[ «1 ] 
XIII. Contributions to Terrestrial Magnetism. — No. XV. 
By General Sir Edward Sabine, R.A., K.C.B., F.B.S. 
Received June 14, — Read June 15, 1876. 
[Plates 17-19.] 
The paper now offered to the Society forms the XV th and last of a series of papers 
printed in the Philosophical Transactions, entitled “ Contributions to Terrestrial Mag- 
netism.” The whole Fifteen Numbers are related to each other as “ Contributions to 
the Magnetic Survey of the Globe.” Four of them (viz. XI., XIII., XIV., 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. XIV.) 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 succession of their 
longitudes, beginning with the meridian of Greenwich. The statements in the intro- 
duction to No. XIII. regarding the different 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 observations within the range of places (land stations) where the rate of secular 
change has been sufficiently established. 
Nos. XI. & XIII., 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, com- 
prised 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 portions of the 
North and South Polar Maps. Therefore, both as regards observations and treatment, 
less abundant as well as less exact evidence may, it may be hoped, suffice. 
MDCCCLXXVI I. 3 U 
