i MAGNETIC APPARATUS. 51 



may be returned to the Smithsonian Institution, indorsed Me- 

 teorology. 



MAGNETIC APPARATUS. 



Few observers will probably be furnished with a regular set of 

 magnetical instruments. A temporary apparatus may, however, 

 be fitted up at comparatively little expense and trouble. For 

 this purpose a steel plate, such as was used a few years since for 

 ladies' busks, may be magnetized, and suspended edgewise in 

 the vertical plane, by a few fibres of untwisted silk, in a box to 

 prevent agitation by the air, furnished with a glass window on 

 one side, through which observations may be made. To render 

 the motions perceptible, a small mirror should be cemented on 

 the side of the magnet opposite the window. In front of this 

 mirror, and at the distance of ten or fifteen feet, an ordinary spy- 

 glass is fastened to a block, and under the glass, to the same 

 block, a graduated scale, with arbitrary divisions marked upon 

 it, is attached. The arrangement is such that the divisions of 

 the scale may be seen through the telescope, reflected from the 

 mirror, and consequently the slightest motion of the needle, and 

 of the mirror cemented to it, gives a highly magnified apparent 

 motion to the scale. The mirror may be formed of a flat piece 

 of steel, highly polished by means of calcined magnesia ; or, in 

 default of a mirror of this kind, a piece of plate looking-glass 

 may be employed, provided one can be procured sufficiently true. 

 The suspension threads should be three or four feet long. The 

 instrument should not be placed very near large masses of iron, 

 and care should be taken not to change the position of any 

 articles of iron which are within the distance of fifteen or twenty 

 feet, otherwise a change in the position of the needle will be 

 produced. For a similar reason the box should be constructed 

 without iron nails. The above described instrument will indi- 

 cate changes in the direction of the magnetic meridian. A 

 similar instrument, deflected at right angles to the magnetic 

 meridian by the torsion of two suspended threads, will furnish 

 an apparatus for indicating changes of horizontal magnetic 

 force. 



