405 



NA TURE 



[August 28, 1 884 



of the chronometers and compasses, or for the more 

 certain guidance of the vessel when in with the land. 

 And for an account of such the " Encyclopaedia " is, we 

 conceive, the proper place : for it is not a work addressed 

 to a youngster learning for the first time how to rough out 

 a course and distance, or to work a meridian altitude or 

 chronometer ; but rather to the general reader wishing to 

 gain some insight into the methods in use, or to the adept 

 who is desirous of a ready reference to those branches of 

 the subject which do not come within the scope of the 

 ordinary text-books. 



It is thus that we do not agree with Capt. Moriarty in 

 the disparagement of lunars, which he would relegate 

 altogether to the examination room; the excellence of 

 chronometers in the present day renders — he seems to 

 say— lunars of no practical use ; and holding this opinion, 

 his notice of the problem is incomplete, and certainly very 

 unpractical. But Sir Charles Shadwell, whose scientific 

 knowledge of the subject is at least equal to that of Capt. 

 Moriarty, has within these last three years pointed out 

 (•' Notes on the Reduction of Lunar Observations," Potter, 

 i :«[> that the lunar method is the only one which gives 

 the seaman an independent astronomical solution of the 

 problem of finding longitude at sea. " Chronometers," he 

 says, " may fail, or go astray ; accident or carelessness 

 may neglect to wind them up ; errors may have been 

 committed in determining or applying their rates, which 

 may not have been discovered till too late to rectify them. 

 In these dilemmas, the sole remedy available for pointing 

 out the real position of the ship in longitude, is a lunar 

 observation." Capt. Moriarty presumably thinks that the 

 rarity of such " dilemmas " renders them of no practical 

 importance : that they do occur, however, is within the 

 cognisance of every navigator; but we fear that under 

 the influence of teaching such as this, the skill to get out 

 of the dilemma in a masterly manner is rapidly becoming 

 scarce : for, as Sir Charles Shadwell has well said, " Con- 

 fidence in the hour of uncertainty cannot be improvised 

 for the occasion ; it can only be the result of habitual 

 practice." We venture therefore to protest most earnestly 

 against the pernicious doctrine to which Capt. Moriarty 

 has lent the sanction of his name. If it is worth a ship's 

 while to carry the cumbrous and costly array of masts 

 and rigging as a stand-by in the event of the huge and 

 powerful engines being disabled, it is surely at least 

 equally worth while to carry the handy and inexpensive 

 skill to observe and compute a lunar, as a check on the 

 very delicate machinery of a chronometer. 



In speaking of these, the most exact of all observations 

 with the sextant, we may call attention to the very in- 

 sidious error which arises from false centering, and which 

 i^ much more common than is generally understood. To 

 detect this and find the corrections for it, Capt. Moriarty 

 recommends the taking and computing a number of equal 

 altitudes, comparing the error of chronometer so found 

 with the known error of chronometer. This method is 

 excessively laborious, but may be shortened somewhat by 

 the construction of a curve of error, as Capt. Moriarty has 

 described. There is also the danger, which he has 

 omitted to notice, of the whole work going round in a 

 vicious circle ; of first determining the error of chrono- 

 meter by a faulty sextant, and of then using that error to 

 correct the sextant. Capt. Moriarty appears not to 



know, or has at any rate neglected to point out, that, 

 for an almost nominal fee, sextants can be tested at 

 Kew by a specially constructed apparatus, the invention 

 (we believe) of Mr. Francis Galton. Many men buy a 

 sextant with as little care as they would buy a gridiron ; 

 but no one who means to use his sextant for purposes of 

 exact navigation should think of concluding the purchase 

 without getting a satisfactory Kew certificate with it. If 

 he does, he richly deserves all the trouble and annoyance 

 which the neglect of this very simple precaution may 

 entail. 



Perhaps the best section of Capt. Moriarty's article is 

 that in which he has treated of the modern application of 

 what is commonly known as Sumner's method of deter- 

 mining a ship's position ; what he says on this subject is 

 admirable ; we can only wish that he had devoted a little 

 more space to it, and given it a full development ; for the 

 theory and the practical use of lines of equal altitude are, 

 as yet, neither sufficiently understood nor attended to by 

 navigators. It would, for instance, have been well to 

 have called attention to the very exceptional value of 

 Venus for these observations ; and to the many special 

 cases which arise when the bearing of a distant peak can 

 be taken. In conclusion, we would heartily endorse Capt. 

 Moriarty's commendation of Raper's " Practice of Navi- 

 gation." It is, beyond question, the best practical work 

 which has yet appeared, and we cannot but express our 

 regret that neither in the Royal Navy nor in the Merchant 

 Service, are its merits properly recognised. The text- 

 book commonly used in the Navy is Inman's, as edited 

 by Jeans ; that used in the Mercantile Marine is Norie's. 

 Either of them is very far inferior, as a practical work, to 

 Raper ; but the difficulty of changing an established text- 

 book has hitherto been found insuperable. 



LETTERS TO THE EDITOR 



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The Electric Light for Lighthouses 



In your interesting article on the experiments on lighthouse 

 illumination now in progress at the South Foreland (p. 362) the 

 writer alludes to the remarkable quenching influence of even a 

 light mist on the electric light as compared with either gas or 

 oil lights, and suggests that the smaller area of the illuminating 

 centre in the case of electricity may have something to do with 

 the matter. 



Apart from this, however, selective absorption by water 

 vapour must play an important part, and a great deal must de- 

 pend on the colour of the beam ; that is to say, the proportion of 

 different rays combined in it. 



MM. Janssen, Angstrom, and others have shown that vapour 

 of water in the atmosphere reduces the intensity of the violet end 

 of the spectrum in a general manner and the red end in a more 

 special manner. An examination of Angstrom's map of the 

 telluric lines in the solar spectrum (see Schellen's " Spectrum 

 Analysis," Huggins' translation, Fig. 95, p. 264) will show that, 

 while the extreme red and all the blue ends of the spectrum are 

 much darkened by absorption lines, there is a zone from between 

 B and C to 8 which is comparatively free from them. This 

 region includes the orange rays, and part of the red and yellow. 

 It is the most luminous part of the spectrum, and contains the 

 rays which will penetrate farthest in a fog. Hence it is that the 



