532 



NATURE 



[October 28, 1909 



the work. This serves as an illustration of the care re- 

 quired and being taken, not only with this form of magnetic 

 instrument, but with every instrument used aboard. 



We have preferred to underestimate our absolute accuracy 

 rather than to overestimate it. In any case, it may be 

 said that the magnetic elements are now being obtained on 

 the Carnegie \vith sufficient accuracy, not only for prac- 

 tical demands, but also for purely scientific ones. 



^^^ere we able to choose the time to observe and wait 

 for fairly smooth sea, the magnetic elements could be 

 determined at sea with an accuracy practically the same 

 as in the determinations for land magnetic surveys. 



Diurnal Variation Correclions. — No corrections for 

 diurnal variation need, in general, be applied. The attempt 

 is being made to get the magnetic elements at such times 

 of the day when these corrections are small or are of the 

 order of the error of observations. Thus, for example, the 

 most favourable condition for the declination work is 

 when the sun is low, i.e. early in the morning or late in 

 the afternoon, and at these times the diurnal variation 

 corrections are small and frequently of opposite sign. 

 Should there be evidence of magnetic disturbance during 

 the observations, or as may appear later from observa- 

 tory records, the observations, if necessary, will be 

 rejected.' 



Geographic Positions. — Equal care is bestowed upon the 

 determination of the geographic positions of the points 

 where the magnetic observations are made. The astro- 

 nomical observations and computations are made in dupli- 

 cate, and at times in triplicate, by the observers, and thus 

 the positions are effectively checked. Six well-tested 

 chronometers are carried aboard. With the methods 

 follo\ved. it would appear that the errors in the final posi- 

 tions assigned will, in general, be less than three minutes 

 of arc in latitude and in longitude. 



Magnetic Results ohtained on the " Carnegie," September i 

 to October i8, iqoQ, in the North Atlantic Ocean. 



No. I in Gardiner's Bay ; No. 14 at St. John's, Newfoundland ; No. 32 i 

 Falmouth Bay. 



'^- Magnetic S/oi-ms and Northern Lights.— \l so happened on board the 

 Carnegie that no magnetic observations were in progress during the severe 

 P'^rtions_ of the magnetic storms of September 25 and October 1S-19. No 

 Greenwich time signals could be obtained at St. Johns from Heart's Content 

 f^n the morning of September 25 on account of heav^- earth currents. 

 Northern lights were seen between S and 10.30, local apparent time, of the 

 evening of September 21 in latitude 43° N. and longitude (>oV W. of Green- 

 wich. A glow of white light with occasional streamers extended upward 

 about 10' above the horizon from a low bank of cumulostratus clouds. The 

 display was in the niagnttic north. The evenings of September 22, 23, 24, 

 and 25 were cloudy or foggy. 



NO. 20S7, VOL. Si] 



Corrections of the Present Magnetic Charts. — The table 

 requires no explanation other than is already given in the 

 headings, with the exception of the last three columns, 

 which exhibit the quantities to be added to or subtracted from 

 the declinations scaled from the lines of equal magnetic 

 declination (" variation of the compass "), first, as given 

 by the British .Admiralty chart, 1907 ; next, by the United 

 .States Hydrographic Ofiice for 1910; and, lastly, by the 

 Deutsche Seevvarte for 1905.' In all cases the values 

 scaled from the charts have been referred to the present 

 time with the aid of the secular variation corrections as 

 given on the respective charts. It will be seen that, in 

 the case of each chart, the corrections are usually less than 

 1°, and, considering the miscellaneous data at the disposal 

 of the makers of the charts and the uncertainty of the 

 secular variation reductions, the general correctness of the 

 charts is most gratifying. 



However, one fact, revealed by the prevalence in sign 

 of the quantities appears to be of sufficient practical 

 importance to require attention. The data given in the 

 table apply pretty closely to the tracks followed by the 

 Transatlantic steamers between New York and England. 



.Along the portion from New York to a point somewhat 

 beyond Sable Island the corrections for each chart are 

 positive, amounting in the maximum to ii° ; they then 

 change sign, with eastwardlv progression, reaching a 

 negative value of nearly 1°. In other words, for the track 

 pursued by the Atlantic liners from England to a point 

 off Sable Island, the present magnetic charts, in general, 

 show too large westerly declination, whereas on the re- 

 mainder of the route to New York the charts give too small 

 westerly declination. 



It can readily be seen what the effect of these systematic 

 errors of the charts would be on the course of a vessel 

 sailing from England to New Y'ork if, during the entire 

 passage, no sun or stars were visible, as sometimes occurs, 

 so that the course of the vessel would have to be shaped 

 entirely by the compass and the log. At the end of the 

 2000 miles of the great circle route the vessel, off Sable 

 Island, w^ould be about thirty miles too far north of her 

 regular track, and if the set of the current were in the 

 same direction the vessel would easily be exposed to ship- 

 wreck. From Sable Island to New York the effect of the 

 chart errors, being reversed in sign to what they were 

 before, would be in the opposite direction, i.e. the vessel 

 would be put out of her true course in a southerly direc- 

 tion. 



For a vessel going eastward the effects are just reversed ; 

 hence on the course from New York to Sable Island the 

 tendency of the chart error would be to set the vessel to 

 the northward, hence again towards the source of danger; 

 thereafter the vessel would be set to the southward of her 

 outlined course. So that, if proper allowance were not 

 made, a captain would have his vessel turned off towards 

 Sable Island or Cape Race, whether he came from the 

 cast or from the west. 



It appears to bo known to some captains, at least, that 

 there is some such systematic change which, judging from 

 conversations had with them by one of the present writers 

 (Bauer), during various .Atlantic voyages, is ascribed by them 

 to a systematic change in their ship deviations. Two cap- 

 tains have told him, independently of each other, that after 

 their compasses had been carefully adjusted they have noticed 

 bv repeated observation a systematic change in the deviations, 

 which reversed in sign when Sable Island was passed. 

 The effects would be opposite for the eastward and the 

 westward cruises. The deviations referred to by these 

 captains are the differences between chart values and those 

 observed with their adjusted standard compasses ; hence 

 they are not pure deviations, but are the sums of ship 

 deviation and chart error. The work of the Carnegie has 

 now proved that the cause of the systematic change and 

 reversal of ship deviations encountered by these painstaking 

 captains was due, in part at least, to systematic errors in 

 the variation ch.arts. 



The corrections for the chart of the lines of equal mag- 

 netic dip (British .Adiniralty Chart of 1907 and the Deutsche 

 Seewarfp of 1905) are generally less than J', being some- 

 tim'^s plus and sometimes negative. 



The lines of equal horizontal magnetic intensity as laid 



t The latest chart of the Deutsche Seewarte is not at present to hand. 



