IN THE LAST HALF-CENTURY. 81 



to the middle of the arc described. As a 

 matter of fact, it is compelled to travel 

 through its left-hand half -swing, and thus 

 virtually to go up hill. Consequently, 

 the ' attractive forces ' of the bob and the 

 earth are now acting against it, and con- 

 stitute a resistance which the charge of 

 kinetic energy has to overcome. But, as 

 this charge represents the operation of 

 the attractive forces during the passage of 

 the bob through the right-hand half-swing 

 down to the centre of the arc, so it must 

 needs be used up by the passage of the 

 bob upwards from the centre of the arc to 

 the summit of the left-hand half-swing. 

 Hence, at this point, the bob comes to a 

 momentary rest. The last fraction of 

 kinetic energy is just neutralised by the 

 action of the attractive forces, and the 

 bob has only potential energy equal to 

 that with which it started. So that the 

 sum of the phenomena may be stated 

 thus : At the summit of either hall-arc of 



6 



