that these extraordinary particles of the solar photosphere ought henceforth to be identified solely by the name given by the discoverer; a name in reality sufficiently accurate, although to some eyes the form of a willow-leaf does not represent literally the appearance of these solar fragments. Among other observers who have devoted particular attention to solar phenomena, we may mention the names of the Rev. F. Howlett, Dr. Selwyn, Professor Phillips, M. Chacornac, and M. Faye, some of whom have deduced very interesting conclusions; for instance, M. Chacornac has published some valuable remarks in the Comptes Rendus relative to the variable luminosity and reflective power of various portions of the solar photosphere, and to the successive envelopes which are supposed to enclose the centre of our system. 810. New chart of the Moon. The importance of an accurate delineation of the lunar surface, on a large scale, has been acknowledged for some time, particularly as many omissions have been found in the chart of Beer and Mädler. The subject has been a popular one. Some of Mr. Nasmyth's drawings and photographs of portions of the lunar surface, from models made by himself, are exceedingly truthful, giving all the effect of light and shade in a most marvellous manner. Mr. Birt has also published accurate delineations on a large scale, but Professor Schmidt has completed a map two metres in diameter, in which the details of the lunar features are given with a minuteness almost beyond conception. A specimen of this map was exhibited at one of the meetings of the Royal Astronomical Society, when it was much admired for the extreme delicacy with which all the details are delineated. 811. Irregular proper motion of Sirius and Procyon.Some very interesting papers on the proper motion of Sirius and Procyon have been communicated to the Royal Astronomical Society, by Messrs. Auwers, Main, Safford, and O. Struve. The apparent variability of the proper motion of these stars had, for some time, attracted the attention of the practical astronomer. With respect to Sirius, M. Calendrelli, of Rome, asserted in 1857, that this variability arose from errors in the composition of the Greenwich catalogues of stars, and not from any peculiar motion of the star itself. Mr. Main has, however, clearly shown that M. Calendrelli was himself in error, and he has also proved that the apparent discordances in question had not their origin in any error of observation or reduction; "but that it depended upon a real fluctuation, most important and interesting, which will render the motion of this star a serious subject for study during the remainder of the present century." We are indebted to MM. Auwers and Safford for complete investigations of the supposed orbit of Sirius, on the assumption that the irregularities in the proper motion are produced by the perturbations of a dark disturbing body. Some excitement was therefore made by the announce ment of the discovery of a companion to Sirius, on the 31st of January, 1862, by Mr. Alvan Clark, of Boston, U. S. The angular position and distance of the companion with respect to Sirius, as observed by M. Otto Struve, are as follows:: The researches of M. Auwers on the orbital motion of Sirius require, in three years, an increase of the distance of the disturbing body of o"55, and a diminution of the position angle of 5°.31. These numbers agree so closely with those deduced by M. Struve from his micrometrical measures, that it can scarcely be doubted that the small object discovered by Mr. Alvan Clark is really the cause of the irregularities in the proper motion of Sirius. The magnitude of this companion is excessively minute, commonly it is recorded as the ninth or tenth, so that only on the most favourable occasions, and then with first-class telescopes, is there any possibility of its being seen. Mr. Newcomb, of Washington, has also discussed the position observations of the companion to Sirius, and has come to the conclusion that this optically minute object is truly a satellite of that brilliant star. There is also strong evidence that the perturbing influence of a faint companion, observed by O. Struve in 1873 and 1874, is the cause of the irregular proper motion of Procyon. 812. Movement of the solar system in space.-On page 416, we have given a brief abstract of the investigations on this subject by different astronomers, and we have shown the remarkable agreement existing between the results, with respect to the position of the apex of solar motion. From this we might infer, that its velocity and direction are as accurately known as any other of the celestial movements. But this is not the case, for it must be borne in mind that many of the proper motions by which the results are obtained, are not altogether certain, and that the problem itself has still some speculation mixed up with it. However, this agreement between the independent determinations of different astronomers, in relation to the point in the heavens to which the direction of solar motion is assigned, has been generally accepted as sufficient evidence for the settlement of the position of this point within a reasonable limit. Most astronomers have followed similar methods of investigation, until Mr. Airy, in 1859, devised a new plan of computation, from a conviction of the inadequacy and defects of the methods hitherto in use. It is out of place here to enter into any detailed account of this new method of Mr. Airy, because we could scarcely do so without giving all the mathematical formulæ. The general principle of the method, however, consists merely in removing the primary geometrical notions from the apparent movements on the surface of a globe to the real movements of the bodies in space. This is performed by treating the linear movements of the sun and of each star by the use of rectangular co-ordinates. The advantages resulting from the adoption of this method are: 1. That it is perfectly complete and independent, requiring no assumption of a point determined by preceding investigations. 2. It gives the proper weight to each observation, subject to the consideration as to the general weightmultiplier to be attached to any class of stars defined by brilliancy or other characteristic, which may enable us to judge of their distance. Mr. Airy first applied this method to 113 stars whose proper motions are large, arranging them into groups according to magnitude, in conformity with the researches of W. Struve, whose assumed relative distances were also adopted. From the most probable of two assumptions, Mr. Airy found the right ascension of the apex of solar motion to be 261° 29′, the north polar distance 65°16', and the velocity of solar motion 1912. This large velocity depends principally on the excessive proper motions of a few of the stars. In undertaking this preliminary investigation, Mr. Airy expressed a wish that it should be considered only as a specimen of the application of a new method, which he hoped would be applied to a larger number of stars. The continuation of this investigation was at once commenced at the Royal Observatory, and in 1863, a second paper was prepared by Mr. Dunkin, at the request of the Astronomer Royal.+ On this occasion, the proper motions of 1,167 stars were employed, 819 of which are situate in the northern, and 348 in the southern hemisphere. As in the former paper, the investigation was made on two hypotheses:-first, by supposing that the irregularities of proper motion are entirely due to chance-error of observation; and second, that they are due solely to a peculiar motion * Memoirs of the Royal Astronomical Society, vol. xxviii. of the stars themselves. From the first of these hypotheses, the following results have been obtained : Annual velocity of solar motion=0"3346. The results on the second and more probable hypothesis are as follows:: Annual velocity of solar motion=0"4103. It will be observed, from the preceding numbers, that the annual velocity of solar motion, or the angular displacement of the sun as viewed from a star of the first magnitude, differs but slightly from that determined from the researches of M. Otto Struve (03392). The mean of the three values gives o"3614. Now, if we assume this to be the probable amount of the proper motion of the solar system in space, and that the average parallax of a star of the first magnitude is o" 209, we shall find, by comparing the annual solar motion with the radius of the earth's orbit, that it amounts to 1729 of such units, or, in round numbers, 158 millions of miles. Thus far it will be seen that the direction of solar motion given by these two investigations agrees generally with that found by former astronomers, and therefore, prima facie, the result may be considered satisfactory. If, then, these proper motions depend in a great measure on a proper motion of the sun, we might expect to find that if we take the sums of the squares of the residua uncorrected for solar motion, and again when corrected, the sums of the latter would be considerably diminished. The following is the result of this comparison :— Sum of squares of Motion in Parallel Sum of squares of Motion in N. P. D. Uncorrected=78"-7583 The small diminution in the corrected numbers is very curious, and it shows that our fundamental suppositions must rest, to some extent, on a slender basis; and the results, notwithstanding the general agreement in the position of the solar apex, warrant the question whether we have much ground to infer that the proper motions of the stars are produced principally by the motion of the sun and its system in space. It may be, and probably such an inference is partially true, that these apparent motions in the positions of the stars are rather due to some compound effect, resulting from causes some of which have yet to be discovered. Again, we may remark that the grounds upon which we have been working are uncertain to a considerable extent, which uncertainty will scarcely be removed until our knowledge of the distances of the stars is increased, or until the proper motions themselves are redetermined from unexceptionable observations made at both epochs with the improved instruments of modern times. Referring to the small diminution in the sums of the squares in Mr. Dunkin's corrected numbers, Sir John Herschel observes: "No one need be surprised at this. If the sun move in space, why not also the stars? and if so, it would be manifestly absurd to expect that any movement could be assigned to the sun by any system of calculation which should account for more than a very small portion of the totality of the observed displacements. But what is indeed astonishing in the whole affair, is, that among all this chaotic heap of miscellaneous movement, among all this drift of cosmical atoms, of the laws of whose motions we know absolutely nothing, it should be possible to place the finger on one small portion of the sum total, to all appearance undistinguishably mixed up with the rest, and to declare with full assurance that this particular portion of the whole is due to the proper motion of our own system." # 813. Observations of the spectra of stars and nebulæ.— Some very important observations of the spectra of the fixed stars and nebulæ have been made during the last few years, in this country, principally by Mr. Huggins and Dr. W. A. Miller. A series of observations has also been made at the Royal Observatory, by Mr. Carpenter, in which the dark lines of the stellar spectra have been micrometrically measured in reference to the principal fixed lines of the solar spectrum.† In comparing the lines of the stellar spectra with those of certain chemical elements, Mr. Huggins and Dr. Miller have found several remarkable coincidences occur. For example, in the spectrum of Aldebaran, coincidences with nine of the elementary bodies were observed, viz. sodium, magnesium, hydrogen, calcium, iron, bismuth, tellurium, antimony, and mercury. In Sirius and a Orionis, five cases of coincidence were found. These comparisons have been made on a number of other stars, in some of which corresponding lines have been observed, whereas in other cases no lines coincident with those of any known chemical element have been noticed. Mr. Huggins has also analysed the light of several nebulæ and clusters. These objects give either a continuous spectrum, analogous * Outlines of Astronomy, eighth edition, p. 704. For an explanation of the spectroscope and of the method of observing Fraunhofer's lines of the solar spectrum, the reader is referred to Lardner's Handbook of Natural Philosophy.-Optics, p. 145. |