354 



NATURE 



[Feb. 7, 1889 



objects in which real nebula; are distinctly associated with the 

 above diffused nebulosity. 



3. Detached nebulosities. — He next mentions six cases m 

 which, instead of the extensive diffusion referred to under the 

 first head, the nebulosity is found detached. 



4. Milky nebula. — He here remarks that when detached 

 nebulosities are small we are used to call them nebulre, and he 

 shows that ihe nebulosities and the nebulae, whatever may be 

 their appearance, as well as those expressly called by him 

 " milky," partake of the same general nature. 



5. Milky ncbiihe with condensations. —He refers to the 

 brightest portions of the nebula in Orion as an indication of 

 •what he means by condensation ; then to another in which the 

 greatest brightness lies towards the middle ; and then he adds : — 

 " By attending to the circumstances of the size and figures of 

 this nebula we find thai; we can account for its greater bright- 

 ness towards the middle in the most simple manner by supposing 

 the nebulous matter of which it is composed to fill an irregular 

 kind of solid space, and that it is either a little deeper in the 

 brightest place, or that the nebulosity is perhaps a little more 

 compressed. It is not necessary for us to determine at present 

 to which of these causes the increase of brightness may be 

 owing ; at all events it cannot be probable that the nebulous 

 matter should have different powers of shining, such as would 

 be required independent of depth or compression " (p. 282). 



6. Nebuba 7vhich are brigJitcr in more than one place. — He 

 associates the general swelling of the nebulous matter about the 

 places which appear like nuclei with the unequally bright places 

 in the diffused nebulosities, and further on he refers to univer-al 

 gravitation "as a cause of every condensation, agglomeration, 

 compression, and concentration of nebulous matter." 



7. Double nebuhr with Joining nebulosity.- — He points out that 

 " in fifteen objects two nuclei or centres of attraction have been 

 observed, and that if the active principle of condensation carries 

 on its operation a diffusion of their at present united nebulosities 

 must in the end be the consequence " (p. 285). 



^.Double nebtilcc not more than z' from each other. — He 

 points out that there are twenty-three of this class. 



9. Double nebula at a greater distance than 2' from each other. 

 — Of these he gives a hundred examples, pointing out that 

 "there are not more than five or six which differ so much in 

 brightness from one another that we can suppose them to be at 

 any very considerably different distance from us " (p. 288), and 

 he further adds that " equal brightness or faintness runs through 

 them all in general." 



10. Treble, quadruple, and sextuple nebula. — He refers to 

 twenty treble, five quadruple, and one sextuple object of each 

 kind. 



11. Very narrow long Jtebuhe. 



12. Extended nebula. 



13. Irregular nebula. 



14. Ncbula_ that are of an irregular round figure. 



15. Round nebulcE. 



16. Nebula- that are remarkable for some peculiarity of figure 

 or brightness. — He ascribes this irregularity to the as yet 

 imperfect concentration of the nebulous mass in which the 

 preponderating matter is not in the centre (p. 300). 



17. Nebula that are gradually a little brighter in the middle. 



18. Ndbtda which are gradually brighter in the middle. 



19. Nebula that are gradually mnch brighter in the middle. 



20. Nebula that are suddenly much brighter in the ?niddle. 



21. Round nebula increasing gradually in brigh'ness up to a 

 nucleus in the middle. 



22. Nebula that have a nucleus. 



23. Round nebula that show a progression of condensation. 



24. Round nebula that are of an almost uniform light. I 



25. Nebula that have a cometic appearance. 



26. Extended nebula that show the progress of condensation. 



27. Nebula that draw progressively towards the period of final 

 condensation. 



28. Planetary nebula. 



In addition. Sir William Herschel in his various papers gives 

 drawings illustrating the classification which has been above 

 referred to (Phil. Trans., vol. ci. Plates 4 and 5, and vol. civ. 

 Plate i.i). A more elaborate set of plates illustrating the 

 various gradations of the different forms will be found accom- 

 panying Sir John Herschel's catalogue (Phil. Trans., vol. cxxiii., 

 1833, Plates 9, 10, II, 12, and 13). In these illustrations will 

 be for.nd some forms of great interest not referred to by the 

 elder Herschel. ' Long parallel nebulre, for instance, with a dark 



streak separating them, and elliptic and ring nebula;. With 

 these exceptions, all the illustrations readily fall into Sir William 

 Herschel's classification. 



In the valuable paper of Sir John Herschel, to which reference 

 has been made, there is evidence to show that he gives up the idea 

 of nebulous matter distinct from stars advocated by his father. 

 He says : "If the nebula be anything more than a cluster of 

 discrete stars, as we have every reason to believe, at least in the 

 generality of cases, no pressure can be propagated through it " 

 ( Hhil. Trans., 1833. vol. cxxiii. p. 502). Coming down to the work 

 of Lord Rosse, we find that as early as 1846 he had convinced 

 himsi'lf almost completely that no such thing as so-called nebu- 

 lous fluid existed. In a letter to Nicol ("Architecture of the 

 Heavens," p. 143) under date March 19, referring to the nebula 

 of Orion, he states that he could "plainly see that all about 

 the trapezium is a mass of stars, the rest of the nebula also 

 abounding with stars and exhibiting the characteristics of 

 resolvability strongly marked." 



The magnificent observations of the nebulae made by Lord 

 Rosse will be found in the Philosophical Transactions (R. S. ) for 

 the years 1S50 and 1861, the latter giving an account of the work 

 done by the 6-foot, and in the Scientific Transactions of the Royal 

 Dublin Society for 1880. In the volume for 186 1, p. 702, Lord 

 Rosse seems rather inclined to withdraw the very definite letter 

 which has been previously quoted, and states that, "When the 

 letter R, meaning that the nebula is resolvable, has been used, 

 he does not attach much importance to the expression of opinion 

 it conveys, because the question of resolvability can only be 

 successfully investigated when the air is steady and the speculum 

 is in fine order." 



This state of uncertainty, however, did not last long, for 

 in 1864 Dr. Huggins and Dr. Miller demonstrated that 

 the spectrum of several planetary and other nebulae which 

 they examined, instead of giving spectra like those of the 

 stars, gave one of bright lines, one of the lines being due, as 

 they asserted at the time, to hydrogen ; the other, as it lay very 

 near a line of nitrogen, was supposed by them to represent " a 

 form of matter more elementary than nitrogen, and which our 

 analysis has not yet enabled us to detect " (Phil. Trans., 1864, 

 p. 444). Then ihay wrote: — "It is obvious that the nebulx 

 (that they had exauined) can no longer be regarded as agglo- 

 merations of suns after the order to which our sun and the fixed 

 stars belong. We have in these bodies to do no longer with a 

 special modification only of our own type of suns, but find 

 ourselves in the presence of bodies possessin;.j a distinct aid 

 separate plan of structure." 



THE INSTITUTION OF MECHANICAL 

 ENGINEERS. 

 'T'HE forty-second annual general meeting of the Institution 

 of Mechanical Engineers took place at 25 Great George 

 Street, Westminster, by permission of the Council of the 

 Institution of Civil Engineers, on January 30 and 31, and 

 February i, the President, Mr. Charles Cochrane, in the chair. 



The three papers down for reading and discussion were : 

 supplementary paper on the use of petroleum refuse as fuel in 

 locomotive engine^ by Mr. Thomas Urquhart, Locomotive 

 Superintendent, Grazi and Tsaritsin Railway, South- East Russia ; 

 on compound locomotives, by Mr. R. Herbert Lapage, of 

 London ; on the latest development of roller flour milling, by 

 Mr. Henry Simon, of Manchester. 



The author of the first paper states that his object is to bring 

 before the Institution the more recent results of his experience in 

 the use of petroleum refuse as a locomotive fuel, now being 

 used on an unprecedented scale on the Grazi and Tsaritsin 

 Railway, Since the publication of the original paper in 

 1884, nothing new in principle has been discovered, and the 

 same appliances have been used, having undergone very slight 

 modifications, dictated by experience and constant ob-ervation. 

 The whole of the 143 locomotives under the author's super- 

 intendence, as well as various stationay boilers of various t3'pes, 

 have been fired with petro'eum refuse, to the complete exclusion 

 of all solid fuel, as well as in all the heating furnaces at the 

 Company's Central Works at Boriooglebsk. The petroleum 

 refuse is burnt in the form of a spray, being blown into the 

 furnace against a brick structure, serving the double purpose of 

 a reservoir for the heat, and against which the spray is broken 

 up. Many experiments were made with a variety of forms of 



