296 



NA TURE 



\_yan. 27, 1 1 



The Coal-Dust Theory 



Those readers of Nature who have followed the various 

 phases of the coal-dust question will be interested to know that 

 the following resolution was carried at the Conference of 

 Miners, which concluded its sittings in Birmingham yesterday 

 morning : — 



" This Conference, believing that recent explosions have de- 

 monstrated that coal-dust is sufficient, without the presence of 

 gas, to cause a serious explosion, is of opinion that a clause 

 should be inserted in the new Mines Act, making it illegal to 

 use blasting-powder, or other intiammable substance, in any part 

 of the tram or trolly-way, unless the dust on the top, the bottom, 

 and the sides of such tram or trolly-way has been properly 

 damped or removed, for a distance of fifteen yards on each side 

 of the hile in which the shot is to be fired." 



This practical method of dealing with part of the difficulty 

 will doubtless lead to the happiest results, if properly carried 

 out. 



The use of dust-tight tubs, or mine-waggons, as a means of 

 preventing the deposition of coal-dust in the first place, and 

 thus avoiding the necessity for so many precautions in dealing 

 with it afterwards, suggested itself to me while I was examining 

 the scene of the explosion in the silkstone-pits at Altofts Col- 

 liery. In order to fulfil the condition of being dust-tight, the 

 niine-vvaggons would require to have no crevices of any kind 

 through which dust could be shaken, and they would require to 

 be so filled that no pieces of coal would roll over their sides on 

 to the floor. 



This alternative method, which I propose to discuss at greater 

 length elsewhere, showing practical results as far as they extend, 

 might ]jerhaps meet the case of those mines in which the use of 

 water is objectionable, on account of its disintegrating effect 

 upon the roof or floor. W. G.'^lloway 



Cardifi", January 15 



Barnard's Comet- at Perihelion 



In spite of the bad weather and the glare of twilight and 

 moonlight, I have made a good number of observations and 

 drawings of this comet ; especially on Deceinber 16, 1886, the 

 day of its passage at perihelion, and consequently of its shortest 

 distance from the sun (nearly two-thirds of the distance of the 

 earth from the sun). 



The head of the comet was a large, brilliant, star-like 

 nucleus, surrounded with a splendid, globular chcvdure. From 

 this sprang two tails : the larger, directed towards the North Pole, 

 was straight, or but a little convex to the east (on which side it 

 was also a little thicker), ending in an extremely faint nebulosity 

 nearly 10" from the head. The smaller tail was much shorter, 

 and directed 50° to the west of the other. The colour of the 

 comet was a beautiful light blue. 



The spectrum of the comet resulted from the ordinary three 

 bands of the hydrocarbons; the green being the strongest, then the 

 yellow, whilst the blue was the faintest. The bands were crossed 

 by the linear spectrum of the nucleus, continuous, but strongly 

 reinforced on the bands, and extended very little beyond the 

 limits of the bands themselves. 



Before the passage at perihelion the comet had the same form, 

 but less developed ; the spectrum also was the same. 



On the morning of December 7 I followed the comet with 

 the lo-inch refractor till twenty-five minutes before sunrise : this 

 proves the great brilliancy of the comet. A. Ricco 



Palermo Observatory, January 9 



Magnetic Theory 



May I trespass upon your space to ask a questiun which I 

 have never seen proposed, but which is so obvious that it must 

 have occurred to many others interested in magnetic theory ? 



In a current field — with closed currents — we have the familiar 

 equations — 



dG 



dJI 



dz ' 



dit , d-c' , dzu dF 



— -f - -+- = O = — -r — 



dv dy dz dx dy 



V^/'-f 47r« = O, &C. 



And if (7, />, c are the tomponents of magnetic force — 



dG dH „ dc db „ 



~ ' ^nli — —-— — iSc. 



In a magnetised mass, A, B, C being components of mag- 

 netisation, if we take 



F = 



III 



C^^&c, //=&.c. 



( „ d „d \dxdvdz 



We have at all external points a magnetic force of the com- 

 ponents a, b, c, where a = — - — , &c. 

 dz dy 



lit volume. 



dG 



dz dy 

 If we introduce the vector whose components per 1 



f, tc, are — - —-, &c. , where A, B, C are continuous and 

 ,/: dy 

 per unit surface are iiB - >nC over surfaces where A, B, C pass 



■ ■ " ■ ifydz 



discontinuously to zero, we get F — 



-iir^ 



&c. 



d// 



dFdG, 



dx dy di 



•'}' 



dy dz 



du dv d'M _ 



dx dy dz 

 and all the equations of the current field are reproduced. 



Only the components a, /3, 7, of the magnetic force at internal 

 points, being derivable from the potential 



are not identical with a, l>, c, as they do not satisfy the equa- 



dc db 

 tions -- --=4ir;<, &c. 

 dy dz 



My question is, what is the physical evidence in favour of the 

 existence of A, B, C and a, j8, 7 ? 



All we know, and can know, about a magnetised mass is 

 derivable from observations of the external field. 



Everything, therefore, that we can know is satisfied by ex- 

 pressing the state of the mass in terms of 11, v, w, and regarding 

 these quantities as the ordinary electric current components. 

 So that a, b, c are components of force everywhere. ' 



My meaning is that if the order of our investigations had been 

 reversed, commencing with current phenomena and so passing 

 on to magnetic, it seems almost certain that we should have 

 attempted to explain the latter in terms of the former. 



Doubtless many of the observed facts of induced magnetism 

 would present grave difficulties, but I do not think these diffi- 

 culties would have driven us to the hypothesis of permanently 

 polarised molecules, or that we should have derived any addi- 

 tional help from such hypothesis. H. W. Watson 



Berkeswell Rectory, Coventry 



Sounding a Crater, Fusion-Points, Pyrometers, and 

 Seismometers 



I had expected to see some confirmation of the remarks that 

 form the chief part of the letter on this subject by Dr. H. J. 

 Johnston-Lavis in Nature of the 30th ult. (p. 197), but as no 

 one has taken up the matter yet, perhaps you will allow me, as 

 for years the chief assistant of the late Robert Mallet, to say 

 that it is quite true that elaborate apparatus was devised by him 

 and made by different instrument-makers, with a view to ob- 

 taining experimental imformation on the whole of the questions 

 and more than those referred to by Dr. H. J. Johnston-Lavis. 

 A preliminary report was presented by him to the British 

 Association in 1863, in which the scope of his inquiry and nature 

 of apparatus were mentioned, and other reports were written by 

 him which I have not by me now. I am also able to say that 

 the whole of the apparatus remained for years, through Prof. 

 Guiscardi, in the University of Naples, and that Mallet wrote to 

 him as to its disposition for the use of others, should occasion 

 permit, just before his death. W. WoRBY Beaumont 



Norwood Road, S. E. 



Folkestone Gault 



Mr. John Griffiths, of Folkestone, the well-known 

 collector of Gault fossils, is without resources, and is perman- 

 ently disabled by rheumatism, brought on by exposure in his 

 daily labours, which have not only enriched the museums of 

 Europe and the United States, but have formed the groundwork 

 of the investigations into the zones and fossils of the Gault made 

 by myself and fellow-workers — the Rev. Prof. Wiltshire, F.G. S. , 

 before my own endeavours, and those of Messrs. F, G. H. 

 Price, F.G.S., and Starkie Gardner, F.G.S., since. Mr. F. G. 



I H. Price, of Messrs. Child's Bank, Temple Bar, W.C. , has 



I kindly undertaken to receive subscriptions. 



I H.M. Geological Survey C. E. De Range 



