1 80 Instructiojisfor the Scientific Expedition 



vations made on terra firma, or on ice, the opportunity should be 

 seized of going through the regular series on ship-board with more 

 than usual diligence and care, so as to establish by actual expe- 

 riment in the only unexceptionable manner the nature and amount 

 of the corrections due to the ship's action for that particular geo- 

 graphical position, and by the assemblage of all such observations 

 to afford data for concluding them in general. 



Fourthly. No change possible to be avoided should be made in the 

 disposition of considerable masses of iron in the ships during the 

 whole voyage ; but if such change be necessary, it should be noted. 



Fifthly. When crossing the magnetic line of no dip it would be 

 desirable to go through the observation for the dip with the instru- 

 ment successively placed in a series of different magnetic azimuths, 

 by which the influence of the ship's magnetism in a vertical direc- 

 tion will be placed in evidence. 



On land, or on ice. — As the completeness and excellence of the in- 

 struments with which the Expedition will be furnished will author- 

 ise the utmost confidence in the results obtained by Captain Ross's 

 well-known scrupulosity and exactness in their use, the redeter- 

 mination of the magnetic elements at points where they are al- 

 ready considered as ascertained, will be scarcely less desirable than 

 their original determination at stations where they have never before 

 been observed. This is the more to be insisted on, as lapse of time 

 changes these elements in some cases with considerable rapidity ; 

 and it is therefore of great consequence that observations to be com- 

 pared should be as nearly cotemporary as possible, and that data 

 should be obtained for eliminating the effects of secular variations 

 during short intervals of time, so as to enable us to reduce the ob- 

 servations of a series to a common epoch. 



On the other hand it cannot be too strongly recommended, stu- 

 diously to seek every opportunity of landing on points (magnetically 

 speaking) unknown, and determining the elements of those points 

 with all possible precision. Nor should it be neglected, whenever 

 the slightest room for doubt subsists, to determine at the same time 

 the geographical position of the stations of observation in latitude 

 and longitude. When the observations are made on ice, it is need- 

 less to remark that this will be universally necessary. 



With this general recommendation it will be unnecessary to enu- 

 merate particular localities. In fact, it is impossible to accumulate 

 too many. Nor can it be doubted that in the course of antarctic 

 exploration, many hitherto undiscovered points of land will be en- 

 countered, each of which will, of course, become available as a mag- 

 netic station, according to its accessibility and convenience. 



There are certain points in the regions about to be traversed in 

 this voyage which offer great and especial interest in a magnetic 

 point of view. These are, first, the south magnetic pole (or poles), 

 intending thereby the point or points in which the horizontal in- 

 tensity vanishes and the needle tends vertically downwards ; and 

 secondly, the points of maximum intensity, which, to prevent the 

 confusion arising from a double use of the word poles, we may pro- 

 visionally terra magnetic /oc?. 



