[Illustration: Figure 1.--A STUDY OF THE FIGURE OF THE EARTH WAS one of the earliest projects of the French Academy of Sciences. In order to test the effect of the earth's rotation on its gravitational force, the Academy in 1672 sent Jean Richer to the equatorial island of Cayenne to compare the rate of a clock which was known to have kept accurate time in Paris. Richer found that the clock lost 2 minutes and 28 seconds at Cayenne, indicating a substantial decrease in the force of gravity on the pendulum. Subsequent pendulum experiments revealed that the period of a pendulum varied not only with the latitude but also regionally, under the influence of topographical features such as mountains. It became clear that the measurement of gravity should be made a part of the work of the geodetic surveyor.] _The history of gravity pendulums dates back to the time of Galileo. After the discovery of the variation of the force of gravity over the surface of the earth, gravity measurement became a major concern of physics and geodesy. This article traces the history of the development of instruments for this purpose._ THE AUTHORS: _Victor F. Lenzen is Professor of Physics, Emeritus, at the University of California at Berkeley and Robert P. Multhauf is Chairman of the Department of Science and Technology in the Smithsonian Institution's Museum of History and Technology._ The intensity of gravity, or the acceleration of a freely falling body, is an important physical quantity for the several physical sciences. The intensity of gravity determines the weight of a standard pound or kilogram as a standard or unit of force. In physical experiments, the force on a body may be measured by determining the weight of a known mass which serves to establish equilibrium against it. Thus, in the absolute determination of the ampere with a current balance, the force between two coils carrying current is balanced by the earth's gravitational force upon a body of determinable mass. The intensity of gravity enters into determinations of the size of the earth from the angular velocity of the moon, its distance from the earth, and Newton's inverse square law of gravitation and the laws of motion. Prediction of the motion of an artificial satellite requires an accurate knowledge of gravity for this astronomical problem. The gravity field of the earth also provides data for a determination of the figure of the earth, or geoid, but for this problem of geodesy relative values of gravity are sufficient. If g is the intensity of gravity at some reference station, and [Delta]g is the difference between intensities at two stations, the values of gravity in geodetic calculations enter as ratios ([Delta]g)/g over the surface of the earth. Gravimetric investigations in conjunction with other forms of geophysical investigation, such as seismology, furnish data to test hypotheses concerning the internal structure of the earth.