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\section{The Gravitational Field}  ``If we pick up a stone and then let it go, why does it fall to the ground?'' The usual answer to this question is: ``Because it is attracted by the earth.'' Modern physics formulates the answer rather differently for the following reason. As a result of the more careful study of electromagnetic phenomena, we have come to regard action at a distance as a process impossible without the intervention of some intermediary medium. If, for instance, a magnet attracts a piece of iron, we cannot be content to regard this as meaning that the magnet acts directly on the iron through the intermediate empty space, but we are constrained to imagine--after the manner of Faraday--that the magnet always calls into being something physically real in the space around it, that something being what we call a ``magnetic field.'' In its turn this magnetic field operates on the piece of iron, so that the latter strives to move towards the magnet.