114 GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. 



afford to neglect the valuable information the magnetic needle affords. 

 Thus although Eiicker and Thorpe have since made a second and more 

 elaborate survey of the British Isles, their remark of 1890 that " the 

 kingdom can be divided into magnetic districts in which the relations 

 between the direction of the disturbing forces and the main geological 

 characteristics are so suggestive as to be worthy of careful statement 

 and further investigation/' not only holds good, but has received 

 confirmation. 



The mining engineer is deeply interested in a knowledge of the 

 declination. Charts of normal lines are of great use to him whether 

 above or below the earth's surface, but especially below when he has 

 no other guide. To such an one a knowledge of regional magnetic 

 disturbance as deduced from surface observations is most important, 

 as it tells him that he is in the neighborhood of magnetic rocks, the 

 disturbing effect of which on his compass needle may be far greater 

 in the depths of his mine and turning it into a treacherous guide. 



We have now considered magnetic observations in a measure from 

 the point of view of the immediate practical results which their scien- 

 tific treatment produces, but who will say hi this great maritime nation 

 that the work of magnetic observers, even if solely to make navigation 

 possible, is not worthy of the fullest consideration"? 



There is besides a vast field of inquiry for the observer of terrestrial 

 magnetism in unraveling the secrets of the earth considered as a mag- 

 net, and the ceaseless change of its magnetic condition which the 

 needle tells us of, for which no immediate practical result can be fore- 

 seen, yet is worthy of the attention of the ablest physicists and most 

 advanced mathematicians. 



Inquiry into the causes of the secular change is one requiring the 

 fullest attention, but observation has not yet done sufficient work. 

 It certainly has done much in certain countries, and for a large portion 

 of the world as regards secular change in the past, and data obtained 

 for predicting future changes for a few years, but only one expedition 

 has examined the Antarctic regions magnetically, and it is doubtful if 

 any substantial progress will be made until a second expedition is 

 made thither, one profiting by the experience of its precursor, and 

 equipped with possibilities for work hardly hoped for by Ross. 



It may be remarked in passing that a remarkable alteration in the 

 amount of the secular change has been noticed in the declination and 

 inclination at the following observatories : Bombay, Batavia, and Hong 

 Kong about the period of the eruption of Krakatoa in 1883. This may 

 be only a coincidence, but may it not also point to the possibility that 

 the changes below the surface of the earth which culminated in that 

 mighty explosion, and may still be at work, have had, and continue to 

 have, magnetic effects which are recorded by the needles at those 

 observatories % 



Critical investigations have for many years been directed to the elu- 

 cidation of the causes of the observed diurnal variations of terrestrial 



