November 1, 1892.] 



KNOWLEDGE 



211 



then, nearly the whole of the solar speed, and the sum has 

 to be further increased by being compounded with a visible 

 or tangential movement of ten miles a second ; so that the 

 Dog-star traverses space probably about twice as quickly 

 as the sun. 



Arcturus furnishes another notable instance of the 

 surrender, on closer enquiry, of a fictitious velocity. Its 

 original seeming approach of fifty-five miles has dmndled, 

 at Potsdam, to 4-6 miles a second — a result in all but 

 perfect agreement with that independently obtained by 

 Professor Keeler with the great Lick refractor. His 

 measures were made upon the D-lmes in the spectrum of 

 the star ; and a similar determmation of Aldebaran by 

 Mr. \Y. W. Campbell shows recession to the amount of 

 MO'5 miles, the Potsdam photograplis giving 80'2 miles 

 per second. The former inferiority of the visual to the 

 chemical method of ascertaining the radial movements of 

 stars thus seems at last abolished : though not always or 

 everywhere. The Lick atmosphere and the Lick telescope 

 form a hitherto unmatched combination, and the signal 

 advantages conferred by them have been illustrated by 

 an important discovery. Owing to the enormous supply 

 of light at his command, Professor Keeler was able to 

 execute micrometrical measures in nebular spectra of the 

 fourth order, the dispersion, equal to that given by twenty- 

 four prisms of 60^, being effected by a Rowland's grating 

 of 14,43H lines to the inch. The upshot was the detection 

 of motion-displacements of considerable magnitudes. Until 

 then nebuliB had appeared absolutely stationary : they 

 yielded no certain sign, telescopic or spectroscopic, of 

 mobility in any direction. The removal of this seeming 

 anomaly constituted an advance of no small moment. 

 The outcome of Professor Keeler's experiments was more 

 decisive than could have been anticipated. The mean 

 velocity towards or from the earth of the eleven nebuliP 

 measured (including that in Orion), proved to be once and 

 a half times that ot the Potsdam stars, or sixteen miles a 

 second." Eight of these showed movements of approach, 

 only three movements of recession. The disparity, how- 

 ever, was doubtless due to their situation mainly in the 

 hemisphere iii front of the sun. The most rapid traveller 

 amongst them is the celebrated planetarv in Draco 

 (N.G.C. G.343 = H. IV., 37), found by Professors Holdeu 

 and Schaeberle to possess a curious helical structure. 

 Prolonged observations have as yet eUcited from it no 

 trace of proper motion ; yet its rate of transport in our 

 direction turns out to exceed forty miles a second. The 

 swiftest motion of recession — twenty-eight miles a second — 

 belongs to a small stellar nebula (N.G.C. 6700) : the well- 

 known " Saturn planetary " in Aquarius advancing at a 

 nearly equal rate. So far as these preliminary trials go, 

 then, nebular take the lead of steUar movements in the 

 line of sight. 



The Orion nebula recedes from the sun. or we should 

 perhaps rather say the sun leaves it behind at the 

 rate of 10-6 miles a second. A solar velocity of 13-(i miles, 

 directed towards a point some ten degrees east of a Lyrse, 

 would completely account for the observed line-displace- 

 ments in the spectrum of that amazing object, and allow 

 us to adopt the plausible hypothesis that the vast system 

 of which it forms the nucleus exists in a state of com- 

 parative rest. It may be worth notice that the three i 

 stars, z, p, and " Orionis, share the virtual retreat from 

 the earth of the chaotic mass adjacent to them on the I 

 sphere, and perhaps not disconnected fi'om them in space. ] 

 Another example of related motion in the stellar members 



* Reducing to the true wave-length of the chief nebular line, as 

 subsequently fixed br Keeler himself at 5003'95 of Angstrom's scale. 



of a nebulous system is most likely met with in a and 

 V Cygni. Both stars are pretty clearly shown in M. 

 Wolfs photographs — as the readers of Knowledge for 

 October, 1801, may remember — to be involved in the out- 

 skirts of the same extensive nebula ; and they are found 

 to be endowed with what may be fairly called a combined 

 movement towards the sun of four or five miles a second. 

 If this were purely a transferred effect of the solar trans- 

 lation, it would imply for it a speed of only 5-3 miles a 

 second, which seems unreasonably small. The alternative 

 supposition is perhaps to be preferred — namely, that the 

 sun tends to overtake the star-and-nebula system, which 

 travels in the same direction, but at a slower pace. Such 

 points, however, can scarcely yet be profitably debated : 

 they can certainly not be decided until a complete solution 

 of the problem of the solar movement is at hand, based on 

 materials derived in equal measure from the southern and 

 northern hemispheres. The nature of the required data 

 could not be better exemphfied than by the specunen-Iists 

 of radial velocities provided from Potsdam and Lick. 



Notices of Boofe.s. 



Lu Pliiitt' Mars et ses conditio)is d'htthitubilite. Par 

 Camille Flammarion. (Gauthier - Villars. Paris.) — M. 

 Flammarion has presented the world with a very important 

 monograph on the planet Mars, to which he has devoted 

 much time and labour. The volume, which is a large 

 octavo and extends to 600 pages, contains over 300 beau- 

 tiful woodcuts, in which M. Flammarion has reproduced 

 580 drawings of the planet by the principal observers of 

 Mars. Nothmg of any importance seems to have escaped 

 him. Beginning with Fontana's very rough sketches made 

 in 1636, and ending with drawings made during the pre- 

 sent opposition, in addition to sketshes and drawings 

 made by other observers, he has also given many beautiful 

 drawmgs of his own, and admirable maps of the chief 

 planetary features. M. Flammarion is of opinion that Mars 

 is or may be inhabited, and that a very large number of 

 the canals and other details delineated have a real exist- 

 ence, and are not due to any optical illusion. But some 

 of the drawings, especially those of the Schiaparelli type, 

 which he gives are very diagrammatic, and so unlike the 

 projection of markings on a sphere that they forcibly 

 suggest that the canals and straight dark lines must be 

 the result of some optical illusion. 



JupiUr and liis Sj/stem. By Ellen M. Gierke. (Edward 

 Stanford, Cockspur Street, London.) — Miss E. M. Clerke 

 has taken the opportunity of the present very favourable 

 opposition of -Jupiter to give the public a lucidly-written 

 account of what is known about the physical condition of 

 the giant planet. Miss Clerke, like her better known 

 sister the authoress of " The System of the Stars,"' 

 writes very plainly and interestingly, and her little book is 

 likely to turn the eyes and the thoughts of a great many 

 fr-esh observers to the changes continually going on upon 

 this magnificent planet. Many of these changes and 

 other phenomena observable on the planet may be easily 

 seen with small instrumental means, especially when 

 •Jupiter is in opposition. Like Mars, -Jupiter is most favour- 

 ably situated for observation when it is in the opposite 

 quarter of the heavens from the sun. It crosses the 

 meridian at midnight, and the Ulumiiiated disc is turned 

 full upon us. The earth is then between the sun and 

 ■Jupiter, and when Jupiter is near to periheUon, as at 

 the present opposition, the earth's distance from the 

 planet is only 360 million mUes, whereas when Jupiter 



