Rotation of Mars.—The diurnal period of this planet is known with far greater certainty and precision than that of any other planet, the Earth excepted. It will be useful to quote the values derived since Cassini’s time:—

h m s
1666.J. D. Cassini24 40
1704.J. P. Maraldi24 39
1781.W. Herschel24 39 21·7[37]
1784.W. Herschel24 37 27
1838.J.H. Mädler24 37 23·8
1845.O. M. Mitchell24 37 20·6
1859.A. Secchi24 37 35
1864.F. Kaiser24 37 22·62
1866.R. Wolf24 37 22·9
1869.R. A. Proctor24 37 22·735
1873.F. Kaiser24 37 22·591
1873.J. F. J. Schmidt24 37 22·57
1883.A. Marth24 37 22·626
1884.W. F. Denning24 37 22·34[38]
1885.H. G. v. de S. Bakhuyzen24 37 22·66

The last of these, by Prof. Bakhuyzen of Leyden, is probably the best. It was based on a large number of observations extending over 220 years, viz. from those of Huygens in 1659 to those of Schiaparelli in 1879.

In a terrestrial day Mars rotates through 350°·8922, according to Mr. Marth’s period. In one hour the axial motion is 14°·6, whereas on Jupiter the horary rate of rotation is 36°·7. At intervals of 40 days (during which Mars completes 39 rotations) the various features on the disk are presented at very nearly the same times as before. Mr. Marth’s ephemerides of this planet are extremely useful to those who study the markings; and these, in combination with the charts and memoirs of Schiaparelli, Green, Terby, and others, greatly facilitate and encourage the renewed study of this object.

Further Observations required.—Favourable oppositions of Mars occur every 15 years, as in 1877 and 1892. It is at such periods that this planet should be sedulously interrogated for new features, or for corroboration of those already known. Rather a high power must be employed—certainly more than 200; and if the telescope has an aperture of at least 8 inches, the observer will be sure to discern a considerable extent of detail. He should compare his views with the various charts previously alluded to, and note any inconsistencies. Fresh drawings should also be made; and if the forms are not well assured on one night, he may confirm them by coming 37 minutes later to his instrument on the following night. Or the collective issue of several nights’ work may be included in the same drawing. The bright spots on the planet should be as attentively studied as the darker regions, and given a place in every drawing; for it is probably in connection with these luminous objects that active changes may be recognized. The “canals” and their duplication form the principal markings to be looked for; though the successful elucidation of these appearances can only be expected in a case where a powerful telescope, a keen eye, and a good atmosphere operate together. Something of them may be seen under ordinary conditions, and they ought to be very generally sought for by amateurs; for it is not always that success is found where the best conditions prevail. The great telescopes at Mount Hamilton, Nice, and other observatories may be expected to command some advantages of light, power, and position; but this need not prevent competition, or induce the idea that common appliances are practically of no avail. Everyone should strive to achieve as much as is consistent with his means and opportunities; indeed there is all the more need for effort and energy in the observer when his tools are seemingly inadequate to a research, and he should endeavour to find, in his own eye and understanding, that power which shall compensate in a great measure for lack of instrumental capacity. Mr. Proctor, in his ‘Old and New Astronomy,’ has justly remarked:—“The directors of Government observatories have usually been much less successful in studying planetary details than those zealous amateurs who take delight in the study of the heavenly orbs and are ready to wait and watch for favourable opportunities.”

Changes on Mars.—Changes have been confidently reported in some of the Martian spots. Instances have been quoted in which particular markings, though very plain at certain times, have scarcely been perceptible at others. Variations in outline as well as in visibility appear to have been witnessed, and the subject is one which merits more extended notice. It has been asserted that the origin of such variations probably lies in the aerial envelope of Mars. In April and May 1888 M. Perrotin, with the great refractor at Nice, failed to re-observe the feature known as the continent “Libya” on Schiaparelli’s chart, and stated that though this formation was plainly visible in 1886, it had ceased to exist in 1888. He suggested that the obscuration was really produced by clouds or mists circulating in the atmosphere of Mars. But Prof. Holden reported, from the Lick Observatory, that the object alluded to was distinctly visible with the 36-inch refractor there at the end of July, and in the same form in which it was drawn by Prof. Schiaparelli in 1877-8. It is to be assumed, therefore, that if any change occurred it was one of transient nature.

There are other questions relating to the physical aspect of this planet which future observers should be able to answer. Do the markings retain their distinctness right up to the limb? Is the opaque crescent of the disk (when Mars is in quadrature) involved in any phosphorescence or glow indicating an atmosphere? Are the bright spots and luminous borders to the continents equally as stable as the dark spots, and do they maintain an equable brilliancy?

The N. hemisphere of Mars needs much further study, as it is not so familiarly known as the S. hemisphere. This is due to the circumstance that, at favourable oppositions, the region of the S. pole is suitably presented for observation. It is only when the planet is comparatively distant, and small in diameter, that his N. hemisphere comes into view.

The difference of inclination under which the features are seen at successive oppositions gives rise to many apparent changes of figure. When the S. hemisphere is exposed to the Earth numerous objects are seen which are quite invisible when the opposite hemisphere is displayed to us. These altering conditions have to be considered in their influences by every student of areography.

Satellites of Mars.—After evading the keen and searching eyes of Sir W. Herschel, and the power of his 40-foot telescope—after eluding the grasp of Lord Rosse’s 6-foot speculum, and the frequent scrutiny of Lassell with his 2-and 4-foot mirrors, the two satellites of Mars were ultimately revealed to Prof. Hall in the 25·8-inch refractor at Washington. These tiny orbs had been enabled to avoid previous discovery by their minuteness and by their close proximity to Mars. Yet as soon as they were known to exist many observers saw them, and in certain cases success was undoubtedly attained with comparatively small instruments. The late Dr. Erck picked up the outermost satellite with a 7-1/3-inch objective, and Mr. Pratt saw it with an 8-1/7-inch mirror by With. But the effect of this eye-straining may just possibly, in one or two instances, have drawn the imagination out of its normal repose. Mr. Pratt’s instrument shows stars in the group ε Lyræ which are invisible in the great Washington telescope and in the 36-inch mirror formerly used by Mr. Common; so that it may well have produced a spectral satellite of Mars. But the satellites are certainly within the occasional reach of moderate means; for they were repeatedly seen with a 9½-inch refractor at the Observatory of Princeton, U.S.A., in October and November 1879. They “were decidedly more easy to see than Mimas,” the innermost satellite of Saturn.