6 THE VOYAGE OF H.M.S. CHALLENGER. 



conclusions, there is another aspect of them which is of great importance to practical 

 navigation. It has been frequently reported that vessels navigating the coasts of 

 certain islands, as well as the mainland, have found their compasses disturbed, such 

 disturbance being imputed to the effects of the visible land. The desirability of either 

 confirming or refuting this impression on the part of seamen by reliable investigation 

 can hardly fail to be appreciated. 



It has been shown, with instruments placed on land within five feet of the soil, 

 that the effect of the local magnetic disturbance in localities visited by the Challenger 

 did not exceed at the most more than 2° or 3° in the declination, and 2ij° in the 

 inclination ; then, remembering the law of magnetic attraction and repulsion, it is 

 impossible that a vessel's compass in such case could be affected at the ordinary 

 distances of such vessel's passing a coast. 



The question, however, is not finally answered by any means by these results, 

 reassuring as they are as far as they go. Thus, near a station on the summit of the 

 bluff,' — Bluff Harbour, in the south island of New Zealand, — an observer found the 

 declination to be as follows: when 30 feet north of it, 9° 36' W., and 30 feet east of 

 it, 46° 44' E. Now supposing the bluff to be submerged some 30 or 40 feet at three 

 miles from the coast, it is not difficult to conceive that a vessel passing near the spot 

 would find her compass considerably disturbed. In point of fact, there is a remarkable 

 instance, among others, of magnetic disturbance proceeding from submerged magnetic 

 land, namely, at Cossack in North Australia. Here H.M. surveying vessel " Meda," 

 when sailing on the line of transit of two objects on the land, in 8 fathoms of water 

 and three miles from the nearest visible land, suddenly observed a deflection of the 

 standard compass amounting to 30°. This remarkable deflection of the needle remained 

 constant for a quarter of an hour, just time enough for the vessel to sail over about a 

 mile in one direction, when immediately after the compass returned to the original 

 point. Bearings of points of land were taken to confirm these results, as previously to 

 this occasion the anomalous behaviour of the compass in the " Meda," and other vessels 

 navigating in the vicinity, had been noticed. 



The question, therefore, with regard to local magnetic disturbance of the compass 

 in ships sailing in the neighbourhood of the land stands thus. That such disturbance 

 undoubtedly exists, that the number of positions where its presence has been proved 

 are comparatively few ; but that it behoves the navigator to be on his guard for such a 

 formidable danger, and, when found, to report all particulars as he would that of a 

 newly-found rock or shoal. 



Before leaving this part of the subject, it may be remarked that the lines of equal 

 value on the accompanying magnetic charts are normal, the disturbances from local 

 effects being confined to such limits as to be too small to be accurately drawn. 



Large as was the Challenger's contribution to these magnetic charts, it will be 



