1819.] on Centrifugal Force. 333 



fugal force, and the cohesion of the parts of the string was the 

 centripetal force. But if there is another centrifugal force that 

 acts directly from the centre, then it is either a force that has 

 been generated without any apparent cause, or it is a force that 

 could in no way contribute to circular motion. 



If this is doubted, let any one fix a stone to the one end of a 

 string, and try to give it the rotatory motion by throwing it 

 straight upward, or forward, from his hand holding the other 

 end of the string (which is a centrifugal force acting directly 

 from the centre), and he will find that the stone, instead of 

 revolving in a circle, will either drop perpendicularly downwards, 

 or descend like the ball of a pendulum, as soon as it reaches the 

 end of the string. The motion, therefore, which is communi- 

 cated to a body that is impelled by a force acting directly from 

 the centre can be only rectilinear, like that of an arrow shot from 

 a bow perpendicularly upwards, and not curvilinear, like that of 

 a sling twirled round the head ; and the force which contributes 

 to circular motion can never be directly from the centre. Thus, 

 no force acting directly from the centre could have communi- 

 cated to Mr. Knight's wheel a motion round its own axis — a mere 

 axis in peiitrochio that can be put in motion only by a power 

 applied in the direction of a tangent to the circumference, which 

 power the water was that fell into the cogs of the main wheel, 

 and put the whole machinery into motion. 



Mr. Meikle's next peculiarity is his maintaining that " when 

 a body is relieved from moving in a circle, its flying off in a 

 tangent is in perfect harmony with, nay a consequence of, the 

 centrifugal force's acting directly from the centre." It would 

 certainlv be edifying to see the proofs of the doctrine which Mr. 

 lMeikle thus advances ; that is, to know how a change of motion 

 may be effected in a line of direction different from that of the 

 force impressed. But till the alleged proofs may happen to fall 

 in my way, I shall consider myself as at liberty to suppose, that 

 when a body is relieved from moving in a circle, its Hying off in 

 a tangent is a consequence of the centrifugal force's acting in a 

 line that is not directly from the centre ; for where a force acts 

 directly from the centre, it can have nothing to do with circular 

 motion. Nor is the flying oft' even in a tangent to be regarded 

 as an invariable and necessary consequence of relieving a body 

 from moving in a circle. The consequence may be very differ- 

 ent. For a body may be relieved from moving in a circle by the 

 destruction of the centrifugal force itself, and then it will not fly 

 off' in a tangent, but will assume a direction depending upon the 

 direction or position of the cause that obstructs its progress. 

 Suppose the motion of a twirled sling to be obstructed by its 

 striking against some obstacle, instead of going completely 

 round, and you have an example in point. But if the body is 

 relieved from moving in a circle by the destruction of the centri- 



