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SYDNEY CHAPMAN, ESQ., M.A., D.SC, ON 



force (declination, dip, or intensity) at a particular place and 

 time. All observations at sea, and those made in the course of 

 magnetic surveys on land, are of this class. The instruments 

 employed are set up at the chosen station, the observation is 

 made, and the instruments are then transported to the next 

 station. 



The observations of the other class are made at fixed obseiva- 

 tories, and consist of a continuous photographic registration of 

 the variations of the magnetic elements. The dark chamber 

 in which this is carried on is usually carefully maintained at a 

 nearly constant temperature, either by being built in the ground 

 as a cellar, or by having thick double walls and internal tempera- 

 ture control. At such observatories absolute observations are 

 also made, usually several times a week, in order to standardize 

 the magnetic registers. These registers are the source of 

 our knowledge of the daily and irregular magnetic variations, 

 including magnetic storms. The absolute observations, at 

 observatories and elsewhere, provide our information as to the 

 main distribution of magnetic force over the earth, and as to 

 its slow secular changes. 



The facts thus obtained are, as I have endeavoured to show, 

 both interesting in themselves and of considerable practical 

 importance. Beyond this, however, lies the question of their 

 significance and interpretation ; the search after prior causes 

 is one from which our minds, as constituted, are unable to refrain. 

 In the case of terrestrial magnetism the results of the search, 

 so far as it has been successful, are of great variety and interest. 

 A comparatively few magnetic observations at the earth's 

 surface have, even in the course of the present brief sketch 

 of the subject, led our thoughts to strange and inaccessible 

 regions. We have considered the tidal action of the moon 

 upon the rarefied gases in high atmospheric layers, 

 the mechanical effects of solar heat in these regions, 

 and likewise the electrical consequences of a portion of the 

 sun's radiation which never penetrates to the earth's surface. 

 The Auroral Lights in arctic skies, no less than electric currents 

 within the earth's crust, and at great depths in the intensely 

 compressed and heated core, have claimed our attention, 

 and continually we have had to refer to the varied 

 emissions from the ever disturbed surface of the sun. Never- 

 theless, though much has been explained, further and great 

 mysteries remain to be unfolded. Chief among these is the 



