THE 



LONDON AND EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



FEBRUARY 1839. 



XV. Comparison of the Magnetic Lines of no dip and of least 

 intensity. By Major Sabine, F.R.S. 



[With a Map, Plate IV.] 



To RicJiard Taylor^ Esq. 

 Dear Sir, 



IN my report on the magnetic intensity of the earth, pub- 

 lished in the last volume of the Reports of the British As- 

 sociation, it is remarked, that " every geographical meridian 

 has a point of minimum (magnetic) intensity ; if these points in 

 different meridians were connected by a line, that line would 

 separate the intensities of the northern from those of the 

 southern magnetic hemisphere. It would be in some respects 

 analogous to the line of no dip; but it would not be a line of 

 equal intensity, as it would consist of intensities vai-ying from 

 unity to the lowest on the globe. Such a line traced on the 

 map is found to differ very considerably in geographical po- 

 sition from the line of no dip," 



It may be interesting to those of your readers who are 

 maffneticians to compare the positions of these two lines; 

 namely, that of no dip, and that of least intensity. They are 

 drawn on the accompanying map, which is divided into two 

 portions, to prevent its being inconveniently long for the oc- 

 tavo size, one portion containing 200 degrees, and the other 

 the remaining 160 degrees. The lines of equal intensity 

 (isodynamicsf of TO, 0-9, and 0-8 are drawn in faint 

 dotted lines, and are taken from the general map in the Re- 

 port referred to. The line of least intensity, now drawn for 



Phil. Mag. S. 3. Vol. U. No. 8G. Feb. 1839. G 



