148 Gen. Sir E. Sabine on Terrestrial Magnetism. [June 15, 



XIII. " Contributions to Terrestrial Magnetism/' — No. XV. By- 

 General Sir Edward Sabine, R.A., K.C.B., F.K.S. Received 

 June 14, 1876. 



(Abstract.) 



The paper now offered to the Society forms the fifteenth and last of a 

 series of papers printed in the Philosophical Transactions, entitled 

 " Contributions to Terrestrial Magnetism." The whole fifteen numbers 

 are related to each other as " Contributions to the Magnetic Survey of 

 the Globe." Eour of them, viz. XL, XIII., XIY. and the present paper, 

 contain the complete statement of this survey in the double form of 

 "Catalogue" or "Tables" and of "Magnetic Maps;" of these maps 

 there are twelve, one for each of the three magnetic elements in each of 

 the four papers. The present paper consists (as did its last predecessor, 

 No. XIY.) of four zones, each 10° in breadth : — 



Zone 1, comprehending from the equator to 10° S. 

 Zone 2, „ „ lat. 10° S. „ 20° S. 



Zone 3, „ „ lat. 20° S. „ 30° S. 



Zone 4, „ „ lat. 30° S. „ 40° S. 



In the Tables the observations are entered in each zone in the suc- 

 cession of their longitudes, beginning with the meridian of Greenwich. 

 The statements in the introduction to No. XIII. regarding the diiferent 

 magnetic elements apply to the present paper, as they did also to the 

 preceding paper. No. XIV. 



The question of correction for secular change next presents itself. 

 Happily the greater part of the observations were made within, or very 

 near to, the " mean epoch," viz. 1840-1845. Sea-observations were not 

 generally corrected for differences of epoch in the previous papers ; but 

 in the present paper such corrections have been introduced for observa- 

 tions within the range of places (land stations) where the rate of secular 

 change has been sufficiently established. 



Nos. XI. &XIIL, published earlier, comprise the northern and southern 

 portions of the globe from either pole to lat. 40°. These are the regions 

 which have long been recognized as offering to the magnetician at once 

 the most arduous and the most important field of research. In the 

 middle or equatorial portions of the globe, comprised in the last and 

 present papers, the magnetic relations are simpler, and the laying down 

 of the lines representing them derives much aid from the adjacent por- 

 tions of the north and south polar maps ; therefore, both as regards 

 observations and treatment, less abundant as well as less exact evidence, 

 it is hoped, may suffice. 



