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.