ii8 



NA TURE 



[Dec. 2, 1886 



stubby brushes which are found in a few cases, and correspond 

 to a repulsive force not more than one-fourth the sun's attrac- 

 tion. Supposing, as he do2s, that the njl repulsion is the sa ne 

 for ea :h atom, the apfianiil repulsion, or repulsive accclera'ioH, 

 would be greater for the lighter atoms, and nearly inversely pro- 

 portional to their molecular weights ; and so he concludes that 

 probably tails of the first type are co-nposed of hydrogen, those 

 of the second type of hydrocarbons, like coil-ga;, anl those of 

 the thii'dof iron, and its kindred metals. As to the second type, 

 the spectroscope speaks distinctly in confirmation. Tails of the 

 first and third types are not common, and are usually faint, and 

 since Br.dichin's result was announced there has been no 

 opportunity for spectroscopic verification in their case. 



I said his investigations had given a mathematical and me- 

 chanical explanation of comets' tails; but the /^/i'Vfl/ question, 

 as to the nature of the force which causes the observed repulsion, 

 remains unsettled, though I think there is no doubt that general 

 opinion is crystallising into a settled belief that it is electrical ; 

 that the sun is not at the same electric potential as surrounding 

 space, and that, in consequence, semi-conducting masses of 

 pulverulent matter, such as comets seem to be, are subject to 

 powerful electric forces as they approach and recede from the 

 central body. At the same time there are those — Mr. Ranyard, 

 for instance — who forcibly urge that the direct action of the solar 

 heat might produce a similar repulsive effect by causing rapid 

 evaporation from the front surface of minute particles, charged 

 with gases and vapiurs, frozen by the cold of outer spice. 

 -> I ought not to dismiss the subject of comets without at least 

 alluding to the numerous unprecedented and interesting pheno- 

 mena presented by the great comet of 1882: first, its unques- 

 tionable relation to, but distinctness from, its predecessors of 

 1880, 1843, and 1668, the three belonging to one brotherhood, 

 of common origin, and all following nearly the same path around 

 the sun. I call special attention to this point, because Miss 

 Gierke, in her new and admirable " History of Astronomy in 

 the Nineteenth Century" (which I hope every one interested in 

 astronomy will read as soon as may be), has, I think, made a 

 mistake regarding it, assigning to the difference between the 

 computed periods of these comets much too great an importance. 



The strange elongation of the nucleus of this comet into a 

 string of luminous pearls ; the faint straight-edged beam ol 

 light that enveloped and accompanied the co Jiet for some time ; 

 and the several detached wisps of attendant nebulosity that were 

 seen by several observers, are all important and novel items of 

 cometary history. 



.t Rfeleoi'S — Time will not allow any full discussion of the pro- 

 gress of meteoric astronomy. It must suffice to say that the 

 whole course of things has been to give increased certainty to 

 our newly-acquired knowledge of the connection between 

 meteor-swarms and comets, and to make it more than probable 

 that a meteor-swarm is the result of the disintegration and 

 breaking-up of a comet. This seems to be the special desson of 

 the Bielids, the re-appearance of which, as- a brilliant star- 

 shower last November, attracted so much attention. In an im- 

 portant paper read before the National Academy of Sciences, 

 last April, Prof Newton pointed out how all the facts connected 

 with the division into two of Biela's comet forty years ago, its 

 subsequent movements and disappearance, and the meteoric 

 showers of 1872 and 1SS5, and especially the peculiar features 

 of this last shower, all conspire to enforce this doctrine. 

 "I mention, doubtfully, in this same connection the recent sup- 

 posed discovery by Denning of what are generally alluded to 

 as "long radiants" : systems of meteors, i.e., which for weeks, 

 and even months, together, seem nightly to emanate from the 

 same point in the sky. One of these radiants, for instance, the 

 first of half a dozen described by Mr. Denning, is about l.f° 

 north of 3 Trianguli, and the shower appears to last from July 

 toiNovember, at the rate of perhaps one or two an hour. 

 - If the fact is real, it follows inevitably that, disseminated 

 through all the space in which the ea.th is moving, and has 

 been moving for several years — not less than 1,000,000,003 

 miles — there are countless meteoroids moving in parallel lines, 

 and with a velocity so great that the earth's orbital motion of 

 19 miles a second is absolutely insignificant as compared with 

 theirs. Their speed must be many hundreds of miles per second. 

 This may be true, but I own I am not ready to accept it yet. 

 The observations indicate directly no extraordinary swiftness. 

 Mr. Proctor, whose mind appears at present to be chiefly occu- 

 pied by the idea that suns and planets are continually bombard- 

 ing their neighbours (or at least do so at some stage of their 

 existence), ascribes such meteors to the projectile energies of 



some of the " great " stars. But there is not time to discuss 

 his notion, and it is hardly necessary, until it has begun to receive 

 somewhat more extensive acceptance. I am not aware that, so 

 far, he has any converts to his theory of comets and meteors. 



.Stars. — Want of time will also prevent any adequate treat- 

 ment of the recent progress of stellar astronomy. 



Two great works in the determination of star places must, 

 however, be mentioned. One is the nearly completed catalogue 

 of all the northern stars, down to the ninth magnitude, begun 

 almost twenty years ago, under the auspices of the .\stronomische 

 Gesellschaft, by the co-operation of some fifteen different obser- 

 vatories. The observations are now nearly finished, and several 

 of the observatories have already redticed and publi^hed their 

 work. A very few years more ought to bring the undertaking 

 to a successful end. 



Another similar work, almost, though not quite, as extensive, 

 is the great catalogue of southern stars, made at the Observatory 

 of Cordova by our own Dr. Goidd and his assistants. He 

 himself, with his own eyes, observed every star of the whole 

 number — nearly 80,000 — his assistants reading the circle and 

 making the records ; and the whole has been reduced, printed, 

 and published within the space of twelve years — a veritable 

 labour of Hercules, for which, most justly, our National Aca- 

 demy has awarded him the Watson Medal. He had already, some 

 years ago, received the gold medal of the English Royal Astro- 

 nomical Society, for the " Uranametria Argentina," an enumera- 

 tion of all the naked-eye stars of the southern hemisphere, with 

 their approximate positions and estimated magnitudes. This, 

 however, was only a sort of preliminary by-play, to pass the 

 time while waiting for the completion of his observatory and 

 meridian-circle. 



We must mention also the remarkable star-charts made by 

 Dr. Peters, of Hamilton College, of which he has already 

 published and distributed at his own expense about twenty, and 

 more are soon to follow. 



But the old-fadiioned way of cataloguing and charting the 

 stars is obviously inadequate to the present needs of astr momy, 

 and a new era has begun. While, hereafter, as hitherto, the 

 principal stars, several hundred of them, will be observed even 

 more assiduously and carefully than ever before, with the me- 

 ridian-circle or similar instruments, the photogr.aphic plate will 

 supersede the eye for all the rest. It is now easily possible to 

 photograph stars down to the thirteenth or fourteenth magni- 

 tude, and to cover a space of 25° square on a single plate. The 

 remarkable thirteen-and-a-half-inch instrument constructed by 

 the Henry Brothers, for the Paris Observatory, and first brought 

 into use last August, does this perfectly. Instruments very 

 similar, but smaller, lately set up at Harvard College, at the 

 Cape of Good Hope, and at Liverpool, while they do not reach 

 so faint stars, cover more ground at a time. 



Negotiations are already under way to secure the co-operation 

 of a number of observatories for a photographic survey of the 

 heavens ; and it is probable that, after some preliminary consulta- 

 tion and before very long, it will be actually in progress. Accord- 

 ing to Struve's estimates, it could be accomplished in about ten or 

 twelve years, even on the Paris scale, by the combined efforts of 

 fourteen or fifteen establishments. Orders have already been given 

 to the Henry Brothers, by Dom Pedro, of Brazil, and Mr. Com- 

 mon, of England, for instruments precisely like the one at Paris. 

 Americans, and New Yorkers especially, may well take a peculiar 

 interest in astronomical photography, since it was at Cambridge, 

 in 1S61, that the first star-photographs were ever made, and here, 

 in New York, Rutherfurd and Draper were among the earliest 

 and most successful workers : in the observatory above us is now 

 mounted the very instrument with which Rutherfurd made his 

 unrivalled pictures of the moon and his plates of the Pleiades, 

 more thjin twenty years ago. 



During the past ten years, stellar photometry has become almost 

 a new science. Its foundations, indeed, were laid by J. Herschel, 

 Seidel, Wolff, and Zollner, before 1870, and the magnitudes of 

 some two hundred stars had been measured, and the law of 

 atmospheric absorption determined. But the great work of 

 Pickering, at Harvard, in the invention and perfecting of new 

 instruments, and his "Harvard Photometry," which gives us a 

 careful measurement of the brightness of all the naked-eye stars 

 of the northern hemisphere, marks an epoch. And he is pushing 

 on, and has already well under way the measurement of the 

 300,000 stars of Argelander's " Durchmusterung. " Nor must we 

 omit to mention Pritchard, of England, whose name has just 

 been joined with Pickering's by the Royal .\stronomical Society, 

 in the bestowal of their gold medal for his wedge-photometer and 



