a centre is very simply illustrated by taking an elastic band 

 and fitting it tightly over a pulley which can be driven at a 

 great velocity, such as that shown in PI. IY. fig. 1, when it 

 will be seen that as the velocity increases the tightness of the 



* For simplicity the body is here spoken of as a whole : and this is cor- 

 rect if the body is infinitely small ; but "if the body is of any size and moves 

 round a centre either outside the body or inside it, then we must consider 

 each particle of the body separately, as the different parts of the body are 

 moving: at different velocities and in different directions. 



