this also might be heated, (though not so soon nor so much as the iron;) by which one may also take notice, that 'tis not necessary, a body should be it self hot, to be calorific. And now I speak of striking an iron with a Hammer, I am put in mind of an Observation that seems to contradict, but does indeed confirm, our Theory: Namely, that, if a somewhat large nail be driven by a hammer into a plank or piece of wood, it will receive divers strokes on the head before it grow hot; but when 'tis driven to the head, so that it can go no further, a few strokes will suffice to give it a considerable Heat; for whilst, at every blow of the hammer, the nail enters further and further into the wood, the motion that is produc'd is chiefly progressive, and is of the whole nail tending one way; whereas, when that motion is stopt, then the impulse given by the stroke being unable either to drive the nail further on, or destroy its intireness, must be spent in making avari-
