MOTION, MECHANICS, ETC. 181 



The pendulum of the clock, which is fixed to the pillar 

 of the instrument, vibrates seconds ; it has only one wheel 

 which immediately acts on the pendulum : the small weight 

 which continues the pendulum's motion, after it has been 

 wound up, is half an hour in descending to the ground. 

 The clock will be sufficiently exact if it keep time with a 

 common well regulated clock for this half hour. 



When the axis of the wheel, abed, has been adjusted 

 horizontal, let two equal weights, A and B, be suspended 

 from the extremities of a silk line of proper length, the 

 thickness of which is no greater than is just sufficient to 

 sustain the weights. When these weights are perfectly 

 quiescent, a small impulse being applied to either, in a ver- 

 tical direction, will set the whole in motion ; which will be 

 continued uniform till one of the boxes arrives at the ex- 

 tremity of the scale. When the box, A, is at the bottom 

 of the scale and quiescent, it must be observed whether the 

 middle line on the scale be everywhere exactly opposite to 

 the line sustaining A ; or, in other words, whether the line 

 in the middle of the scale be in the same vertical plane with 

 the line which sustains A. If it be not, the lower extremity 

 of the scale must be moved along the arm of the base until 

 the adjustment is correct. It is also to be observed, whether 

 the line be everywhere at equal perpendicular distances from 

 the middle line on the scale : if it be not, the lower extremity 

 of the scale must be removed further from, or nearer to the 

 silk line, until the distances are everywhere equal. The 

 middle line on the scale will now be vertical, and the circular 

 frame must be so constructed, that the box, A, may pass 

 centrally through it, when the adjustments are correct. 



In letting the box, A, begin to descend at any beat of the 

 pendulum, the observer must not wait until he hears the 

 beat, at which he intends A's descent shall begin ; for, in 

 this case, A's descent will always commence too late ; the 

 proper method is to attend to the beats of the pendulum, 

 until an exact idea of their succession is obtained : then the 

 extremity of the rod being withdrawn from the bottom of 

 the box, A, directly downwards at the instant of any beat, 

 the descent will commence at the same instant. 



Having now sufficiently explained the instrument, we shall 

 proceed to the construction of some experiments with it. 



Let two equal weights, A and B, be suspended by a line, 

 joining them, and going over a fixed pulley. If any weight 

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