August 1, 189G.] 



KNOWLEDGE. 



183 



bead was towards the stern. I was dreadfully ill that 

 journey. 



Returning next day by another steamer I again followed 

 the directions, but on this occasion the cabin was aft and 

 the engine room amidship, so my bead was towards the 

 bow. This time I was much better, but there was very 

 little sea on. 



As engine rooms vary in position they are no guide, and 

 I want to know which way of the ship is the right one. 



A Landsman. 



[We have shown our correspondent's letter to Mr. Moy, 

 who says that "A Landsman" appears to prove the 

 correctness of his theory and experiment, which, in Mr. 

 Moy's case, was tried on a paddle steamer, in which the 

 engines and boilers were amidships, and the position of 

 the centre of gravity was therefore clearly amidships. In 

 "A Landsman's" first experiment he appears to have been 

 on a screw steamer, with the engine room well aft. If 

 loaded, the centre of gravity would be a little abaft the 

 centre of the vessel, and the position of his berth would 

 render inoperative the effect arising from longitudinal 

 oscillation ; a berth much further forward would have 

 efiected his purpose. — Eds. K.] 



NOTE ON A PHOTOGRAPH OF THE NEBULA 

 M 8 IN SAGITTARIUS. 



By Prof. Kavasjee D. Naegajh-.ala, M.A., F.R.A.S. 



THIS nebula was very carefully examined by Sir 

 •John Herschel in the years 1836 and 1837, during 

 his voluntary exile at the Cape of Good Hope. It 

 has been fully described on pages 14-16 of the 

 Cape Obsen-'itions, and a half-page drawing of it 

 has been assigned the place of honour in the plates. 



bir John Herschel describes it in the following words : — 

 " Its brighter portion may be described as consisting of 

 three pretty distinct streaks or masses of nebula of a milky 

 or resolvable character, arched together at their northern 

 extremities so as to form some resemblance to the arches 

 of an italic letter m very obliquely written, and this is the 

 aspect under which it strikes the eye on a cursory view. 

 On closer attention these streaks are seen to be connected 

 and run into each other below (or to the south) by branches 

 and projections of fainter light, and to form three distinct 

 basins, insulating oval spaces — one entirely, the others 

 comparatively dark. Northwards, a great effusion of faint 

 nebula runs out, insulating a larger and more Ul-defined 

 basin of great extent and irregular form, which in some 

 measure communicates with the best defined and darker 

 of the three oval spaces already spoken of." 



The stars, both of M 8 proper and of the cluster h 3725 

 just following it, arc involved in the nebula, and Herschel 

 has counted and assigned places to one hundred and eighty- 

 six stirs, ranging in maguitudo from six to sixteen, in the 

 region involved. Almost every one of these stars can be 

 recognized on the photograph taken with an exposure of 

 two hours forty minutes. 



The nebulosity, however, impressed on the plate is much 

 more extensive than that portrayed by Herschel, and the 

 star 7 Sagittarii, showu as being absolutely free from any 

 nebulosity in the drawing, is distinctly surrounded by 

 nebulous matter in the photograph. This difference in 

 visually recognizing the extent of the lu'liulous matter may 

 very likely be due not only to the continuous action of the 

 photographic plate, but also to the diH'orence in the focal 

 ratios of the two instruments employed. Kir J. Herschera 

 reflector was of eighteen and a quarter inches diameter and 



twenty feet focus, giving a focal ratio of a little over 1:13, 

 while the focal ratio of the reflector employed for photo- 

 graphing was only 1:7-4. 



An exposure of about four hours, in my opinion, would 

 involve 7 Sagittarii in the same common nebulous envelope 

 as M 8 and h 3725. This view is further supported by 

 a photograph taken with one hour's exposure with an old 

 portrait combination by Ross of two and a quarter inches 

 diameter and about seven inches back-focus, in which the 

 nebulosity completely extends right beyond 7 Sagittarii. 



But the chief point of interest centres in the nucleus of 

 the nebula, which, in the words of Herschel, " is decidedly 

 not stellar, and resembling much more that of the nebula 

 in Andromeda than any other I (Herschel) can call to 

 mind as a term of comparison." Piesumably, therefore, 

 he had most carefully observed this nucleus, and delmeated 

 it faithfully with his great skill. In the drawing the 

 nucleus is shown decidedly concave towards the folloicing 

 side, but it is as much clearly convex in the photograph. 



The photograph taken with an exposure of two hours 

 forty minutes has the stellar images not quite circular; 

 but another, taken with an exposure of one hour thirty 

 minutes, has stellar discs much more perfect, and on it, 

 too, the nucleus is distinctly convex towards the folloicing 

 side. The photographs are mainly in agreement with 

 Herschel's drawing ; but to what cause could be assigned 

 this gross discrepancy in the form of the nucleus is a 

 question on which I cannot presume to enter. 



The photographs were taken in the principal focus of a 

 sixteen and a half inches reflector, of one hundred and 

 twenty-two inches focal length. 



STOCK-TAKING OF THE VARIABLE STARS. 

 By Lieut.-Colonel E. E. Markwick, F.R.A.S. 



THE advance in the discoveries of variable stars, both 

 by photography and direct observation , is proceeding 

 so rapidly that it seems desirable to take stock 

 or count of those very interesting bodies and see 

 bow we stand at present. It is evident that the 

 theory or theories of the variable stars must hold an 

 important place in the larger theory of the universe. If 

 we can interpret correctly what is going on in a star or 

 sun, this wUl be the key to similar phenomena occurring 

 in countless other suns ; and hence we are gaining an 

 insight into a part, and an important pai-t, of the interior 

 economy of the universe of worlds. 



This was not so in the past. The variable stars (there 

 being then comparatively few known) were looked upon 

 rather as peculiar cases or exceptions, the great multitude 

 of stars being regarded as absolutely unchangeable. But 

 now the number of the variables is so considerable, and 

 the phenomena connected with them, as shown by the 

 spectroscope, are so extremely complicated, that the 

 attention of astronomers is being more and more directed 

 to them. 



Some years ago Dr. Gould announced his opinion that 

 " a very large proportion of the fixed stars exhibit fluctua- 

 tions of brightness"; and although at present, either with 

 photometers or photography, regular variation of less 

 than half a magnitude is exceedingly difficult to prove, 

 still the progress of the optical and mcchinical arts may 

 in the future give us the means of detecting with certainty 

 variation so smill as the one-tenth of a magnitude. 



The study of variable stars, m the matter of variation 

 of light only, does not, like other branches of astronomy, 

 require very elaborate apparatus to detect variation of 



