vestigate the properties of the space surrounding the large body. In this investigation the only variables are the force and the distance from the active centre of , the force being always directed either towards or from that point.
The force may thus be considered as an attribute of space, and it becomes possible, even if itself is inaccessible, to determine its magnitude and location by measuring the direction and magnitude of the force experienced by unit matter in different parts of space. It is not even necessary that the measurements should be made on unity of active matter; any convenient quantity of active matter in the small body will serve. All we need do is to reduce the measured force in the ratio of the actual amount of active matter used to its unit value. It is in this manner that astronomers, by observing disturbances in the orbit of a known star, can predict the existence of some heavenly body not yet discovered by the telescope. The astronomical problem is exceedingly complicated because of disturbances from other active masses for which allowance has to be made, but in studying the same problem as applied to electric charges no such complication need
