16 Photography at Greenwich Observatory. 



coincident with the poles of the earth's axis, neither do they 

 indicate the points of greatest intensity in the magnetic force, 

 which is nearly three times as great in some parts of the earth 

 as in others. The dip, like the declination, is subject to 

 secular, annual, and diurnal variations, but our limits will not 

 allow us to enter upon the details. It may, however, be men- 

 tioned that at London in 1720 the dip was 74° 42 , and in 

 1830 69° 38'. It is now about 68°, so that it continues 

 diminishing. 



In 1836, principally by the exertions of Humboldt, the 

 British Government consented to co-operate in the formation 

 of magnetic observatories, and accordingly established and 

 equipped those at Toronto, St. Helena, the Cape of Good 

 Hope, and Hobart Town. The East India Company established 

 four more in their territories, while magnetic and meteoro- 

 logical departments were added to the observatories of Green- 

 wich and Dublin, and an expedition to the Antarctic Seas was 

 sent out under Sir J. C. Ross, to obtain corresponding obser- 

 vations in high southern latitudes. Some of these observa- 

 tories were kept up for a limited period only, but others remain 

 in full operation ; and to the accumulated mass of observations 

 thus procured, as reduced and discussed by General Sabine, 

 we are now indebted for an immense increase of our know- 

 ledge of terrestrial magnetism and its laws. 



The present Astronomer Royal has always taken a deep 

 interest in magnetism, and is himself one of our best authori- 

 ties on the subject, and in the year 1837 he became anxious 

 that our national observatory should take part in the efforts 

 then making to improve the science. Accordingly, at his 

 request, an additional piece of ground was enclosed in the 

 observatory domains, and in the following year the magnetic 

 observatory erected. It is built of wood, and fastened with 

 wooden pins, the metal iron being carefully excluded, except 

 in the case of a portion of a stove and those parts of the clocks 

 and instruments where it is absolutely necessary. The posi- 

 tion of the building is about 170 feet from any other part of 

 the observatory, and 34 feet from the nearest shed or erection 

 of any kind. It is in the form of a cross, originally 40 feet in 

 each direction, by 12 feet wide and 10 feet high; but the 

 northern arm of the cross has lately been lengthened 8 feet. 

 From the exterior of the observatory grounds the building is 

 easily recognized, as in front is placed the tall mast used to 

 collect atmospheric electricity, and conduct it to the various 

 pieces of apparatus placed in the window of the northern arm 

 of the building. The remainder of this arm forms the com- 

 puting room of this department of the observatory. The 

 instruments were placed in the remaining three arms of the 



