VI. TEREESTKIAL MAGNETISM. 255 



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. 



1229b. A self-registering Barometer. 



Chas. Brooke, F.R.S. 



1229 c. 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. 



1229e. Apparatus for showing the longitudinal vibrations of 

 a row of particles, (1) stationary, and (2) progressive. 



Chas. Brooke, F.R.S. 



The vibrations shown are those constituting the first harmonic subdivision 

 of a pipe closed at one end. 



1230. Photographic self-registering Declination Mag- 

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

 Ronalds. 



Kew Committee of the Royal Society, Kew Observatory. 



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This instrument was superseded by the improved Kew magnetographs , 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. 



Professor Heis, 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 



