Page:Popular Astronomy - Airy - 1881.djvu/263

Rh because there are other forces besides gravity acting on the pendulum, that is to say, the clock weights acting through the train of the clock wheels. It is necessary to have a detached pendulum. Now I wish to know how many vibrations that pendulum would make in a day. It is troublesome to find out. Possibly the pendulum will not swing for a whole day.

In the experiments made in an expedition directed by the Spanish Government, a man was stationed on each side of the pendulum, to count 60 vibrations at a time; and they continued to count the vibrations as long as the pendulum continued sensibly in motion. When they had got through a great number of 60's, they observed the time which a clock showed. It was a very tedious method indeed.

In every other instance in modern times, the vibrations of the detached pendulum have been compared with the vibrations of a clock pendulum. The mode adopted in the English and French expeditions was this: a detached pendulum is placed in front of a clock; a person is watching with a telescope; he watches when the two pendulums are going the same way; he remarks whether the vibrations of the detached pendulum recur quicker or slower than those of the clock pendulum; he sees that the vibrations separate more and more, till the two pendulums actually move in opposite ways; after this, they begin to move more nearly in the same way, and at length move exactly in the same way; perhaps the number of vibrations elapsed between these two agreements of motion may be 500. If you can determine the time when the two pendulums swing the same way, you find how long it is before one pendulum gains two vibrations upon the other. Then the calculation