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MAGNETIC OBSERVATORY AT 



operations. The upper part of the building is occupied by a similar 

 ante-room and a laboratory, where the preparation of the sensitive 

 paper and other photographic operations are performed. A soapstone 

 stove, with copper pipe, supplies in winter the necessary warmth to 

 the room. The windows have close shutters, with an opening cor- 

 responding to a yellow pane of glass, which admits sufficient light for 

 operating in the room, while it in no degree affects the sensitized paper. 

 No sunlight whatever penetrates to the instrument-room, which is 

 only dimly lighted by the reflection at the ceiling of the burners used 

 for the registration. 



GENERAL DESCRIPTION OF INSTRUMENT ROOM. 



The annexed diagram, Fig. 1, illustrates the disposition of the 

 instruments. D is the declination magnet; H the horizontal force, 

 and V the vertical force magnetometer; the letter n designates the 

 north end of each magnet; the declination magnet being suspended 



Plan of instrument room. 



freely in the magnetic meridian, the horizontal force magnet in a 

 bifilar torsion-balance, with its north end to the west, and the bal- 

 anced vertical force magnet attached to a beam resting on knife- 

 edges, with its north end to the east. The line joining the centres 

 of D and H makes an angle of 35° 16' with the magnetic meridian, 

 according to Lloyd's theorem, for the position in which small changes 

 of direction in one magnet will have no effect upon the other. In 

 6, b, b, on the gas-burners, from which the recording pencils of light 

 emanate, which are reflected by concave mirrors attached to the 

 magnets, and concentrated upon the record cylinders r, r, r. Bach 

 magnet carries, besides, a small plane mirror, by means of which 



