182 Instructions for the Scientific Expedition 



Another, and as it would appear, a better mode of conducting sucli 

 a research, would be, when in the presumed neighbourhood of a 

 focus of maximum intensity, to run down two parallels of latitude 

 or two arcs of meridians separated by an interval of moderate extent, 

 observing all the way in each, by which observations, when com- 

 pared, the concavities of the isodynamic lines would become appa- 

 rent, and perpendiculars to the chords, intersecting in or near the 

 foci, might be drawn. 



Two foci or points of maximum total intensity are indicated by 

 the general course of the lines in Major Sabine's chart in the South- 

 ern Hemisphere, one about long. 140° E., lat. 47° S., the other 

 more obscurely in long. 235° E., lat. 60° S., or thereabouts. Both 

 these points are certainly accessible ; and as the course of the Expe- 

 dition will lead not far from each of them, they might be visited 

 with advantage by a course calculated to lead directly across the 

 isodynamic ovals surrounding them. 



Pursuing the course of the isodynamic lines in the chart above 

 mentioned, it appears that one of the two points of minimum total 

 intensity, which must exist, if that chart be correct, may be looked 

 for nearly about lat. 25° S., long. 12° W., and that the intensity at 

 that point is probably the least which occurs over the whole globe. 

 Now this point does not lie much out of the direct course usually 

 pursued by vessels going to the Cape. It would therefore appear 

 desirable to pass directly over it, were it only for the sake of deter- 

 mining by direct measure the least magnetic intensity at present ex- 

 isting on the earth, an element not unlikely to prove of importance 

 in the further progress of theoretical investigation. Excellent op- 

 portunities will be afforded for the investigation of all these i)oints, 

 and for making out the true form of the isodynamic ovals of the 

 South Atlantic, both in beating up for St. Helena, and in the pas- 

 sage from thence to the Cape ; in the course of which, the point of least 

 intensity will, almost of necessity, have to be crossed, or at least ap- 

 proached very near. 



Nor is the theoretical line indicated by Gauss as dividing the 

 northern and southern regions, in which free magnetism may be 

 regarded as superficially distributed, undeserving of attention. That 

 line cuts the equator in 6° east longitude, being inclined thereto 

 (supposing it a great circle) 15°, by which quantity it recedes from 

 the equator northward in going towards the west of the point of in- 

 tersection. Observations made at points lying in the course of this 

 line may hereafter prove to possess a value not at present con- 

 templated. 



As a theoretical datum, the horizontal intensity has been recom- 

 mended by Gauss, in preference to the total, not only as being con- 

 cluded from observations susceptible of great precision, but as afford- 

 ing immediate facilities for calculation. As it cannot now be long 

 before the desideratum of a chart of the horizontal intensity is supplied, 

 the maxima and minima of this element may also deserve especial 

 inquiry, and may be ascertained in the manner above pointed out. 



The maxima of horizontal intensity are at present undetermined 

 by any direct observation. They must of necessity, however, lie in 



