THE SMITHSONIAN INSTITUTION. 249 



tion of the heavens included on the map, may he marked by a line, 

 the lengtli of whicli will show tlic path of the meteor; the course 

 should be indicated by an arrow, and the time recorded. 



MAGNETIC APPARATUS. 



Few observers will probably be furnished with a regular set of 

 luagnetical instruments. A temporary apparatus may, however, be 

 fitted up at comparatively little expense and trouble. For this pur- 

 pose 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 ])revent agitation by the air, 

 furnished with a glass window on one side, through which observa- 

 tions may be made. To render the motions perceptible, a small 

 mirror should be cemented on the side of the magnet opposite the 

 Avindow. In front of this mirror, and at the distance often 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 five or 

 six 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 indicate 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 hori- 

 zontal magnetic force. 



ELECTRICAL APPARATUS. 



To ascertain whether any change takes place in the electrical state 

 of the atmosphere during the appearance of an aurora, the end of a 

 long insulated wire, suspended from two high masts or two chimneys 

 by means of silk threads, may be placed in connexion with a delicate 

 gold leaf electrometer. Any change in the electrical state of the at- 

 mosphere, simultaneous with the aurora, will be indicated by the 

 divergence of the leaves. Two slips of gold-leaf attached by a little 

 paste to the lower end of a thick wire, passing through a cork in a 

 four-ounce vial, will answer for this purpose. The arrangement of 

 the leaves will be best made by a bookbinder, who is expert in the 

 management of gold-leaf. 



The map when filled, together with any written observations, may 

 be returned to the Smithsonian Institution, endorsed Meteorology. 



