450 



NATURE 



[March 7, 1889 



'Between Lynx and Auriga 98 



Near v Virginis 175 



,, K Cephei 300 



Meteor-Showers. 

 R.A. Decl. 



" .. 46° N. 

 .. 10 N, 



.. 80 N. 



Slow ; bright. 

 Slow ; bright. 



GEOGRAPHICAL NOTES. 

 M. RoLLAND, a French naturalist, charged with an official 

 mission to Madagascar, has sent in his Report to the Minister 

 of Public Instruction. M. Rolland sums up his geographical 

 observations by remarking that, notwithstanding its apparently 

 simple contour, the topography of Madagascar is exceedingly 

 complex. Behind the line of lagoons which border the coast, 

 and which, except that the water is salt, remind one of the 

 etangs of Languedoc, the hills begin to rise, and increase in 

 height towards the interior. Behind these, again, the mountains 

 rise by stages to a height of over 6500 feet. The surface is cut 

 up by innumerable ravines, at the bottom of which are torrents, 

 which rush on their way towards the Indian Ocean. This chain 

 forms the backbone of the island, and consists mainly of Primary 

 and crystalline rocks. When it is crossed, the Mozambique 

 Channel is reached. The two slopes, east and west, are very 

 unequal in extent. The former, which M. Rolland has ex- 

 plored to a considerable extent, occupies more than one-third 

 of the total area of Madagascar. A broad valley, that of 

 the Mangoro, runs north and south, parallel to the great central 

 chain and the coast. Unfortunately, the Mangoro is not navig- 

 able, even for canoes. The two other most important rivers are 

 the Manangoro and the Mangataka ; and these three rivers, with 

 innumerable streams, render this part of the island one of the 

 best-watered regions on the globe. The climate varies consider- 

 ably from one zone to another. On the east coast the temper- 

 ature oscillates between 13° and 30° C. ; on the west coast, it 

 never descends below 17°; in Imerina province it ranges from 

 5° to 25°. M. Rolland refers in some detail to the well-known 

 characteristics of the fauna of Madagascar, and to the abund- 

 ance of mineral treasures, especially iron, copper, and lead ; 

 but, he states, the natives carefully conceal the localities of the 

 beds. 



Lieut. Vans Agnew has undertaken a journey to the Upper 

 Salween and South- Eastern Tibet, with the object of attempting 

 the solution of the problem of the course of the Lu River — 

 whether to the Irawadi or the Salween — propounded by General 

 J. T. Walker in his paper read to the Royal Geographical 

 Society on April 25, 1887. The Council of the Society have 

 sanctioned a contribution of ^100 towards the expenses of the 

 expedition. Lieut Vans Agnew leaves India for the Salween 

 in the course of the present month. 



At the February meeting of the Berlin Geoijraphical Society 

 Dr. A. Schenck read a report on his recent journey in Nama 

 Land and Herero Land, South-West Africa. He showed that 

 the whole country between Walfisch Bay and the Orange River 

 is— in consequence of the purely mechanical decomposition of the 

 prevailing granitic rock, which is taking place under the great 

 daily variations of temperature, causing in many places the dis- 

 integrated surfaces to be eaten away in the form of a crust — 

 covered over with a sea of sand and granitic shingle, from which 

 the highest elevations stand out like islands. The country is 

 not suited for agricultural colonies. The coast and the 

 interior stand in contrast with regard 10 the season of 

 rainfall. While on the coast the rain falls mostly in 

 winter, the rainfall in the interior occurs only in sum- 

 mer, and nearly always in the form of thunder-showers, 

 which, as Dr. Schenck believes, are caused by the con- 

 densation of the moisture-laden air, which is brought to this 

 part by the warm, humid, north-east winds from the more equa- 

 torial regions of Africa, through coming into contact with the 

 cool south-west winds blowing from the coast to the interior. 

 As to the configuration of Great Nama Land, Dr. Schenck 

 gives the following notes. After the hilly coast-region between 

 Angra Pequena and Aos is passed, a broad valley- like depression 

 is reached, filled up with drift- sand. East of the depression the 

 country ascends and forms a stony, desolate plam, out of which 

 rise isolated peaks or longer mountain-chains running in a north 

 and south direction. The whole of this district, as far as Aos, 

 forms a connected mountain system composed of ancient rocks, 

 granite, and gneiss, which has been buried by the sand from 



which the higher parts stand out. Beyond Aos the traveller 

 enters upon the steppe region, which is divided into detached 

 plateau districts. Beyond Aos and the river-bed of the Goa- 

 gib, on which the station of Bethanien is situated, the Huib \ 

 plateau stretches away to the north, as far as the region of 

 Khuias, and to the south to a point a few miles north of the 

 Orange. A long series of table-mountains, resembling in form 

 truncated cones, mark the western escarpment of this plateau ; 

 the former are composed of granite and gneiss, and are covered 

 with limestone and sandstone, horizontally laid down. East of 

 Bethanien, and corresponding with the line of a long geological 

 fault, is the escarpment of another plateau ; it is about 5000 feet 

 in height. It descends to the Great Fish River on the east ; on 

 the other side of the river, the plateau character of the country 

 is continued to the Karas Plateau, which extends into the brush 

 steppe of the Kalahari. Further details concerning this interest- 

 ing region will be found in the March number of the Proceedings 

 of the Royal Geographical Society. 



THE FORCES OF ELECTRIC OSCILLATIONS 

 TREATED ACCORDING TO MAXWELL'S 

 THE ORV. BY DR. H. HER TZ. ^ 



IL 



Note by the Translator. 

 TT is to be noted that Hertz follows the French system of 

 -*■ wave-lengths and periods. Had I noticed this before the 

 diagrams went to the engraver, I would have altered it, and inter- 

 preted his T as iT, &c., throughout. As it is, I have left 

 them everywhere as in the original. My elaborate attempt 

 to evade a literal translation of Doppclpiinkt was quite unneces- 

 sary. Prof. Karl Pearson has sent me a reference to Maxwell's 

 definition of "double-point" in vol. i. Art, 129, first edition of 

 " Electricity and Magnetism."' — O. J. L. 



In order now to ascertain the distribution of force for the 

 remaining parts of space we may use graphic representation, 

 constructing for definite times the lines of electric force, viz. 

 the curves Q = const., for equi-distant values of Q. 



Since Q itself is the product of two factors, of which one de- 

 pends only on ;-, the other only on 0, the construction of these 

 curves presents no great difficulty. 



We decompose every value of Q for which we want the 

 curve into two factors in different ways ; we determine the 

 angle 6 for which sin- is equal to the one factor, and by 

 means of an auxiliary curve that value of p for which the function 

 of p contained in Q is equal to the other factor ; we thus get as 

 many points as we please of the curve. When one attempts to 

 carry out the construction one perceives many small processes 

 which it would be prolix to detail here. We will content our- 

 selves with examining the results of such construction, as exhibited 

 in Figs. I, 2, 3, 4. 



These figures represent the distribution of force at the times 

 ^ = o, ^T, 5T. |T ; and also, by suitable inversion of the arrows, 

 for all future times which are similar multiples of \T. At the 

 origin i-; shown, in the correct aspect and about of the right 

 proportional size, the arrangement by which in our earlier 

 experiments the oscillations were excited. 



The lines of force are not indicated right up to the picture 

 because our formulas regard the oscillators as infinitely short, so 

 in the neighbourhood of a finite oscillator they are insufficient. 



Let us begin a study of the figures with Fig. i. Here, when 

 t = o the radiation is in the condition of its strongest develop- 

 ment, but the poles of the straight oscillator are not electrically 

 charged — no lines of force start thence. Such lines of force begin, 

 however, noA' from the time /' = oto start out from the poles ; they 

 are inclosed in a sphere which expresses the value Q = o. In 

 Fig. I this sphere is indeed still vanishingly small, but it enlarges 

 itself quickly, and by the t = ^T it tills already the space K^ 

 (Fig. 2). The distribution of lines of force inside the sphere is 

 approxinialely of the same kind as correspond to a static electric 

 charge on the pole. The speed with which the spherical surface 

 Q = o spreads out from the origin is at first much greater than 



i-[or "z*"]; in fact, the latter velocity would only correspond to 

 A 



' Translated and communicated by Dr. Oliver Lodge. Continued from 

 p. 404. 



