216 Variation of Magnetic Declination and Inclination. [M;ir. 1, 



dynamic substance within the earth's crust are afforded (1) by the 

 dipping needle making only one downward or upward motion (out- 

 side the space comprised within the north and south limits of the 

 magnetic equator) for the outward and return march of the declina- 

 tion needle, as instanced in the continued diminution of the dip in 

 the British Isles during the westerly outward and return march of the 

 declination needle since the year 1723. (2) From the secular changes 

 of the dip in opposite directions about the same meridian in the 

 northern and southern hemispheres, as instanced in the dip diminishing 

 in the British Isles and increasing at the Cape of Good Hope and 

 St. Helena for the same epoch (prop. X). (3) The rapid increase of 

 the dip about the nodes of the magnetic equator, as first indicated by 

 Sabine in the Gulf of Guinea and St. Helena. Before the time of 

 my experiments no attempt had been made to assign a cause for 

 the large and rapid change in the dip (about 17 minutes annually) 

 on this part of the terrestrial surface until I found that the results 

 obtained on the magnetarinm agreed very closely with the observations. 

 Subjoined are tables of the declination and inclination at St. Helena 

 and on the magnetarium. 



On working the inclination backwards on the magnetarium 

 chronologically, it will be seen from the Table VI that about the 

 year 1747 the dip changed the sign from south to north. As no 

 observations were made on the dip at St. Helena previous to the 

 year 1825, there is no record of this interesting fact, nor has it 

 hitherto been deduced from theory. 



A further confirmation of the agreement of the results obtained on 

 the magnetarium with actual observations has recently been brought 

 under my notice in the bulletin issued by the United States Coast 

 and Geodetic Survey, No. 23, March 16th, 1891. In this publication 



Table VII. Secular Changes of the Declination and Inclination at 

 Ascension Island and on the Magnetarium. 



* Annual differential motion of internal sphere 22'*5 = 0*375 from zero at 

 London in the jcur 1657. 



