Ascertaining the Force of Gravity.
A pendulum which swings once in a second at the base of a tall tower will require for the same travel a little more than a second when borne to the top of the tower, because then further from the centre of the earth. Still greater will be the difference in its periods as it swings first at the base of a mountain and next at its summit. A pendulum, therefore, is a means of learning the force of gravity at a given place, and without sacrifice of accuracy it is well that it should be as small as possible. In 1890, Professor T. C. Mendenhall, then superintendent of the United States Coast and Geodetic Survey, designed a pendulum one fourth the length of those previously used, and of admirable precision. Afterward pendulums were built of dimensions further reduced to about two and one half inches in length, with periods of oscillation of one fourth of a second. Such pendulums are easily carried to stations difficult of access, and have been employed on the summits of high mountains, including Pike’s Peak: their indications agree well with those of the larger and somewhat cumbersome apparatus previously used.