296 SEC. 9. MAGNETISM. 



1229b. Self-registering Barometer. 



Clias. Brooke, F.R.S. 



1229c. Photographic Apparatus, for registering simulta- 

 neously the variations of both the above instruments. 



Chas. Brooke, F.R.S. 



1229d. Self-registering Bifilar Magnetometer, with 

 compensation for changes of temperature, and warm- water envelope 

 for testing the same. Chas. Brooke, F>R.S. 



The compensation is effected by diminishing the lower interval of the 

 double suspension, by means of the differential expansion of glass and zinc, 

 in proportion to the diminished magnetic energy of the bar, due to elevation 

 of temperature. 



Photographic apparatus for registering the variation of the above, by means 

 of a reflected pencil of light. 



1230. Photographic self-registering Decimation Mag- 

 net, constructed in 1846, at the Kew Observatory, by Mr. Francis 

 Ronalds. 



Kew Committee of the Royal Society. 



Described in the Philosophical Transactions for 1847, vol. I. 



The magnet, 2 feet long, when in use, was suspended by a silken skein, 

 9 feet long, on its under side ; it carries a brass bar, from one end of which 

 hangs a perforated metal plate, which, moving in front of a lamp, permits a 

 pencil of light to fall upon a daguerreotype plate, carried slowly upwards by 

 a clock suitably arranged. 



The magnet is surrounded by a damper, made by electrotyping a frame of 

 mahogany with copper. Both are enclosed in double wooden cases, having 

 both surfaces covered with gold paper. 



This instrument was superseded by the improved Kew magnetographs, 

 which have been in almost continuous action since 1858. 



1231. Instrument for the determination of the position 

 of the point of convergence of the rays of the Aurora 



Borealis, both when it is below the horizon and also when it is 

 above the horizon at the appearance of the Corona. 



Prof. Hcis, Munster. 



The ball, resting in the pan, can after a few trials be brought into such 

 position that several diverging pencils of the aurora borealis on the northern 

 or the southern sky are, when properly viewed, covered by the rod which 

 passes through the centre of the ball. The point of this rod, which can be 

 moved up and down in the ball, shows, when the instrument is set according 

 to the astronomical meridian, the azimuth and altitude of the converging point 

 of the aurora pencils. This point of convergence does not exactly coincide, 

 as the exhibitor has shown at the time of the great display of aurora borealis, 

 Feb. 4th, 1872, with the point towards which the inclination needle directs. 

 From the deviation of the two points, the height of the aurora can be cal- 

 culated. 



The instrument, which is easily manipulated, is much recommended to 

 arctic explorers. 



Instrument for navigators in the Arctic Regions for ascertaining the con- 

 nexion of the Northern Lights with terrestrial magnetism, and for determining 



