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INTRODUCTION. The year 1877 will be remembered by the discovery of the two faint satellites of Mars more than by any other one event. A possible satellite of Mars has often been sought for by D’Arrest, Tuttle, and others, but an imagined analogy placed such a satellite or satellites farther from the planet than either of the bodies discovered by Professor Hall. These satellites may be said to be fatal to the analogies ordinarily quoted, and to show how vague a guide these really are. The outer satellite is almost unique in the solar system, and the inner one is quite so, in its relations of distance and mass with its primary. A further account of these will be found in the proper place.
The changes of spectrum of Schmidt's Nova Cygni are of great importance, and are certainly not yet understood; and the same may be said of Dr. Henry Draper's discovery of oxygen in the sun, even the nearer consequences of which have not yet been worked out.
The astronomical expedition of Mr. David Gill to Ascension Island is particularly noteworthy; and by advices from Mr. Gill up to October 8, it appears probable that success is secured. These and other subjects are spoken of briefly under their appropriate heads, but it must be remembered that the accounts here given are necessarily the barest summaries, and are intended mainly to call attention to work which has been done, in order that a reference to the originals may be made when desired.
NEBULÆ AND THE NEBULAR HYPOTHESIS. Lord Rosse is preparing for publication all the observations of nebulæ made at Birr Castle with his six and three feet mirrors during the last thirty years. Those drawings which have already been published in the Philosophical Transactions will not be republished. The whole will probably be divided into three or four parts, each comprising 6 h. or 8 h. of right ascension. The editing is done by Dr. Dreyer, Lord Rosse's assistant, and the work is to be published by the Royal Irish Academy. It is expected to be soon in the hands of astronomers.
Dr. Dreyer is also engaged in preparing a supplement to Herschel's general catalogue of nebulæ, etc., and any additions or corrections to this should be sent to him.
The Melbourne reflector has for some time been employed for the purpose of re-drawing all nebulæ previously figured by Sir John Herschel. Forty-nine nebulæ have been carefully drawn by the observers, and the results of the work are about to be published. Mr. Ellery speaks of the lithographic copies of these drawings as fully successful.
M. Stephan, of Marseilles, gives the places of thirty new nebulæ discovered by him, making 185 in all found at Marseilles. The first 125 of these will appear in Dreyer's new catalogue of nebulæ.
Dr. Koch, of the Leipzig Observatory, has published a micrometric investigation of the cluster G. C. 1712 for his inaugural dissertation. It contains an interesting résumé of previous work of this kind. Other work of the kind is now in hand at this observatory.
The cluster Gamma Argūs, respecting which Gilliss reported changes since Sir John Herschel's observations, has