The Laws of Motion. 219 



the power of gravity in their ascent. Thus, if I 

 were to throw the bullet up to the steeple of 

 Westminster Abbey, I must give it just as much 

 force as it acquired in its descent. Thus again, 

 the body D in rolling down the inclined plane, 

 A B (Plate XVIII. fig. 83) will acquire suf- 

 ficient velocity by the time it arrives at B to carry 

 it up nearly to C ; and if the plane were per- 

 fectly smooth, and the air gave no resistance, it 

 would carry it up quite to that point: it is upon 

 this principle the pendulum is constructed. You 

 all know, I conceive, that a simple pendulum 

 consists of a bob or ball fixed to a small string or 

 wire. If therefore the bob (fig. 84) is let go at 

 a, it will fall to d, and by the velocity it acquires 

 in the fall it will rise to c : this is called an oscil- 

 lation ; and if a pendulum were put in motion in 

 a space quite void of air, and free from all resist- 

 ance from friction on the point of suspension, it 

 would move for ever. Pendulums vibrate in pro- 

 portion to the square roots of their lengths, and 

 the vibrations of the same pendulum are always 

 performed in the same space of time. Hence 

 their great utility in measuring time ; for a pendu- 

 lum of thirty-nine inches, one-fifth will vibrate an 

 aliquot part of the time the earth is turning on 

 its axis, that is, l-86400dth part, or sixty times 

 in a minute. Near the equator, however, pendu- 

 lums move slower than near the poles ; and they 

 are also subject to variations and irregularities 

 from heat and cold, which causes the metals, of 



