p2 LECTURE IX. 



rest or ascend, it is clear, from the laws of equilibrium, that no motion derived 

 from gravitation can take place: if it may descend, it must either continue 

 to descend for ever, with a finite velocity, which is impossible, or it must first 

 descend and then ascend, with a vibratory motion, and then the case will be 

 reducible to that of a pendulum, where it is obvious that no new motion is 

 generated, and that the friction and resistance of the air must soon destroy 

 the original motion. One of the most common fallacies, by which the super- 

 ficial projectors of machines for obtaining a perpetual motion have been de- 

 luded, has arisen from imagining, that any number of weights ascending by 

 a certain path, on one side of the centre of motion, and descending in the 

 other, at a greater distance, must cause a constant preponderance on the side 

 of the descent : for this purpose, the weights have either been fixed on hinges 

 which allow them to fall over at a certain point, so as to become more distant 

 from the centre, or made to slide or roll along grooves or planes, which lead 

 them to a more remote part of the wheel, from whence they return as they 

 ascend: but it will appear on the inspection of such a machine, that although 

 some of the weights are more distant from the centre than others,^ yet there is, 

 always a proportionally smaller number of them on that side on which they 

 have the greatest power; so that these circumstances precisely couuterbalancci 

 each other. (Plate VI. Fig. 78.) 



