Royal Institution. 



219 



tation the conclusion did not seem so sure. As some further rela- 

 tions of the sun and the earth would have finally to be submitted, 

 the audience were reminded, by the use of Arago's idea, of the rela- 

 tive magnitude of the two ; for, sup})osing that the centres of the 

 two globes were made to coincide, the sun's body would not only 

 extend as far as the moon, but nearly as far again, its bulk being 

 about seven times that of a globe which should be girdled by the 

 moon's orbit. 



For the more careful study of the magnetic power a torsion- 

 balance had been constructed, which was now shown and its mode 

 of operation explained. The torsion wire was of hard drawn pla- 

 tinum, 24 inches in length, and of such diameter that 28*5 inches 

 weighed one grain. It was attached as usual to a torsion -head and 

 index. The horizontal beam was a small glass tube terminated at 

 the object end by a glass hook. The objects to be submitted to the 

 magnetic force were either cylinders of glass with a filament drawn 

 out from each, so as to make a long stiff hook for suspension from 

 the beam ; or cylindrical bulbs of glass, of like shape, but larger 

 size, formed out of glass tube ; or other matters. The fine tubular 

 extremities of the bulbs being opened, the way through was free 

 from end to end ; the bulbs could then be filled with any fluid or 

 gas, and be re-submitted many times in succession to the magnetic 

 force. The source of power employed was at first a large electro- 

 magnet ; but afterwards, in order to be certain of a constant power, 

 and for the advantage of allowing any length of time for the obser- 

 vations, the great magnet, constructed by M. Logeman upon the 

 principles developed by Dr. Elias (and which, weighing above 100 

 lbs., could support 430 lbs. according to the report of the Great 

 Exhibition Jury), was purchased by the Royal Institution and used 

 in the inquiries. The magnet was so arranged that the axis of 

 power was five inches below the level of the glass beam, the interval 

 being traversed by the suspension-filament or hook spoken of above. 

 The form and position of the terminations of soft iron are shown in 

 plan by the diagram upon a scale of -^q, and also the place of the 

 object. All this part is enclosed 

 in the box which belongs to, and 

 carries the torsion-balance, which 

 box is governed by six screws 

 fixed upon the magnet table ; and 

 as both the box and the table have 

 lines and scales marked upon them, 

 it is easy to adjust the former on 



the latter so that the beam shall be over and parallel to the line 

 a, e with the point of suspension over c ; or, by moving the whole 

 box parallel to itself towards m, to give the point of suspension any 

 other distance from the angle c. As already said, the objects were 

 constructed with a suspension filament of such length as to make 

 them coincide in height with the angle in the magnetic field. When 

 suspended on the beam, they were counterpoised by a ring or rings 

 of lead on the further arm of the beam. These when required were 



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