772 Mr. Charles Ghrcp [March 4, 



WEEKLY EVENING MEETING, 



Friday, March 4, 1910. 



Sir James Crichton Browne, M.D. LL.D. D.Sc. F.R.S., 

 Treasurer and Yice-President, in the Chair. 



Charles Chree, Esq., M.A. Sc.D. LL.D. F.R.S., Superintendent 

 Observatory Department, National Physical Laboratory. 



Magnetic Storms. 



§ 1. In the case of terrestrial magnetism, the ordinary scientific 

 procedure of passing from the more known to the less known has been 

 Ijy no means universally followed. 1 shall, however, observe it 

 to-night, and shall preface my description of the complicated phe- 

 nomena exhibited in magnetic storms l)y a brief account of the 

 comparatively regular phenomena which are of daily occurrence. 



The magnetic needle has been described with poetic licence as 

 " true to the pole," and few, I suspect, are aware how little it deserves 

 this reputation. The earliest known information on this point in 

 England dates from 1580, when Boroughs, observing at Limehouse, 

 found the needle to point 11:^:' to the east of geographical north. 

 During the next 2 A centuries it kept moving to the west, reaching 

 its extreme position of 245' to west of north in 1<S18. It has since 

 reti'aced its path, and now at Kew points only a little more than 16" 

 to west of north. The declination, or angle made with the true north, 

 is at present diminishing about one degree in 10 years, so the poet's 

 words may be true of England before the year 2100. 



§2. Besides this slow secular change, there are daily changes, 

 which are continuously recorded at a number of observatories. The 

 method of ol)taining a continuous record of the direction of the 

 compass needle is very simple. A magnet suspended by a fine filjre 

 carries a mirror, which reflects a beam of light. As the magnet turns 

 the mirror moves witli it, and the reflected light, by a well-known 

 optical principle, moves through twice the angle described by the 

 mirror. The reflected light falls on photographic paper at a fixed 

 distance, and the movement of the spot of light on the paper is pro- 

 portional to the angle through which the magnet and mirror have 

 moved. From a fixed mirror, adjacent to the movable one, light is 

 also reflected on to the paper, which is wound on a drum turned by 

 clockwork at a uniform rate. When the paper is unwound and 

 developed, it shows a straight line trace due to the light from the 



