When we add to this circumstance the extreme faintness of Uranus, we cannot wonder that Herschel should have been unable to speak very confidently on many points of interest. His measures of the planet's globe were sufficiently satisfactory, and, combined with modern researches, show that Uranus has a diameter exceeding the earth's rather less than four and a half times. Thus the surface of Uranus exceeds that of our globe about twenty times, and his bulk is more than eighty times as great as the earth's. His volume, in fact, exceeds the combined volume of Mercury, Venus, the Earth, and Mars, almost exactly forty times. But Sir W. Herschel was unable to measure the disc of Uranus in such a way as to determine whether the planet is compressed in the same marked degree as Jupiter and Saturn. All that he felt competent to say was that the disc of the planet seemed to him to be oval, whether he used his seven-feet, or his ten-feet, or his twenty-feet reflector. Arago has expressed some surprise that Herschel should have been content with such a statement. But in reality the circumstance is in no way surprising. For as a matter of fact Herschel had been almost foiled by the difficulty of measuring even the planet's mean diameter. The discordance between his earliest measures is somewhat startling. His first estimate of the diameter made it ten thousand miles too small (its actual value being about thirty-four thousand miles); his next made it nearly three thousand miles too great; while his third made it ten thousand miles too great. His contemporaries were even less successful. Maskelyne, after a long and careful series of observations, assigned to the planet a diameter eight thousand miles too small; the astronomers of Milan gave the planet a diameter more than twenty thousand miles too great; and Mayer, of Mannheim, was even more unfortunate, for he assigned to the planet a diameter exceeding its actual diameter of thirty-four thousand miles, by rather more than fifty thousand miles. It will be understood, therefore, that Herschel might well leave unattempted the task of comparing the different diameters of the planet. This task required that he should estimate a quantity (the difference between the greatest and the least diameters) which was small even by comparison with the errors of his former measurements.

But besides this, a peculiarity in the axial pose of Uranus has to be taken into account. I have spoken of the uprightness of Jupiter's axis with reference to his path; and by this I have intended to indicate the fact that if we regard Jupiter's path as a great level surface, and compare Jupiter to a gigantic top spinning upon that surface, this mighty top spins with a nearly upright axis. In the case of Uranus the state of things is altogether different. The axis of Uranus is so bowed down from uprightness as to be nearly in the level of the planet's path. The result of this is that when Uranus is in one part of his path his northern pole is turned almost directly towards us. At such a time we should be able to detect no sign of polar flattening even though Uranus were shaped like a watch-case. At the opposite part the other pole is as directly turned towards the earth. Only at the parts of his path between these two can any signs of compression be expected to manifest themselves; and Uranus occupies these portions of his path only at intervals of forty-two years.

Herschel would have failed altogether in determining the pose of Uranus but for his discovery that the planet has moons. For the moons of the larger planets travel for the most part near the level of their planet's equator. We can, indeed, only infer this in the case of Uranus (for even the best modern measurements cannot be regarded as satisfactorily determining the figure of his globe), but the inference is tolerably safe.

For six years Herschel looked in vain for Uranian satellites. His largest telescopes, supplemented by his wonderful eyesight and his long practice in detecting minute points of light, failed to reveal any trace of such bodies. At length he devised a plan by which the light-gathering power of his telescopes was largely increased. On the 11th of January, 1787, he detected two satellites, though several days elapsed before he felt justified in announcing the discovery. At intervals, during the years 1790-1798, he repeated his observations; and he supposed that he had discovered four other satellites. He expresses so much confidence as to the real existence of these four bodies, that it is very difficult for those who appreciate his skill to understand how he could have been deceived. But he admits that he was unable to watch any of these satellites through a considerable part of its path, or to identify any of them on different nights. All he felt sure about was that certain points of light were seen which did not remain stationary, as would have happened had they been fixed stars. No astronomer, however, has since seen any of these four additional satellites, though Mr. Lassell has discovered two which Herschel could not see (probably owing to their nearness to the body of the planet). As Mr. Lassell has employed a telescope more powerful than Herschel's largest reflector, and has given much attention to the subject, no one has a better right to speak authoritatively on the subject of the four additional satellites. Since, therefore, he is very confident that they have no existence, I feel bound to represent that view as the most probable; yet I am unable to pass from the subject without expressing a hope that one of these days new Uranian satellites will be revealed.

The four known moons travel backwards; that is, they circle in a direction opposed to that in which all the planets of the solar system, and all the moons of Jupiter and Saturn, as well as our own moon, are observed to travel. Much importance has been attached to this peculiarity; but in reality the paths of the Uranian moons are so strangely situated with respect to the path of Uranus, that the direction in which they travel can hardly be compared with the common direction of the planetary motions. Imagine the path of