gravity, and you see there [pointing to the scales] a good deal of water gravitating towards the earth. Now here [exhibiting a small piece of platinum[1]] is another thing which gravitates towards the earth as much as the whole of that water. See what a little there is of it—that little thing is heavier than so much water [placing the metal in opposite scales to the water]. What a wonderful thing it is to see that it requires so much water as that [a half-pint vessel full] to fall towards the earth, compared with the little mass of substance I have here! And again, if I take this metal [a bar of aluminium[2] about eight times the bulk of the platinum], we find the water will balance that as well as it did the platinum; so that we get, even in the very outset, an example of what we want to understand by the words forces or powers.
I have spoken of water, and first of all of its property of falling downwards. You know very well how the oceans surround the globe—how they fall round the surface, giving roundness to it, clothing it like a garment; but, besides that, there are other properties of water. Here,
