January 17. 1895J 



NATURE 



271 



corollaries ; the diagrams of instruments scarcely give 

 any ideas of what they are intended to represent ; and 

 the descriptive part of the subject might have been 

 omitted without much sacrifice. The ground covered is 

 that which is ordinarily understood by an elementary 

 treatment of mathematical astronomy, dealing chiefly 

 with the considerations relating to the positions, move- 

 ments, dimensions, and distances of the various heavenly 

 bodies, but includes also some very scanty references 

 to their telescopic appearances. On the whole, the 

 various points are clearly, though shortly, e.xplained, 

 but there is much to suggest that the author would 

 be all the better for some little observatory practice ; 

 for example, his method of determining the angular 

 value of a micrometer by means of the sun (p. 48) is 

 scarcely practicable, and a sun-spot 13,000 miles long is 

 by no means to be classed as one of the largest spots (p. 

 68). It may be pointed out, also, that a single observer, by 

 observing at intervals of twelve hours, gets better results 

 for the parallax of Mars than two working in the way 

 indicated on p. 115. A ship's mean time at sea, too, is 

 usually determined by one observation near the prime- 

 vertical, and not by the method of equal altitudes. 



In less than a dozen pages the author attempts to give 

 an idea of the classification of the stars, and of "the 

 principal discoveries which have been made in modern 

 times, chiefly by means of spectroscopic analysis, with 

 their nature and physical condition ' (p. 203). The 

 omission of the solar prominences in an account of the 

 phenomena of a solar eclipse, is a good indication of the 

 very feeble character of this chapter. 



The book is intended specially for students preparing 

 for University examinations, and by such it may be 

 found useful. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Exploration at Ruwenzori. 



rp.RH.^PS it may interest your readers to give a short account 

 of Ruwenzori, where I have now ^pent four months. The 

 mountain is a very difficult one to study, on account of the 

 difficulty of reaching the most interesting part. Taking a sort 

 of botanical section from the shore of the Albeit Edward 

 Nyanza, one finds first a scries of gra'^sy plains covered with 

 Andropogous some two feet high, and in certain months sup- 

 porting large herds of elephants. Kudu, and Lurw.iU ante- 

 lopes. This is in part the old level of the lake, and in part 

 gravel and sand brought by the numerous rivers ; in places it is 

 doited by -Vcacia and Ihe iree Euphorbia, which has something 

 of the appearance of an enormous chandelier. After leaving 

 this plain, one comes to a series of small hills from 4000 'o 

 5000 feet in height, which have been apparenily cut out of the 

 mountain by the numerous riveis and streams. Some of these 

 are covered wiih patches o( culiivatiun, banana plantations, 

 &C. ; usually these are hidden from the main road. When one 

 reaches the mountain proper, one finds up to 7000 feet a steep 

 ascent covered with grass and small shrubs, usually three to 

 four feet high. The valleys in this part are usually very steep 

 V-shaped trenches, and cultivation is abundant everywhere, 

 sometimes over 7000 feet, and in the Wakondja country the edible 

 Arum is grown up to 7400 feet or more. This heighi, 7000 

 to 8000 feet, marks the beginning of the forest. Ii is composed 

 of deciduous trees, someiimes with a very thick undergrowth ; 

 sometimes it is pretty open, with a profusion of fern and moss 

 on the old trunks, and creepers in some places. I have found 

 tree ferns and Hegonia, but usually the flowers are rather pale 

 in colour, or quite inconspicuous. At 8(>oo feet another distinct 

 change takes place, and a wilderness of decaying young and 

 mature bamboos replaces the trees. Here and there these are 

 hung with creepers, but the predominant feature is the wetness of 

 everything. Moss covers almost every trunk below, and amongst 



NO. 13 16, VOL. 51] 



the rools are only very watery plants, such as Urticacete. At 

 9600 feet anolher change takes place ; bamboos disappear ccm- 

 (ilelely, and tree heather I akt sits place. In a drypartof ihemoun- 

 lainonefindsa charming little violei, a Cardamine, Galiums, Epi- 

 lobium, Rubus, &c. In wetter places one finds a regular peal-moss 

 with Sphagnum, beautiful orchids, and short heather ; in another 

 place one will find enormous trees of heather, usually gnarled 

 and twisted in growth — tree Senecios, tree Hypericums, &c. 

 This region seems to extend to the snow (which, I am sorry to 

 say, I have not been able to reach). On my highest attempt, I 

 could see the heather trees apparently higher than the snow. 

 On anolher attempt to get to the summit, I found what seems to 

 me .■Itchemilla alpina'. One feature of the mountain is the 

 extreme scarcity of animals and birds. In the lower forest there 

 are bushbuck, baboons, and two other sorts of monkeys ; one 

 the magnificent black and white-furred kind from which grena- 

 diers' shakos were made, and another with short slatey fur, which 

 is new to me. 



Peihaps the commonest birds are the sunbirds ; one green, 

 yellow, and crimson, I have seen above 10,000 feet, and 1 have 

 also seen (though 1 am almost afraid to say ii) a robin, and a 

 goldfinch. 



.\s 10 the geology of the mountain, I do not care to risk an 

 opinion at present, but I have taken many specimens which I 

 hope may solve some of the que-lions. I think glaciers must 

 have extended seven to eight miles down two of the valleys, but 

 there was no evidence to my mind of any extensive glaciation. 

 I think I am right in saying that the Salt Lake is nothing but 

 an extinct volcanic crater, and in several places along the east 

 side of the mountain there are others, or a smail chain of vol- 

 canoes ; usually the chain radiates from the centre of the 

 mountain. 



I hope to start to-morrow for Ujiji, my object being to see 

 whether a practicable route exists from Tanganyika to the Albert 

 Edward Nyanza. I only hope I shall be able to bring my col- 

 lections saely home. G. F. Scott Elliot. 



Salt Lake, Ruwenzori, August 2, 1894. 



The Alleged Absoluteness of Motions of Rotation. 



I MUST confess that discussions upon mathematicil meta- 

 physics appear to me to be somewhat unpractical. They are 

 suggestive of the case upon which Serjeant Snubbin was en- 

 gaged, which related to a right of way " leading from some 

 place which nobody ever came from, to some other place which 

 nobody ever went to." Nevertheless, I propose to offer some 

 remarks upon this subject. 



That aiW;//^ motions of translation and rotation exist appears 

 to me too clear for argument ; but whether our senses are 

 capable of taking cognisance of them and reducing them to 

 exact measurement, is quite anolher matter. The view 

 advocated by Prof. Greenbill appears to be that motions of 

 rotaiion are determinate — that is to say, they are capable of 

 exact measurement within the limits of experimental error ; 

 v/hereas the contrary is the case with motions of translation. 

 Mr. Love, on the other hand, holds that neither kind of motion 

 is determinate in the above sense. Now a knowledge of the 

 absolute value of the velocity of translation of any object in- 

 volves a knowledge of the magnitude and direction of the sun's 

 velocity in space ; and until the latter has been determined, 

 which has not yet been done, the former is necessarily unknown. 

 It therefore follows that all motions of translation of which our 

 senses are capable of taking cognisance are relative. 



On the other hand, the motion of rotation of any oV"ject is 

 independent of the motion of translation of the sun or any other 

 body. If, therefore, it were in our power to construct a system 

 of axes which either move parallel to themselves or whose 

 angular moiion is known, it «ould be possible to determine ihe 

 absolute value of ihe angular velocity of any object. liut even 

 if it were possible to devise an experiment by which such a 

 system of axes could be obtained, our results would only be 

 accurate within the limits of experimental error. It may, 

 therefore, be well to point out that, without inventing any new 

 experiment, the angular velocily of any object may be 

 accurately determined within the limits of experimental errorxn 

 the following manner. 



.Select two stars, X and V, whose proper motions are so 

 minute ihat they have never been detected by the most refined 

 observations. Through the sun and the two stars draw a plane, 

 S X V, and through the sun draw a line, S /., perpendicular to 



