March 2, 1891.] 



KNOWLEDGE. 



51 



such mistake can have been made, for Mr. Russell's 

 photograph of the 2nd of October contains a trace made 

 by the planet Mars during the exposure, while the photo- 

 graph we have here reproduced does not. There seems, 

 therefore, to be no doubt that we have two independent 

 photographs, and we seem to liave evidence of very rapid 

 change in the brightness of the southernmost of these 

 two star-clusters. 



While the nebulosity in the upper part of Mr. Russell's 

 picture corresponds generally with that in the upper part 

 of Mr. Barnard's picture, making allowance for differences 

 ■of sensitiveness of the plates used, that in the middle and 

 lower parts does not. The brightest" nebulous region in 

 Mr. Barnard's picture is on the right-hand side of the tree- 

 like form which stretches across the middle of the plate at 

 the base of its lowest right-hand branch. Hut this bright 

 region is entirely wanting in Mr. Russell's photographs, as 

 also is the very bright nebulous region to the left-hand 

 side of the base of the great tree-like structure. It is, of 

 course, possible that such differences might be due to the 

 nebulosity of the upper part of the tree-like structure 

 being caused by a stippling produced by larger stars than 

 those which give rise to the nebulous appearance on the 

 lower part of the plate, and that while the small stars have 

 left their trace on Mr. Barnard's plate, only a larger grade 

 of stars have impressed themselves sufficiently to leave a 

 developable trace on Mr. Russell's plate. It is also pos- 

 sible that the lower part of the nebulous mass may shine 

 with a dift'erent kind of light from that with which the 

 upper part shines, and that while Mr. Barnard's plates 

 were sensitive to both kinds of light, Mr. Russell's were 

 only sensitive to the one. But it is also possible that we 

 may here have e\'idence of the existence of a vast variable 

 nebula which undergoes changes in the relative brightness 

 of its parts with surprising rapidity. 



I sliould like to call attention to the fact that the 

 branching tree-like form shown in Mr. Barnard's picture 

 (the upper part of which appears in Mr. Russell's pictures) 

 seems to afl'ord us e\-idenceof the projection of matter into 

 a resisting medium just as certainly as the tree-like forms 

 in the great Orion Nebula and the tree-like forms f in the 

 Corona bear witness to explosions on a colossal scale, 

 which have taken place below their bright bases, causing a 

 stream of matter to be projected upwards, which stream 

 has subsequently been divided and its branches deflected 

 from their original course by a resisting medium. If there 

 were no resistmg medium and the only force acting on 

 the projected matter was gravity towards the region from 

 which the explosion took place, the streams would have 

 the form of trajectories, and they could only bo projected 

 into conic sections. 



The actual existence of the great tree-form on Mr. Bar- 

 nard's picture seems to be confirmed by the arrangement 

 of the stars in lines along its branches, which is best 

 shown in the small photographs published in the August 

 number of Knowleduk for 1890, and in the Munthli/ Xoticcs 

 of the Royal Astronomical Society for March IK'JO. To see 

 the stars in these small pictures they should be examined 

 with a magnifying glass. The brighter streams of stars 

 will be recognized in Mr. Russell's photographs after they 

 have onco been seen in Mr. Barnard's. 



We seem, therefore, to have evidence that there is a 

 resisting medium which occupies a vast region of the 



* The ne.irly circular white patch at the top of tho picture ie, as 

 was explained in the July number, duo to an over-exposed image of 

 the planet Jupiter, with the Trilid Nobula below it. Jupiter has 

 moved away and the Tritid Nebula remains on Mr. Hussell's 

 pictures. 



t See Knowledge for May 1889, p. HO. 



Milky Way ; and perhaps the whole nebulous circle which 

 surrounds the sky is not one vast nebula. The resisting 

 medium need not be gas ; dust mo\'ing in space, or larger 

 particles, would equally offer resistance. The variability 

 in brightness over so vast a region, if substantiated by 

 future photographs, will need us to assume the existence 

 of forces travelling far more swiftly than Ught or elec- 

 tricity, and giving rise to the synchronous dimming or 

 glowing of the light-giving matter. 



Picture No. III. represents the Nebecula Major (the 

 larger Magellanic cloud), taken by Mr. Russell on the 17th 

 of October 1890 with an exposure of 7h. 3m. In a private 

 letter enclosing me the silver print of this picture Mr. 

 Russell says : " This negative has brought out the grandest 

 spiral structure in the heavens. Herschel estimated this 

 object to cover 48°. It is now shown to be one great 

 spiral structure supported, as it were, by two smaller 

 ones in which Mars onhj are visible. One is situated on 

 the North follawlnii, and the other on the South priredimj 

 side of the great spiral." These spirals are just visible 

 on our plates, but they are not so well shown as on the 

 silver print or on the transparency which Mr. Russell has 

 kindly sent me. 



Picture No. IV. represents the Nebecula Minor, taken 

 on October 11-15, 1890, with an exposure of 8 hours. It 

 is also spiral in structure, though not so clearly so as the 

 Great Nebecula. Within it and around it are some curious 

 streams of small stars, all of about the same magnitude. 

 One of such streams, shown in our plate on the upper 

 left-hand side of the chief cloudy mass, is like a double W. 



M r. Russell has also sent over a most interesting contact 

 print from a negative of the Coal-sack region. Instead of 

 being a completely closed space, it is seen to be open on 

 the south side, and very numerous small stars are seen to 

 be scattered over three-fourths of its area. It is only at 

 its northern jiart that there is the absolute absence of stars 

 so frequently referred to by Mr. Proctor. 



jSToticrs of Boofes. 



The Flii/aical Properties of Gaxex. By Arthur L. Kim- 

 uALL, of Johns Hopkins University. (William Heinemann, 

 London. 1890.) Professor Kimball's book will be wel- 

 comed as giving, in simple, untechnical language, and in 

 a manner easily to be comprehended by the non-mathe- 

 matical section of the community, the reasoning by which 

 physicifts have been led from the properties of gases as 

 they were discovered by experiment to the present gene- 

 rally accepted kinetic theory of their constitution. After 

 having given in brief outline a historj' of the discovery of 

 some of the more important gases and their behaviour 

 under pressure and expansion by heat, Prof. Kimball 

 deals with the easily condensable vapours, and the gases 

 which do not obey Boyle's law. He then treats of air- 

 pumps and diffusion and occlusion. .\vagadro's law 

 that equal volumes of all gases, under tlie same con- 

 ditions of temperature and pressure, contain the same 

 number of molecules, is illustrated and explained in a 

 manner that must make it clear to the most obtuse. 

 Crookes's experiments with high vacua and radiant 

 matter are also well explained and illustrated. It 

 is, perliaps, a pity that Prof. Kimball does not go a 

 little further and show, as he might have done in an ele- 

 mentary manner, how the average velocity of the molecules 

 of a gas may be determined from its density when the 

 pressure which it exerts at a known temperature is 

 measured, and how the number of molecules may be 



