VI. TERRESTRIAL MAGNETISM. 253 



side, a light brass bar, which moves a little shutter in front of an oil lamp, 

 allowing a pencil of rays to pass through a hole in it. The light is then 

 thrown, by means of a lens, upon a daguerreotype plate, which is steadily 

 drawn upwards by means of a clock. 



The curves upon the daguerreotype plates were sometimes etched in, and 

 engravings subsequently Avorked off, or tracings were made upon sheets of 

 gelatine, which, being preserved, allowed the silvered plates to be repeatedly 

 used. 



This instrument was superseded by the improved magnetographs erected at 

 Kew by Mr. Welsh in 1857, which have since remained in almost continuous 

 action. 



The suspension frame originally fitted has been replaced by one not belong- 

 ing to the instrument when in use. 



1222. St. Helena Magnetometers. 



(1.) Declinometer instrument and telescope, used at St. Helena, 

 1840-1849. 



(2.) Bifilar magnetometer and telescope, used at St. Helena, 

 I84O-1849. 



(3.) Vertical force magnetometer, used at St. Helena, 1840- 

 1 849. Kew Committee of the Royal Society, Kew Observatory. 



The three magnetometers, the declination, horizontal force, and vertical 

 force instruments, respectively, Avere made by Grubb, of Dublin, and formed 

 one set of those used in the Colonial Magnetic Observatories, founded by the 

 Government in 1840. The instruments were described in the Report of the 

 Royal Society Committee of Physics, &c. 



These instruments were erected at St. Helena in 1840, and constantly 

 observed from that date until 1849. 



The declinometer consists of a magnet bar, suspended by fibres of untwisted 

 silk, and carrying a collimator arrangement of lens and scale, the whole 

 being enclosed in a cylindrical casing, perforated with windows, through 

 which the scale is vieAved by means of a telescope. 



The bifilar is a somewhat similar arrangement, but the support of the 

 magnet is formed of two parallel wires, Avhich are tAvisted so as to bring the 

 magnet into a position at right angles to the meridian. 



The vertical force magnetometer is a light magnet 12 inches long, carrying 

 a brass frame with cross wires at each end ; it is supported by a steel knife 

 edge, bearing on agate planes, and its movements are observed by micro- 

 scopes, fitted with micrometers, by Avhich the position of the cross wires on 

 the magnets is read. 



1223. Declination Compass, 'used by Sir J. Richardson and 

 Capt. Pullen. 



Kew Committee of the Royal Society, Kew Observatory. 



It consists of a square glazed box, containing a compass card, which is 

 formed of a light metal divided circle, and two spring needles, connected to 

 an agate cup in the centre. This is mounted so that it can either be suspended 

 by a silk thread, or rest upon a point in the ordinary manner. 



Two microscopes are fixed vertically above it, so that the divisions on the 

 circle may be read by them, concave metallic reflectors being fitted to them 

 for the purpose of illuminating the scale at the time of reading off. 



