NA TURE 



[Dec. 30, 1886 



case lies in the fact that the antipodal point corresponding, being 

 S. lat. 19° 2i' and E, lonj. 115° 38', lies about 100 miles north 

 by east of Barrow Island, off the west coast of Australia. It 

 is further interesting to note that the line which joins this point 

 with the southern point of Barrow Island fairly represents the 

 direction of the coast-line at this point, and leads to the pre- 

 .■•umption of the existence of main lines of faulting there having 

 that direction. To this relation of antipodean points in connec- 

 tion with earthquakes I have already had occasion to call 

 attention. J- P- O'Reilly 



Royal College of Science, Dublin, December 22 



Barnard's Comet 



This comet has been observed here with the 74-'nch refractor, 

 with power 50, than which no higher power could be used with 

 advantage. 



On December 19, at i8h., it appeared as bright as a 2nd 

 magnitude star similarly situated ; the nebulous head was about 

 10' in diameter, with central condensation of perhaps half a 

 minute. The " position " of the principal tail was estimated to 

 be 0° : it remains of a parallel breadth throughout, and does 

 not increase in breadth as it recedes from the head of the comet ; 

 this constant breadth is equal to the diameter of the nebulous 

 head, that is to say, 10'. By sweeping, this tail may be traced 

 to a distance of some 10° from the head. 



The secondaiy tail is inclined at an angle of between 30° and 

 40° 10 the principal one, and fades away rapidly at a distance of 

 perhaps 1° from the head ; it is well defined on the preceding 

 side, but on the following side it melts away into a nebulous 

 mass connecting it with the principal tail for some distance from 

 the head. 



On the 27th, at I9h., the comet was decidedly less bright 

 than on the 19th, but the same general description applies. 



The "positions " of the two tails were measured, and were : 

 the principal tail, 15° '5 ; the secondary, 338°; the included 

 angle, 37°'5 ^ °°'5' The secondary tail did not appear as well 

 defined, on the following side, as on the I9tli. 



Finlay's comet presents no visible feature of interest. 



Wentworth Erck 



Shankill, co. Dublin, December 28 



Electricity and Clocks 



In the absence of any details, apparently Mr. Wilson's 

 simplest plan would be to insulate the hammer and bell of his 

 "small striking clock," and arrange that a galvanic current 

 should pass through both, when they come in contact by the 

 act of striking : this current of course to be directed to a lar^e 

 electro-magnet, to raise the hammer for striking on his bigger 

 bell. .Should the striking of Mr. Wilson's smaller clock be on 

 a gong with a leather-beaked hammer, a separate attachment 

 must be made for contact. Henry Dent Gardner 



Lee, S.E. , December 26 



P. S. — If a longer contact be desired, the hammer whilst at 

 rest should repose upon a weak spring, and be kept away from a 

 banking ; when the hammer rises, contact will ensue between 

 the spring and banking, and last until the hammer falls again. — 

 H. D. G. 



Seismometry 



There are one or two points in Prof Ewing's letter on the 

 above subject in the last number of N.ature which seem to call 

 for a few words of reply. 



(i) As to the alleged inconsistency between what I wrote in 

 iSSi and what I wrote in my la t leiter. The remark quoted 

 referred to a light pivoted frame carrying at its centre of per- 

 cussion, relatively to the axis through its pivot, a pivoted mass. 

 There was no "if need be " about this mass : it was an essential 

 part of the system. I believe the remark I then made was 

 perfectly correct and in no way inconsistent with my remarks in 

 1 886. 



(2) .A.S to the vertical-motion instrument, the lever with 

 spring joint used at Comrie in 1842 does not at all resemble the 

 rigid lever working on knife edges and supported by springs as 

 introduced by me and used by Prof Ewing. On the question 

 of compensating the lever by the addition of negative stability, 

 I have nothing to add to what I stated in my last letter. 



{3) As to the publication referred to by Prof Ewing, the 

 memoir printed by the Tokio University is probably, from the 

 circumstances of its publication, hardly known to anybody. 

 The " EncycIopa;dia Britannica" article is not, in my opinion, 

 a fair account of what has been done in seismometry. 



Thomas Gray 



7, Broomhill Avenue, Partick, December 27 



The Recent Weather 



At Cardross, half-way between Dumbarton and Helensburgh 

 on the Clyde, at about 25 feet above sea-level, in an outer lobby 

 with a temperature of say 45° to 50° F. at 9 a.m. on Wednes- 

 day, the 8th inst., the mercurial barometer stood at 27'5I inches, 

 which, with reduction of say o'02 added for elevation, and say 

 0'03 subtracted for temperature, would make it 27'50 inches. 

 On January 26, 1884, it s'ood at 27^39 inches, which with like 

 reductions would give 27*38 inches. These are nearly as low 

 as those you refer to in your number for last week. 



Cardross, Dumbarton, December 23 R. B. W. 



OBSERVATIONS OF NEBUL/E AT ARCETRn 

 IX/r TEMPEL observes under difificulties. TheArcetri 

 •'•'-'■ * Observatory possesses, it is true, two fine refractors 

 by Amici, one of 11, the other of 9^ inches aperture; 

 but neither is, properly speaking, available for astro- 

 nomical use. The smaller is rudely set up on an open 

 and uneven terrace, exposed to every gust of wind, and, 

 at the most, serves to display the wonders of the heavens 

 to curious visitors. Amici I. is duly ensconced in a re- 

 volving dome, but clockwork motion is wanting ; the 

 circles, both of declination and right ascension, are (strange 

 as it iTiay seem) undivided ; and when the necessarily 

 somewhat unwieldy instrument is, with infinite pains and 

 without so much as the aid of a handle, pointed towards 

 the object sought, there is actually no means of clamping 

 it in the position so laboriously arrived at ! That M. 

 Tempel, under circumstances so discouraging to him 

 and disgraceful to the responsible authorities, should 

 have executed a number of valuable drawings of nebute, 

 should have re-observed many such objects neglected, or 

 even believed to have disappeared, since the elder 

 Herschel's time, besides discovering a good proportion of 

 new ones, gives astonishing proof of his keenness, zeal, 

 and accuracy. All the more, nevertheless, there is 

 reason to regret that qualities so rare should be employed 

 at such cruel disadvantage for want of the judicious 

 expenditure of a couple of thousand francs. 



The paper before us is accompanied by reproductions 

 of two admirable drawings by the author, one of the 

 Orion, the other of the " Crab " nebula (Messier I.). The 

 latter is of especial interest, as disclosing a feature un- 

 noticed by any previous observer. This is a dark cleft 

 right through the central condensation, dividing it along 

 the major axis into two spindle-shaped nebute. Incipient 

 fission would seem to be indicated. On the same plate 

 with his own, IVI. Tempel has engraved five earlier draw- 

 ings of the object, by J. Herschel, D'.Arrest, Lassell, 

 Secchi, and Lord Rosse. The comparison is instructive, 

 if somewhat disheartening ; for, assuredly, no two of the 

 six confronted delineations could be supposed, on an 

 unprejudiced inspection, to have been inspired by one 

 original. Yet the nature of that original sufficiently ex- 

 plains the discrepancies. The apparent form of nebute 

 depends upon almost evanescent gradations of diffused 

 faint light, and differs, for each individual eye, with its 

 sensitiveness to them. And since personal equation, 

 as regards such gradations, is shown by many proofs 

 to be enormously large, a vast amount of detailed varia- 

 tion in the representation of the objects exhibiting them 

 becomes intelligible. It is, then, a circumstance of pecu- 



* " Ueber Nebelflecken. Nach Bsobachtungen angestellt in den Jahren 

 1876-79 aufder KCiiigl. Sternuarle zu Arcetri bei Florenz." Von Wilhelm 

 Tempel. Abhandlungen der Konigl. Bohm. Gesellschaft der WibS;n- 

 schaften. VII. Folge, i Band. (Prag, 1885.) 



