The acceptance of the ether as the medium of propagation of all kinds of forces across space does not explain the mechanism of its action, but, by ascribing to the ether certain properties, we are able to express in concrete figures, by the use of any convenient system of measurement, the results of experimental investigation. The general law of action at a distance has been proved by direct experi ment and by astronomical observation to be as follows Let two active masses be con centrated in two points a certain distance apart; then the force acting between them, that is, the force which is being transmitted from one point to the other by the inter vention of the ether, is proportional to the product of the two masses and inversely pro portional to the square of their distance. In the case of gravity this force is always at tractive, that is, tending to bring the masses nearer together; in the case of electricity or magnetism it may be attractive or repulsive according to the nature of the active masses.