570 



.VA TURE 



[October i i, 1894 



LETTERS TO THE EDITOR. 



\Thc Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers oj, rejectiii 

 manuscripts intended for this or any other part ('/Nature. 

 A^ notice is taken of anonymous communications. ] 



Wilde's Theory of the Secular Variation of 

 Terrestrial Magnetism. 



Some prominence is given in Nature of August 9 to.i 

 litter of L. A. Bauer, .1 magnetic computer of the United States 

 Coast and Geodetic Survey, on the above su!>ject. Absence 

 from home and other circumstances have prevented me from re- 

 plying to this lct:er before now. It is not my intention to engage 

 in controversy » ith a critic who roundly comlemns my theory 

 and experiments without reason, and at the same time naively 

 announces that, by means of formula; e'^tabiished by himself, he 

 has succeeded in lepresenting the observations many times 

 better than the raa^netarium results. 



My principal object in writing is to correct an error I have 

 made in stating that no observations had been made on the dip 

 at Sr. Helena previous to the )ear 1S25. The error is all the 

 more inexcusable as^I have in my possession Hansteen's " Mag- 



netismus der Erde," with its valuable atlas-folio of charts of the 

 njaRnelic variation, from which I,. A. Bauer has derived his 

 information of the (lip at St. Helena for the epoch 1700-17S0. 

 Had I said that no reliable observations h.id been made before 

 1825, the statement would hardly have called for any notice. 

 It is well recognised by magneticians that observations of the 

 dip, up to the early part of the present century, were much more 

 difficult to make than those of the declination, on account of 

 instrumental imperfections and the unrecognised influence of 

 vertical masses of ship's iron and local geological formations. 

 Sabine, in his "St. Helena Observations," vol. 2, p. Ixv., shows 

 a difference of between 2" and 3" at Longwood and Sister's 

 Walk, from the latter cause, and this illustrious magnetician 

 did not vcniiire to give observations of the dip further back than 

 the epoch 1825. 



I regret to observe that L. A. Bauer, in his intolerance 

 of the magnetarium results, has inserted in his tabic guesses 

 of hii own for observations, which arc very wide of the 

 troth. 



In order that my remarks on the dip at St. Helena 

 may be belter understood, I will, by your permission, repro- 

 duce the table of L. A. Bauer, although that of Sabine 

 and the Admiralty chart are more in ngrcement with mv 

 rc«ult«. 



NO. 1302, VOL. 50] 



It will be observed that for the epoch 1825-1890, the dip 

 increased no less than t6^'25, or fifteen minutes annually. 

 Hansteen's gre.it work was published in 1S19, and his curve of 

 the inclination for the epoch 17S0, showing lo°'5 S., at St. 

 Helena, was laid down from the observations of Ekeberg, La 

 Perouse, and Cook, which indicate a diminishing dip for the 

 epoch 1771-1775. Hansteen's chart of the inclination for 1700 

 is exquisitely drawn, but ou a scale so small that St. Helena is 

 not shown thereon. The position of the island is, however, 

 easily found from the latitude 15° 55' 

 S., and long. 5^43' W., or 12' E. of 

 the meridian of Ferro. 



As Hansteen's historical work on 

 the magnetic variation is very rare, 

 I have had his chart of the inclination 

 for the epoch 1700 reproduced by a 

 photographic process, with the posi- 

 tion of Si. Helena dotted thereon, for 

 the benefit of your readeis. 



It will be seen from the chart that 

 the magnetic equator, or line of no 

 ilip. is about 1° north of St. Helena, 

 so that the amount of south dip would 

 not be more than z" for the epoch 

 17CO. Hansteen would appear to 

 hive laid down bis curve of tlie dip 

 for 1700 from the iirevioiis ol servi- 

 I ions of Ekeberg and La Caille, which 

 indicate a decrease, reckoning back- 

 wards, from 1 771 to 1754; but he 

 nowhere states in the text of his work, 

 that his cuive was diawn from an ob- 

 scrvation made at St. Helena in 170O. 

 The amount of dip, li"'5, shown 

 by L. A. Bauer for 1770, is the 

 same as that observed by Cook in 

 1775, and is an obvious blunder in his 

 reading olT the position of Si. Helena 

 I ' below the isoclinal of lo" south, 

 instead of from the magnitic equator. 

 1 shall leave others to judge how far any statemenls of L. 

 A. Bauer on the subject of terrestrial magnetism in general, 

 and on the magnetarium in particular, are entitled to credit 

 when he places his erroneous compulation under ihe heading 

 of observed inclinations standing against the distinguished names 

 set forth in his table. 



It may be of inlerest lo magnelicians to know that the secular 

 changes shown in my i.ibles of the declinaliin and inclination 

 at London, Cape of Good Hope, Si. Helena, and Ascension' 

 Island are obtained conjoinlly on the niagnetnrium without 1 

 change of resislance in Ihe secondary circuit ; together with the , 

 great eastern and western lines of no declination, the Asiatic 

 oval of small westerly declination, the oval of small easterly 

 declination in the Pacific, the magnttic equ.itor, and many other 

 agreements with the obseivalionF, all for the epoch 18S0. 



I regret that 1 have not been aide to accoiii|ilisli more ; but in 

 reply to the remark of L. A. Bauer that although 1 possess the 

 records of secular variation stations in ilie United .Slates, I have' 

 made no published attempt to reproduce them, I may sav that ' 

 two years ago I presented a magnetarium to the United .Stair 

 Coast and Geodetic Survey, at considerable cost and trouble 1 

 myself, wiih full instructions for working It, and have no doubi 

 that the dcparlmcnl will make good use of the In'trument. 

 Alderley Edge, September 28. Hknry Wii.hR. 



