in themselves, or of the attraction of the earth "by the corporeal mass thereof, as by a collection of bodies of the same nature." If it be so, he says, "it will follow that the nearer all bodies approach to the earth, the stronger and with the greater force and velocity they will tend to it; but the farther they are, the weaker and slower:" and his experiment consists in comparing the effect of a spring and a weight in keeping up the motions of two "clocks," regulated together, and removed alternately to the tops of high buildings and into the deepest mines. By clocks he could not have meant pendulum clocks, which were not then known, (the first made in England was in 1662,) but fly-clocks, so that the comparison, though too coarse, was not contrary to sound mechanical principles. In short, its principle was the comparison of the effect of a spring with that of a weight, in producing certain motions in certain times, on heights and in mines. Now, this is the very same thing that has really been done in the recent experiments of professors Airy and Whewell in Dolcoath mine: a pendulum (a weight moved by gravity) has been compared with a chronometer balance, moved and regulated by a spring. In his 37th aphorism, Bacon also speaks of gravity as an incorporeal power, acting at a distance, and requiring time for its transmission; a consideration which occurred at a later period to Laplace, in one of his most delicate investigations.
(197.) A well chosen and strongly marked crucial instance is, sometimes, of the highest importance; when two theories, which run parallel to each other (as is sometimes the case) in their explan-
