PRESIDENTIAL ADDRESS. SECTION A. 21 



almost everywhere, which will make it necessary in time to alter the whole 

 System : At present it may suffice to Advertise, that about C. Bonne Esper- 

 ance, the West ^Variation_j;increases at the rate of about a Degree in Nine 

 Years." 



As you see, the lines in this first magnetic map are very 

 regular and show no disturbances. They are derived from 

 observations at a comparatively small number of places, and 

 to-day we should classify them as terrestrial rather than true 

 isogonics. 



It is not necessary to give a complete list of the various 

 magnetic charts of the world published since Halley's time, in 

 which the isomagnetics in the African seas are given. Such a 

 list can be consulted in Hellmann's latest work.* The first 

 magnetic map dealing solely with Africa dates from 1798. It 

 gives the isogonics, for the epoch 1793. in the seas round 

 Africa; South Africa comes in with the rest of the continent. 

 It is entitled, " Chart of the Lines of Magnetic Variation in the 

 Seas Round Africa. James Rennell, May i8th, 1798," and is 

 published in an account of Alungo Park's Travels, in an appen- 

 dix by Major Rennell. t 



In the explanatory note accompanying the map, Rennell 

 says: " It appears on inquiry that the quantity of variation is 

 no more known within the continent of Africa than within that 

 of New Holland. And it happens, moreover, that the lines of 

 equal quantities of variation do not run across Africa with that 

 degree of regularity and parallelism which takes place over 

 a great part of the Atlantic and Indian Oceans (at least this is 

 what appears clearly to my judgment) ; so that it becomes neces- 

 sary to enquire what quantity prevails in the surrounding seas, 

 and what the general direction, as well as the particular nature 

 and tendency of the curves of the lines of equal quantities. 

 . . . . The theoretical part belonging to the interior of 

 Africa is founded on a supposed continuation of those lines of 

 equal quantities, whose tendency has been already ascertained 

 in the surrounding seas." 



Before we pass to what has been done in the mainland of 

 Africa for the stitdy of the magnetic state of the continent, it 

 may be as well to give a short account of the stage magnetic 

 cartography of land areas has reached, and to what extent a 

 magnetic map represents the results of observations. The 

 charts so far put before you show beautifully rounded lines, 

 which enable us to see at a glance what the value of a particular 

 element, such as the declination, ought to be at any given place 

 at the epoch for which the map is drawn; the magnetic maps 

 giving the results of the earliest surveys in European and 

 American countries have the same flat and regular appearance. 

 As , however, the number of stations at which observations were 

 made was increased it became apparent that the magnetic 



* Hellmann : Magnetische Kartographie Veroffentlichungen des Konig- 

 lichen Preussischen Meteorologischen Instituts. Abh. Bd. III., No. 3. 

 Berlin, 190Q. 



t The writer is indebted to Dr. Flint for permission to photograph the 

 copy of this chart contained in the Parliamentary Library. 



