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The long series of his communications to the Royal Society of London opened May 11th, 1780, with a discussion of his observations, begun in October, 1777, of Mira, the variable star in the neck of the Whale. As to the theory of its changes, he agreed with Keill that they could best be explained by supposing rotation on an axis to bring a lucid side and a side obscured by spots alternately into view. A second paper by him on the Mountains of the Moon was read on the same day. He measured, in all, about one hundred of these peaks and craters.

In January, 1781, there came an essay stamped with the peculiar impress of his genius, entitled “Astronomical Observations on the Rotation of the Planets round their Axes, made with a view to determine whether the earth’s diurnal motion is perfectly equable.” It embodied an attempt to apply a definite criterion to the time-keeping of our planet. But the prospect is exceedingly remote of rating one planet-clock by the other. Herschel’s methods of inquiry are, however, aptly illustrated in this curiously original paper. His speculations always invited the control of facts. If facts were not at hand, he tried somehow to collect them. The untrammelled play of fancy was no more to his mind than it was to Newton’s. His ardent scientific imagination was thus, by the sobriety of his reason, effectively enlisted in the cause of progress.