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



ocean lines passing from nearly 40° N. through the Sandwich Islands and Tahiti to 

 40° S., nearly at right angles to the curves of equal magnetic inclination. 



Having cleared the Magellan Strait, the voyage was continued to the Falkland 

 Islands and Monte Video, thence in an easterly direction until the outward track was 

 crossed, about 300 miles to the westward of Tristan da Cunha, turning in a northerly 

 direction by Ascension until the outward bound track was again crossed to the north- 

 ward of the terrestrial equator. From the Cape de Verde Islands the last part of the 

 voyage covered new regions westward of the Azores, and then on to England. At 

 Sheerness this voyage of three and a half years' duration was completed, and the final 

 observations made on board the ship as before starting. The instruments were then 

 transferred to Kew for examination and re-determination of the constants. 



Of the portability and working of the absolute instruments used during the voyage, 

 there is little to be added to wdiat is generally well known concerning them, as they 

 were of the Kew pattern. Of the three Fox circles used at different times during 

 so long a voyage with the ship so much at sea, subjecting the instruments to the 

 jarring effects of a steamship's screw, it may be well to record here the results of 

 the experience gained. On referring to the numerical results in Narrative, Vol. II., it 

 will be found that index errors of the needles used in these circles became very large ; 

 this probably arose from the axles and the jewelled holes in which they worked losing 

 their circular form. These errors would be principally apparent in the observations of 

 the inclination, and point to the necessity of frequent comparisons on land with the 

 Kew dip circle. 



With the intensity observations, less dependence upon comparisons with the 

 Unifilar magnetometer on land was required, for although the deflectors lost a certain 

 amount of magnetism during the voyage, as shown by the tables of weight equivalents 

 taken at different intervals, the observations with weights were so often taken at the 

 same time as the deflectors, that by a simple calculation the period when the change 

 took place in the magnetic moment of the deflectors could be nearly found. This 

 was important, for the method of observing the intensity with deflectors was more 

 largely adopted than that by weights ; besides, in cold and damp weather, there 

 is, in addition to the object of keeping the needle as little exposed as possible, a greater 

 facility in the manipulation. Again, if deflectors are made of proper steel and 

 carefully preserved from touching, either when in use or packed in the travelling 

 box, there should be little difficulty in ensuring the permanence of their magnetic 

 moment. 



With regard to the jarring effects of the screw, much experience has been gained in 

 late years in overcoming it in the case of compasses placed on board ships with engines 

 of very large power and driven at high rates of speed. There seems to be no difficulty 

 in applying such experience to the suspension of the gimbal table on which the Fox 



