But to return to the main point; there may have been, perhaps, a little difference between the times of the two balls: indeed there certainly was, and we know the cause of it. It is the resistance of the air, which acts more effectively on light bodies than on heavy. For two solid balls such as we have used the difference is not great: but if we took a feather or other very light body we know that it would fall very slowly. This does not mean that gravity acts less on a feather than on a piece of lead, but that the air resists its fall more effectively. If there were no air, the feather would fall just as quickly as the balls of wood and lead. I cannot show you this by exhausting all the air from this room, because we want some of it to breathe, but if you will kindly be content with a much smaller drop, we can do the experiment with this tall jar. It is an experiment which has been done many times before and is called the "coin and feather" experiment. The coin used to be a golden guinea, but I fear none of my audience is likely to have such a coin with them, and perhaps they might not even like to lend me a sovereign, so I must use one of my own. We put a feather alongside it in a little trap, rather like that which released the balls, and then we close the tall jar over them and exhaust all the air. In old days this was done with a hand-pump, and required some exertion, but with the beautiful resources of this Institution we have merely to attach a tube and turn a tap; in a few seconds we have a nearly perfect vacuum. And now, if we release the trap, you see that the coin and feather fall together. Directly we turn the tap again and let the air in, the feather blows about while the
