July 2, 1891] 



NA TURE 



209 



OUR ASTRONOMICAL COLUMN. 



The Capture Theory ok Comets. — The last three numbers 

 of the Bulletin Astronomique (April-Jane) have contained papers 

 by M. L. Schulhof, " Sur les Grandes Perturbations des Cometes 

 Periodiques," which place beyond doubt the idea that the 

 periodic comets of our system are captured by the perturbing 

 action of planets. The main object of the research was to develop 

 the relations existing between the elements of the comet's orbit 

 before and after its entrance within the sphere of activity of 

 the disturbing planet. With the criteria obtained, and some 

 results previously formulated by M. Tisserand, it is possible to 

 decide the question as to the identity of two comets of which 

 the time of revolution of one is known, even when the cnmet is 

 believed to have passed several times within Jupiter's sphere of 

 activity between two apparitions. This result is of the highest 

 importance, for it is only by such means that individual comets 

 can be identified. They cannot be recognized by their appear- 

 ance, as they possess no peculiar characteristic that can be 

 telescopically observed. 



M. Schulhof suufgests that, in the light of recent work, periodic 

 comets should not now be classified according to their aphelion 

 distances, but arranged in groups the mean aphelion distance of 

 which approximates to the length of the semi-major axis of one 

 or other of the planets. Such a division has been made for 

 comets having periods between lo and 10,000 years. From the 

 tabulated results, it appears that four comets have aphelion dis- 

 tances which differ but little from the aphelion distance of 

 Mercury. The Venus group numbers seven, the earth's group 

 ten. Mars possesses four comets, and Jupiter twenty-three. 

 Saturn has a family of nine, Uranus eight, and Neptune five. 



Wolf's Periodic Comet (b 1891). — The following ephe- 

 meris is from one given by Prof. Berberich in Edinburgh Circular 

 No. 17. From Astronomische Nachrichten, No. 3042, it appears 

 that Dr. Spitalier observed this comet on May 2; that is, before 

 Prof. Barnard. The brightness on the date (May 4) of re- 

 discovery by the latter observer has been taken as unity. 



Ephemeris for Berlin Midnight. 

 R.A. Decl. Log A. Log ; 



Bright- 

 ness. 



h. m. s. ■ o 



July 6 ... I 4 34 ... -{- 26 30-3 ... 01916 ... 0-2305 ... 3-08 

 ,, 10 .. I 15 8 ... 27 15 ... o-i8oo ... 02270 ... 330 

 ,, 14 ... I 25 49 ... 27 28-3 ... 0-1683 ... 0-2237 ... 3-54 

 „ 18 ... I 36 35 ... 27 50-5 ... 0-1565 ... 02206 ... 3-80 

 „ 22 ... I 47 24 ... 28 76 ... 0-1446 ... 0-2178 ... 406 

 ,, 26 ... I 58 15 ... 28 19-3 ... 0-1326 ... 0-2152 ... 4-34 

 ,, 30 ... 2 9 7 ... 28 25-2 ... 0-1204 ••• 0'2I27 ... 464 



Aug. 3 ... 2 19 57 ... 28 25-1 ... 0-1081 ... 0-2105 ... 4-96 

 ,, 7 ... 2 30 42 ... 28 18-6 . 0-0957 ... 0-2085 ••• 5"3i 

 „ II ... 2 41 21 ... 28 5-2 ... 00832 ... 0-2068 ... 5-67 

 ,, 15 ... 2 51 51 ... 27 44-7 ... 0-0707 ... 0-2053 ••• 605 

 ,, 19 ... 3 2 10 ... 27 i6-6 ... 00581. ... 02041 ... 644 

 „ 23 ... 3 12 14 ... 26 40-9 ... 0-0455 ... 02032 ... 6 85 

 ,, 27 ... 3 22 o ... 25 57-3 ... 00329 ... 0-2026 ... 7-28 

 M 3* ••• 3 31 26 ... 25 5-6 ... 00204 ••• 02022 ... 7-72 

 Sept. 4 ... 3 40 28 ... 24 5*6 ... 0-0080 ... 0-2021 ... 8-i8 

 „ 8 ... 3 49 I ... + 22 57-1 ... 99957 ••• 0-2024 ... 8-64 

 The comet is now in Pisces, and in the beginning of Septem- 

 ber will pass through the Pleiades. M. Bigourdan, of ParisObser- 

 vatory, observed it on June 12, and remarked that it was "une 

 nebulosile ronde, d'environ 20" de diametre, de grandeur 13-3." 



YORUBA AND GAZ ALAND. 



A T the meeting of the Royal Geographical Society, on 

 "^*- Monday, two papers were read : one by Mr. Alvan 

 Millson, on the Yoruba country, West Africa, and the other 

 by Mr. Denis Doyle, on a journey in Gazaland, in South-East 

 Africa. 



The ancient kingdom of Yoruba may be taken as one of the 

 most interesting of the great tribal divisions of West Africa, 

 between the Gold Coast and the Niger. 



Landing at Lagos, the only natural harbour on a thousand 

 miles of coast, a narrow entrance with a 15-feet bar leads into 

 the intricate chain of waterways which extends, with few and 

 slight interruptions, for 500 miles from the Volta river to the 

 Benin branch of the Niger Delta. From the east and west, 



from the Benin river and the waters of the Dahomian frontier, 

 the coast of the gulf is backed by intersecting channels of fresh 

 water flowing steadily from either hand towards the Lagos 

 outlet. In many places these narrow and brimming channels 

 are separated fiom ihe onslaught of the Atlantic rollers by no 

 more than five or six level yards of shifting sand ; the spray 

 from the ocean drifts over ihem, and the loar of the surf is 

 heard by the native as he glides over their calm surface in his 

 fragile canoe. These so-called "lagoons of the Bight of Benin " 

 form but a small portion of the littoral river systems of West 

 Africa ; for from Cape Palmas to Cape Three Points the long 

 Kroo coast is lined by inland waters for the greater part of 300 

 miles, and beyond the rocky .spurs of the beautiful Gold Coast 

 the Dahomian shores have the same remaikable formation. At 

 right angles to this network of channels numerous rivers flow 

 down fror.i the uplands of the interior, tarrying m their rapid 

 streams vast quantities of sand and mud with which they busily 

 build out the land. At fir.-t sight it seems strange that so many 

 and such powerful streams, flowing strongly towards the sea 

 should suddenly be turned aside from their courses by so narrow 

 and fragile a barrier of shifting sand. To the influence of the 

 sheltering headlands which jut out towaids the south; to the 

 rapid Guinea current which tears away the face of their rocky 

 shores and hurries towards the east a ceaseless stream of sand ; 

 to the almost tideless ocean, and the absence of high winds, for 

 the strength and duration of a West African tornado are but 

 slight as compared with the hurricanes of the West Indies or 

 the gales of our stormy coast ; and above all to the enormous 

 growths of floating papyrus aud water-grass which line the 

 inner banks of the lagoons, and prevent the swollen waters from 

 breaking through into the ocean, are due the formation and con- 

 tinual development of this strange delta system. For these 

 rivers are in most instances choked for many miles by a floating 

 papyrus-sod bound together by wild water-figs and palm-wine 

 palms {Raphia vinifera), and when the floods come down from 

 the interior great masses of this floating vegetation are torn away 

 and carried down to the lagoons and onwards towards the sea. 

 Hundreds of acres of these j^rass islets are annually carried down 

 from each of these rivers, and are driven against the banks of 

 the littoral lagoons, where they lodge and grow, and eventually 

 become anchored in their places by more permanent vegetation. 

 In this manner the lagoon sides are padded for hundreds of 

 yards, and even, in some instances, for two or thiee miles in 

 depth on either hand, and their banks are protected from the 

 wash of the current and the weight of the accumulated waters. 

 By this means the frail barrier of sea-sand is strengthened, and 

 the inland waters, although they frequently rise to a height of 

 5 to 6 feet above the sea-level, are effectually prevented from 

 bursting through their banks. Not only are these growths a 

 permanent protection to the land, but by their very nature, 

 floating as they do on the surface of the water, they rise and 

 fall with the floods, and are always ready with their assistance 

 at the right time and place. Were all the rivers which feed the 

 lagoons freed from their natural obstructions, as is the case with 

 the Ogun river near Lagos, the interior to a distance of from 

 thirty to seventy miles would be thrown open to commerce, and 

 the wonderful system of inland navigation which fosters the 

 coast traffic would be still further developed. 



Mr. Millson went on to describe a journey from the coast to 

 the interior, the country rising from terrace to terrace. He then 

 spoke at some length of the Yoruba people and country. 



About eighty miles from the coast, at Oda Ona Kekere, 

 the dense forest suddenly gives place to open cultivated 

 land, and a densely peopled country. Some three miles 

 to the north of Odo Ona Kekere, from the crest of a 

 rising in the undulating land the great city of Ibadan — 

 the London of Negroland — comes full in view, extending for 

 over six miles from east to west, and for more than three from 

 north to south. Surrounded by its farming villages, 163 in 

 number, Ibadan counts over 200,000 soul.s while within the 

 walls of the city itself at least 120,000 people are gathered. Its 

 sea of brown roofs covers an area of nearly 16 square miles, and 

 the ditches and walls of hardened clay which surround it are 

 more than 18 miles in circumference. Its houses are built round 

 courtyards with a single entrance, and form in themselves no 

 mean defence against native inroads. Their walls of thick 

 "adobe" are blank on the outer face, and the thatched roofs 

 are made of a light covering of palm leaves and grass in order to 

 avoid the danger of extensive conflagrations. In the winding 

 rocky streets which intersect these large compounds in every 



NO. 1 131, VOL. 44] 



