iv Transactions of the South African Philosophical Society. 



the average temperature along a line of latitude, and then to note on 

 the map for each place the difference between the actual tempera- 

 ture of the place and the mean temperature of its latitude. Such a 

 map shows what are called temperature anomalies. 



In the case of a map showing the results of a magnetic survey the 

 state of a region is shown by isomagnetic lines. The map before 

 you shows the lines of true dip for South Africa for July 1, 1903. 



The smaller maps show the dip in South Africa at other periods. 

 These have been derived by calculation, and do not give the devia- 

 tions due to local disturbances. The first thing that strikes one in 

 comparing the different maps is the fact that there are great dis- 

 placements of the isomagnetic lines from one period to another ; 

 secondly, the lines in the larger map are very tortuous in some 

 regions, in others they are bunched together, and in others still they 

 are spread widely. The consideration of the causes of these arrange- 

 ments of the lines brings out one of the differences between earth 

 magnetism and meteorology. The latter embraces results which 

 arise from the effect of our sun on the land surface and its adjoining 

 waters, and which is uninfluenced by the earth's crust below a depth 

 of a few yards. On the other hand, earth magnetism gives us 

 results which after the sun's effect — shown in the daily and annual 

 variations — is eliminated, depend on the nature of the earth's crust 

 to some distance below the surface. This effect is shown very 

 clearly if we again present the results at different parts in the form 

 of deviations from the mean magnetic state as derived from all the 

 observations in the region. In such a map certain places have a 

 greater dip than the mean ; that is to say, these places are in South 

 Africa magnetically equivalent to weak south poles, which pull down 

 a freely suspended magnet more than the other parts of the region. 

 When the observation of the other magnetic elements are treated in 

 the same way it is then possible to determine the position of the 

 disturbing body and its extent. In Sweden and in America surveys 

 of this nature have been made to locate iron ore deposits. 



Another physical experiment allows us to obtain information con- 

 cerning the density of the material under the earth's surface. The 

 attraction of the earth on a body is known to be different at different 

 parts of the earth after the necessary allowance has been made for 

 the shape of the earth. The gravitational irregularities in a par- 

 ticular region can be shown on a map representing the results of a 

 gravitational survey of that region. The map might have on it iso- 

 gravitational lines, or it may have marked on it the gravitational 

 anomalies. The latter are present in the neighbourhood of moun- 

 tains, and even in plains which on the surface give no indication of 



