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But notwithstanding all that the knowledge and skill of Huyghens could do, his most perfect instruments were still at the mercy of atmospheric changes. It has been said, that the time of a pendulum's vibration depends upon the length of its suspending-rod. This length is measured, not down to the bottom of the weight, but to the centre of its mass.
A hot day, therefore, lengthens the metallic suspending-rod of a pendulum, and carries the centre of its weight to a greater distance from the point of suspension. By this means, the period of each vibration is of necessity lengthened. An increase of temperature to the extent of ten of Fahrenheit's degrees, will make a second's pendulum with a brass rod lose five vibrations in a day.
Now, suppose that a careful and patient observer, aware of the exact length of the suspending-rod of a vibrating pendulum, were to set himself down to count how many beats it would make in a given period, he would thenceforward be able to assign a fixed value to each beat, and would consequently have acquired an invariable standard whereby he might estimate short intervals.
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