February 17, 1898] 



NATURE 



Z77 



specially refer to his work on Jupiter. It is well known that this 

 very difficult object not only demands instruments of much 

 optical perfection, but also calls for the exercise of the highest 

 qualities which constitute a consummate astronomical observer. 

 Mr. Denning, using a lo^-inch reflector by Browning, made 

 admirable sketches of the planet, in which he represented a 

 remarkable amount of detail with a skilful pencil. But he was 

 not content with work of this kind merely, however valuable 

 such work may be. He watched the revolutions of a large 

 number of spots individually, and determined as accurately as 

 possible their times of transit over the central meridian of the 

 planet. By such observations he ascertained the period of 

 rotation of a large number of the most notable spots. Thus he 

 illustrated the very interesting fact that the individual spots were 

 animated by large proper motions on the surface of the planet. 

 The famous red spot, so well known to every astronomer, has 

 been in particular most diligently followed by Mr. Denning, and 

 he has demonstrated the remarkable circumstance that its period 

 of rotation varies from year to year. The results at which he has 

 thus arrived are in close accordance with the observations of 

 other accomplished astronomers. 



(3) Researches on Meteors. 



But the great work of Mr. Denning's scientific career, and 

 that which has been mainly instrumental in deciding your Council 

 to confer on him the distinction of the Gold Medal, has been 

 connected with the very interesting subject of luminous meteors. 

 The labours of Mr. Denning have been extensive enough to cover 

 nearly every branch of the subject of meteoric astronomy. It 

 would be difficult, indeed I may say it would be impossible, to 

 render any adequate account of it within the limits of an address. 

 I am, therefore, obliged to restrict myself to those more salient 

 points which have specially been under the consideration of 

 your Council. 



The papers on luminous meteors communicated to the Royal 

 Astronomical Society by Mr. Denning, and printed in the 

 Monthly Notices, number altogether forty-two. In bulk they 

 would, if collected together, constitute two-thirds of an ordinary 

 annual volume in that form of the Society's publications. These 

 papers are illustrated much more copiously than has usually been 

 the custom with graphic and instructive figures. 



A considerable number of these memoirs contain valuable 

 lists of meteoric radiants derived either exclusively from Mr. 

 Denning's own work at Bristol, or from his own observations in 

 comparison with the work of other astronomers in the same 

 field. 



Mr. Denning's first published list of radiants appeared in 1876. 

 It contains the determinations of twenty-seven radiant points, 

 derived from his own observations of nearly 900 meteor tracks 

 which he had himself mapped at Bristol between 1871-76. In 

 this early list the periods of the recurrence of the showers are 

 occasionally indicated by single days or short periods. But 

 usually the month of recurrence is alone given. The list con- 

 tains careful determinations of the radiant-points of the chief 

 annual showers, such as Quarantids in 1873, the Lyrids in 1873 

 and 1874, the Perseids in 1871 and 1874, the Orionids in 1874, 

 and the Andromedids in 1872. It is interesting to note that 

 showers were found in correspondence with all but four radiants 

 in the list contained in Mr. Greg's first general Star Shower 

 Catalogue. This was published in comparison with Dr. Heis' 

 similar list of the same year in the British Association volume of 

 Reports for the year 1874. The four radiants which Mr. Denning 

 was not able to corroborate are, it ought to be observed, marked 

 as doubtful by Mr. Greg. 



The latest list of radiants which has been issued by Mr. 

 Denning, and the longest paper that he has as yet commmuni- 

 cated to the Monthly Notices, was published in 1890. This im- 

 portant work gives remarkable evidence of the unflagging 

 dilligence of our medallist. It contains a list of nine hundred 

 and eighteen radiant points, deduced from the observations of no 

 fewer than 9177 meteors, mapped at Bristol, between the years 

 1873 ^"^d 1889. This list, it should be remarked, includes re- 

 peated observations of the same radiants. This is the case not 

 only with regard to the chief showers, but also with regard to 

 various minor showers, whose yearly dates of recurrence cannot 

 in the present state of our knowledge be assigned with any 

 certainty. 



Though many of these showers are but sparsely characterised 

 by meteors, or, to use the more technical expression, are 



NO. 1477. VOL. 57] 



showers of great tenuity, they have not escaped the diligent 

 scrutiny of our medallist. In such cases the dates are generally 

 assigned to the single days when they have had apparently a 

 maximum abundance. A column of supplementary dates have 

 been added, which seldom range over more than a week. It 

 will thus be seen that this catalogue presents an extensive 

 series, not so much of averaged results as of exact and valuable 

 individual determinations. 



In other papers by Mr. Denning, mean results have been 

 given which may be regarded as expressing, with regard to many 

 important showers, the present state of our knowledge. For 

 the chief meteoric showers a list of this kind was published in 

 1887. In it will be found mean positions for 1880, derived from 

 fifteen years of observations of the radiant-points of the nine 

 chief yearly showers. These contain the seven well-known 

 periodic swarms of January, April, August, October, November 

 (two showers) and December, adding to these the striking and 

 very regularly visible displays of May 6 and July 28. 



The most important contributions which our medallist has 

 made to the general problems connected with luminous meteors 

 are connected with the long duration of certain meteoric 

 radiants. The fact of long persistency of radiant-points, and of 

 close assemblages about the points of groups or compact families 

 of simultaneous or successive meteor streams, is as old as Heis' 

 first essays in meteor showers. It is, however, to Mr. Denning's 

 persistent inquiries that we are mainly indebted for our know- 

 ledge of this subject. In his important paper of December 

 1884, Mr. Denning writes: "The fact of stationary radiants 

 exhibiting visible activity during several months is a phenomenon 

 so unaccountable and so utterly opposed to the approved 

 theories as to the orbits of shooting stars, that it must receive a 

 most crucial examination before it can be accepted." 



The long-continued labours of Mr. Denning on this important 

 subject have demonstrated the existence of these enduring 

 radiants. The theoretical difficulties connected with the subject 

 may be still not altogether removed, but we can hardly refuse 

 assent to Mr. Denning's words when he says : " It must be con- 

 ceived that a well-attested fact of observation, however hard to 

 reconcile with known theories, ought on no account to be dis- 

 regarded on account of its nonconformity." 



It is, of course, known that the Perseids from the August 

 shower are found, not only on the special nights with which the 

 swarm is chiefly associated, but they are also displayed on many 

 preceding and following nights. Mr. Denning has traced 

 meteors of this group for the twenty-six nights from July 25 to 

 August 19, and their radiant advanced in that interval over a 

 distance of 40 degrees. In one of those admirable diagrams by 

 which the interest of Mr. Denning's papers is so greatly in- 

 creased, he gives a curve of the ordinary number of Perseids 

 from one on July 25 to a maximum of 57 on August 10, ahd 

 then declining to I on August 19. In his paper of 1890 Mr. 

 Denning shows that the range of the Perseids is even wider 

 still. I, cite this case of the Perseids, because the gradual shift 

 of the radiants as days and weeks passed by is, of course, no 

 more than should be expected from the change of the place of 

 the earth in its orbit. The extraordinary fact is that in the case 

 of certain other showers, which are visible for weeks or months, 

 the radiant undergoes no appreciable change in position. Mr, 

 Denning selected for discussion in his " Memoir" of 1884 six 

 special showers. Among these we may particularly note the 

 o-j8 Perseids, which show a constant radiant from July 6 to 

 November 30. 



Such a paper as that to which I am now referring must be 

 regarded as a classic which every one who is interested in the 

 fascinating subject of meteors would do well to study. It is 

 full of interesting facts and suggestions. We learn that in the 

 catalogues published up to this date there are no fewer than 

 2100 radiants resulting from the projected paths of upwards of 

 6200 meteors ; many of these are, however, duplicate observations 

 of identical showers, and Mr. Denning adds that he does 

 not believe the total number of well-defined streams would 

 exceed 350. 



In his introduction to the great catalogue of 1890, Mr. 

 Denning has given a statement of his methods of work. 



" My plan of work may be briefly described as follows. 

 All the observations were made in the 6pen air and from the 

 garden adjoining the house. Attention was almost invariably 

 given to the eastern sky. In mild weather I sat in a chair with 

 the back inclined at a suitable angle, but on cold frosty nights 

 I found it expedient to maintain a standing posture and some- 



