VI. TERRESTRIAL MAGNETISM. 295 



It is a glazed box 4 inches square, standing on a levelling stand, and carry- 

 ing a brass suspension tube 6 indies in length. It also has an ivory circle 

 fixed to its bottom. 



There are two magnets 2 7 inches long, each of which when not in use 

 is kept in a separate little copper box, where, fitted to an armature, it is 

 embedded in iron filings. 



1225. Dip Circle used by Sir James C. Boss. 



Kew Committee of the Royal Society. 



A Robinson dip circle, with four 6-inch needles, supported on agate planes, 

 and read off direct on the circle of the instrument. 



1225a. Apparatus for discovering the Magnetic Poles 



in Magnets and Electro-Magnets, by Professor Th. Petrouchevsky. 



Imperial University of St. Petersburg. 



A fine metal wire is stretched within the magnetic meridian, above a sensi- 

 tive marine compass. Within a distance of 6 m from the compass, the pole 

 of which is to be determined, and below the same wire, a magnet is placed, 

 so that its two arms are horizontal, and perpendicular both to the meridian 

 and to the wire. The loaded needle of the compass usually deviates through 

 the effect of the magnet, but in a special case it may be made to remain 

 within the plane of the meridian. To effect this the line passing through the 

 two curved magnet poles must pass also within the plane of the meridian, a 

 thing easily accomplished. In the case of electro-magnets, besides the two 

 bobbins forming part of the electro-magnet, a third bobbin is used, intended 

 to compensate the effect of the first two upon the loaded needle. The de- 

 scription of this method, which presumes that both poles are equidistant from 

 the extremities of the magnet, as also the results of experiments, are published 

 in the Russian work " Cours de Physique Experimentale," by M. Petrouchevsky, 

 and since in the " AnnaJen der Physik," by Poggendorf. 



1226. Forbes' Hemispheres for illustration of Lectures on 

 the Earth's Magnetism and Temperature. 



University of Edinburgh. 



1227. Gambey's Declination Compass. 



Conservatoire des Arts et Metiers, Paris. 



1228. Self-registering Bifilar Magnetometer, with ap- 

 paratus for determining the temperature-correction of the magnets 

 employed in several automatic instruments, from the displacements 

 of the photographic trace due to observed changes of temperature. 



Chas. Brooke, F.R.S. 



1229. Self-recording Magnetometer. 



Chas. Brooke, F.R.S. 



Rough home-made apparatus, by which the first automatic records of mag- 

 netic variation by reflected light were obtained. The cylindrical lenses are 

 water-lenses. 



1229a. Self-registering Balanced Magnetometer, with 

 compensation for changes of temperature, and warm water en- 

 velope for testing the same. The compensation is effected by 

 the weight of the column of mercury in a thermometer tube. 



Chas. Brooke, F.R.S. 



