similar manner to the ether itself. We might, in fact, consider them as ethereal manifestations without any attempt to explain the exact nature and mechanism of these manifestations. Such a conception is quite compatible with the practical use of our general formula; it simply means that we must look upon as a kind of ethereal coefficient, the numerical value of which has to be found experimentally.
The conception of an ethereal coefficient, stated in this general way, is perhaps a little difficult to grasp. To make the matter clear I start by applying it to the familiar phenomenon of gravity, and then proceed to investigate the more unfamiliar phenomena of electric and magnetic forces. At the outset we must agree on the units we are going to use in giving a numerical expression to the attractive force between two bodies. If one of these bodies is the earth and the other a stone, this force is simply the weight of the stone. If the two bodies are the earth and the moon, the force is that which just balances the centrifugal force experienced by the moon in flying round in her orbit. The formula for the attraction given on p. 14 refers to masses concentrated in points, and it might
