88 Some Practical Difficulties about Pendulums. 



hecce the first necessity is to become intimately acquainted 

 with them. 



There are two points in the pendulum which are of im- 

 portance, the centre of gravity and the centre of oscillation. 

 These two centres are in different positions, and the more 

 distant the more disturbance is produced in the going of the 

 pendulum. Hence to bring them as near each other as possible 

 is to be aimed at. The pendulum is, therefore, made of a 

 slender rod, to which a heavy weight is attached ; but a heavy 

 weight will cause the rod to stretch, and hence a constant ten- 

 dency to go slow. Next comes the influence of changes in the 

 temperature, to overcome which the gridiron pendulum and 

 the mercurial pendulum have been invented, each of which 

 has its peculiar disadvantages. The regularity of the 

 oscillations of the pendulum is further affected by the density 

 of the atmosphere, so that when the barometer is high the 

 oscillations are of longer duration than when the barometer is 

 low. The amount due to this cause is nearly half a second per 

 day for a barometric change of one inch. To overcome this, 

 Dr. Robinson added a barometric tube to the pendulum rod in 

 different ways ; but, as one of his methods interfered with the 

 corrections for temperature and another would have the effect 

 of increasing the distance between the centres of gravity and 

 oscillation, I have tried in this pendulum to overcome the 

 difficulty by placing a barometer with an independent cistern 

 in the middle of the pendulum. Whether this will give satis- 

 faction, however, must yet be tried. 



