430 SCIENCE PROGRESS. 



charts, showing the simultaneous values of the several mag- 

 netic elements over the whole or a portion of the globe. 1 

 Such maps may be divided into two classes, viz. : ( i ) maps 

 on which the values of the declination are indicated with 

 sufficient accuracy for the requirements of travellers by land 

 or sea, (2) maps (up to the present almost exclusively of 

 land) on which the values of the magnetic elements are 

 given with all possible precision. The latter class is chiefly 

 of scientific value, as showing the relations between the mag- 

 netic and geological constitution of the districts surveyed, 

 and giving information on other similar points. It is hardly 

 perhaps apparent at first sight that the successful construc- 

 tion of these maps is largely dependent on the existence of 

 fixed magnetic observatories keeping a record of magnetic 

 history ; such, however, is the case, as the following con- 

 siderations will show. Maps of the first class, however, if 

 correct when drawn, would soon be useless unless allowance 

 were made for the secular change of declination. The 

 necessary data are partly supplied by observations made 

 at sea, but as these are not very accurate it is most impor- 

 tant that they should be confirmed by more exact work at 

 numerous magnetic observatories. In the construction of 

 maps of the second kind magnetic observatories play a still 

 more important part. 



Exact magnetic observations can only be taken by 

 highly trained observers with first-class instruments, and 

 without an enormous expenditure it is impossible to multiply 

 largely either observers or instruments. Thus the carry- 

 ing out of a magnetic survey over a large area is a work 

 not of days but years, and the data obtained for construct- 

 ing a chart refer to such different times that allowance must 

 at least be made for the secular variation of the elements. 



Again the diurnal variation in some of the elements is 

 very considerable. At Kew, for instance, on an average 

 the declination magnet points eight minutes of arc further 

 to the westward at 2 p.m. than it did six hours previously. 



1 Cf. Captain Creak's Report on the Magneiical Results of If. M.S. 

 " Challenger 1 '. 



