/456 MAGNETIC VARIATION. 



on accurate surveys, in the course of a few years, in some cases, are so 

 affected by this change in the Variation as to endanger the safety of a 

 vessel where they have been implicitly trusted to. The appreciation of 

 this change, which became manifest simultaneously with the necessity for 

 improved Compasses and improved methods of using them, placed the 

 magnetic element in charts on a fresh basis. 



One most important result of this movement was the appointment by 

 the Admiralty of the late Captain E. J. Johnson as superintendent, in 

 1842, of the Compass Department. The great improvement in Compasses 

 dates from this appointment, and the investigation of the difl&cult and 

 varying problems of Local Deviation have been since pursued by eminent 

 men, among whom may be noticed Professor G. B. Airy, Dr. Scoresby, 

 W. Walker, R.N., Archd. Smith, Esq., and many others. These researches 

 have been mainly directed, as before observed, to the effect the ship's iron 

 has on her Compasses. 



Captain F. G. Evans, R.N., wuo succeeded Captain Johnson, drew up a 

 far more perfect chart of the geographic distribution of the Magnetic 

 Variation than had hitherto appeared; former charts having become of 

 impaired value from the lapse of time, and from the imperfection of the 

 observations on which they were based. It is from this chart, brought 

 down to the period of 1917, that the accompanying illustrative chart has 

 been constructed. 



(467.) The Isogonio Lvies, or those upon which the Variation is of the 

 same amount, on this chart, will represent this element, generally as near 

 as the ordinary ship's Compass will show it, and will serve to draw atten- 

 tion to any unsuspected change in the Magnetism of the ship, besides 

 affordinfT the sailor some information when observations cannot be had. 



(468.) The Variation of the Compass in all parts of the coasts of the 

 North Atlantic Ocean are given with the Tables of Geographic Positions 

 at the commencement of this work. It is for the open ocean that the 

 illustrative chart and these notes are mainly intended, and on the chart is 

 inserted the amount of annual change in different parts, so that the 

 approximate Variation may be ascertamed m luture years by applying the 

 necessary correction. 



Around Iceland, the Compass is in many localities unreliable, especially 

 near the shore and over shoals ; frequent observations must therefore be 

 made here for ascertaining the error. 



(469.) But it must not be supposed that this annual change is regular 

 and of the same amount in each year. By the accurate observations which 

 are now self -recorded, the connection between these changes and appa- 

 rently remote causes has been identified. One of these, at the first glance 

 a very singular one, is that the spots in the sun, if absent or present in 

 large quantities, have a marked magnetic influence on the Declination, 

 thus demonstrating the source from which the Magnetism of the earth is 

 chiefly derived. As the Greenwich Observations will illustrate our subject 

 as well as any, and this volume might be filled with interesting remarks on 

 this subject, the notices will be limited to the following extracts from those 

 Observations, as being BuflBcient to impart a notion of the ever-varying 

 amount of the Magnetic Variation and Dip. 



