2 lO 



NA TURE 



[July 2, 1891 



direction, are countless market booths and occasional market 

 places, where the inhabitants can purchase native produce, food, 

 and European luxuries. In the same way, by the sides of the 

 country roads, are built at irregular intervals varying from one to 

 six miles, long low sheds close by some well or running water, where 

 the farm women sit and " make their market." In the farms 

 which extend throughout the country from horizon to horizon as 

 one journeys through it, save where the land is too poor, or the 

 fear of war has desolated the neighbourhood, can be heard the 

 crowing of cocks, the barking of dogs, the shrill laughter of 

 children, and the vociferous clamour of native homestead gossip. 

 For among natives, as among seafaring folk at home, a hundred 

 yards or so is no impediment to polite conversation. From this 

 custom arises the disadvantage that the voices of the people 

 being naturally pitched for distant communication cannot readily 

 be restrained or focussed for nearer ranges of social intercourse. 

 The consequent turmoil and shrill cries are apt at first to un- 

 settle the nerves of an inexperienced traveller, but a few weeks' 

 residence in the country not only accustoms one to their manner 

 of speech, but inures one's system to the sudden shock of their 

 sonorous voices. 



Northward from Ibadan, which may be described as the 

 centre of the chief military and commercial power in Yoruba, 

 two days' journey — about 40 miles — through many villages, and 

 a landscape dotted far and near with oil-palms {Elu;is gidneensis) 

 along a road thronged with travellers, brings one to the capital 

 of central Yoruba, Oyo (Awyaw). On leaving Ibadan, Mr. 

 Millson passed, in the course of a morning's march, over 4700 

 men, women, and children, hurrying into the great city from the 

 farm villages with loads of maize, beans, yams, yam flour, sweet 

 potatoes, fowls, pigs, ducks; or driving cattle, sheep, and goats ; 

 or mounted on small native horses which amble quickly along 

 under the combined influence of an Arab ring bit and an armed 

 spur which leaves its traces in deep scores along the flanks of 

 the poor animals. Far and wide the land has for generations, 

 and indeed for centuries, been cultivated by these industrious 

 natives. The hatchet, the fire and the hoe, have removed all 

 traces of the original forest, save indeed where a dark trail of 

 green across the landscape shows where the valley of some 

 narrow watercourse or larger river is hidden among trees. For 

 two or three years at most the land is allowed to lie fallow, while 

 for three or four years double or treble crops are raised with no 

 further cultivation than an occasional scrape with a hoe, and | 

 during its fallow time no further care is taken of it than to let a 

 rank growth of reedy grass spring up some 6 or 8 feet in 

 height. Among this grass can be seen the seedlings and young 

 plants of a new forest, which would rapidly take possession were 

 the land to be permanently deserted. In spite of this careless 

 and exhausting method of cultivation the crops maintain an 

 excellent average, and the same plot of ground serves for 

 generations to support its owners. 



Mr. Doyle, who accompanies King Gungunhana's two envoys 

 to this country, described his journey from the Mashonaland 

 plateau down through Gazaland to the mouth of the Limpopo. 

 At first the journey was through a broken plateau country, 

 rising to 5000 feet and over, and well adapted for farming 

 operations. After fourteen days' travel, the country suddenly 

 drops from a level of 4000-5000 feet to 860 feet above sea-level. 

 For many miles the altitude was no more than 300 feet, and 

 as it was the rainy season when Mr. Doyle and his com- 

 panions passed through, they found the country almost entirely 

 a swamp. The actual distance travelled was between 700 and 

 800 miles, which was traversed in forty-six days. 



THE CONDITION OF SPACE. 

 T^HE question of the condition of inter-planetary space, with 

 special reference to the possibility that it offers a resist- 

 ance to the passage of the heavenly bodies, has for long occupied 

 the attention of astronomers, but is even yet far from receiving 

 a satisfactory or definite solution. Three hypotheses seem to be 

 more or less in vogue : — 



(i) That it is filled with "ether," differing entirely in its 

 properties from ordinary matter, and offering no resistance to 

 the passage of solid or gaseous bodies. Radiant energy is 

 transmitted by the vibratory motion of the ether, and possibly 

 also the force of gravitation is transmitted by a rotatory motion, 

 though, as Laplace points out, the velocity of the gravitation 

 mu-t be at least 7,ooo,ooD times that of light. 



NO. II 3 I, VOL. 44] 



(2) That it is filled with an ether more analogous to ordinary 

 matter, which offers resistance, or with a highly rarefied gaseous 

 medium similar in constitution to our atmosphere. 



(3) That it is filled with ether, through which innumerable 

 solid bodies of comparatively small size fly singly or in swarms. 

 When they encounter one another, a gas, or a planet, they 

 become luminous, and present the appearance of fireballs, 

 meteorites ; shooting-stars, meteors ; comets, meteoric swarms ; 

 meteoric dust gives rise to the phenomenon of the aurora 

 borealis. This theory has recently been much extended and 

 admirably advocated by Prof. J. Norman Lockyer, in " The 

 Meteoritic Hypothesis." 



If the first hypothesis be true, and space off"ers no resistance 

 to the passage of the planets, Laplace has shown {Mem. Acad. 

 des Science!:, 1784) that any change in their orbits will be 

 periodic, or, in other words, that, with only slight variations 

 from time to time, the present condition of the solar .system will 

 continue indefinitely. 



If the second hypothesis be true, the resistance, however 

 slight it may be, will tend to retard the motion of the planets. 

 In the case of the earth the friction between the outer layers of 

 the atmosphere and the medium will retard the rotation of the 

 earth, and increase the length of the day. There will also be a 

 resistance to the motion of the earth in her orbit, which will tend 

 to decrease the velocity, and therefore to lengthen the year ; but, 

 on the other hand, if the tangential velocity be decreased while 

 the attraction of the sun remains the same, the earth will fall 

 towards the sun, the mean distance will decrease, and therefore 

 the time of revolution will be shortened. 



If the third hypothesis be true, the rain of meteorites will 

 have no effect on the rotation of the earth, but will tend to 

 lessen the orbital velocity. 



Laplace has discussed some consequences of the second hypo- 

 thesis in "Mecanique Celeste," vii. 6, on secular variations in 

 the movements of the moon and earth which might be produced 

 by the resistance of an etherial medium spread round the sun. 

 He assumes that the density of the medium is a function of the 

 distance from the sun, and that the resistance varies as the 

 square of the velocity. He concludes that the accelera- 

 tion produced by the resistance of a fluid ether on the 

 mean motion of the moon is, up to "the present time," in- 

 sensible ; and that the acceleration produced by the same 

 ether on the motion of the earth would be less than i/ioo of 

 that caused to the motion of the moon. These results are ex- 

 tended to other planets and to comets in x. 7, where it is shown 

 that the distance at perihelion remains unchanged, and the only 

 alteration in the orbit is a decrease in the length of the major 

 axis and in the eccentricity. 



The question is discussed from a mathematical point of view 

 in several text-books {e.g. Tait and Steele, "Dynamic-; of a 

 Particle," pp. 279, 379), but in all cases the mathematics are 

 somewhat difficult, and various assumptions have to be made to 

 render the solution possible. 



In the case of the earth, if the resistance of the medium be 

 small, the orbit may be considered to be circular, more espe- 

 cially as it follows from Laplace's results that the error intro- 

 duced decreases with the time, since the orbit becomes more 

 nearly circular. The following brief abstract of the popular 

 treatment suggested by G. A. Hirn in his "Constitution dt; 

 I'Espece Celeste," pp. 104-108, with the substitution of English 

 values, and the extension of the results to the meteoric hypo- 

 thesis, may be not without interest at the present time. 



Many of the data are so uncertain, that the rough approxima- 

 tions by which mathematical difficulties are avoided probably 

 produce no great loss of arithmetical accuracy in the results. 



The vis viva of the earth at the end of any period is equal to 

 the vis viva at the commencement of the period, less the vis 

 viva lost owing to the resistance of the medium, and increased 

 by the vis viva due to the fall towards the sun. Transposing, 

 and dividing by M/2 — 



V," = Vo^ -f Vs- - Vi-. 



Writing S for the attraction of the sun, and resolving along 

 the radius vector A — 



v.yAo = S, . •. Vo' = SA. 



After a time L 



r/A = S 



A^- 

 Ar' 



= SAo-/Ai. 



