14 Photography at Greenwich Observatory. 



constantly changing, that but for this method of obtaining a 

 perfect and unerring record most of their manifestations would 

 be altogether lost, or if observed at all, it would be at intervals 

 only, and accomplished by an expenditure of labour amounting 

 to drudgery, which might have been better applied. 



The science of Terrestrial Magnetism is perhaps not one 

 of the most attractive, and those readers of the Intellectual 

 Obseevee who have studied it will doubtless, for the sake of 

 others not familiar with the subject, pardon a few words of 

 explanation as to what we desire to observe, and the instru- 

 ments employed in observing, with the methods of using 

 them, before we describe the photographic processes forming 

 the principal object of this paper. 



Everybody is aware of the directive property of the mag- 

 netic needle as used in the mariner's compass, but all may not 

 equally be cognizant of the fact that the direction of the needle 

 is not always and everywhere to the true or astronomical north, 

 but varies very considerably from it at various places on the 

 earth's surface ; and that even when the variation is deter- 

 mined for any one place at any particular time, this variation 

 is not constant, but is always changing. For instance, at 

 London in the year 1550, upwards of 300 years ago, the needle 

 pointed 11° 17' E. of the true north; about 1660, rather more 

 than 200 years ago, it had returned to its normal position, and 

 then coincided with the astronomical meridian ; after which it 

 began to move westward, attained its greatest deviation of 

 24° 27' W. in 1815, and is now slowly returning to the north, 

 its present variation being about 20° 30' W. 



At Paris the variation has undergone nearly the same 

 changes, but at other places, Jamaica for example, it has 

 remained constant. At Liverpool it is almost 1° 30' greater 

 than at London ; at Edinburgh about 2° 5' greater ; at Yar- 

 mouth and Dover about 40' less. In Siberia and some parts 

 of North America the variation is at present towards the 

 east. 



In addition to this steady secular variation, or declination, 

 as it is now termed, there are other alterations of the direction 

 of the magnet constantly progressing. One of these is diurnal, 

 and causes the north end of the needle in our hemisphere to 

 move eastward during the early morning hours ; it will arrive 

 at its extreme easterly elongation between 7 and 8 a.m. ; it 

 will then begin to move westward, reaching the extreme west 

 point between 1 and 2 p.m., and then returning in an easterly 

 direction. This variation is governed by the solar time at 

 every place, and is clearly traceable to the sun's position, 

 which produces another fluctuation in these diurnal changes, 

 whereby for half the year, that is, from spring to autumn, the 



