326 



NATURE 



[Feb. 22, 1872 



THE RECENT AURORA, AND A NEW FORM 

 OF DECLINOMETER 



ON Sunday night, the 4th of February', we saw here 

 the mag-nificent coloured Aurora BoreaHs, which 

 has been described in Nature, in the newspapers, and 

 which, I see from telegrams, has been observed at very 

 distant stations. Indications of the aurora were noticed 

 here soon after sunset ; but about 6.45 p.m. the whole 

 eastern portion of the sky became illuminated with red 

 light, at first faint, but rapidly becoming more and more 

 intense, while yellow streamers began to shoot up from 

 the north-eastern arc of the horizon nearly to the zenith. 

 About the south-west there was also much red and yellow 

 light ; it was spread over a large apparent area, but was 

 not so intensely bright or so strongly coloured as that 

 which lay to the north-east. It too, however, possessed 

 splendid broad, yellow streamers. The display lasted in 

 full beauty till about 7.20, but long after that time much 

 red and yellow light with occasional streamers was to be 

 seen. 



It is strange that the phenomena of the Aurora Borealis 

 still remain so little imderstood. It would add much to 

 our knowledge, if those who witness these displays would 

 make sketches of the appearances at the lime when vei-y 

 definite forms of the streamers are observed, noting also 

 the time of the observation very carefully, and the position 

 of well-known stars and constellations. A comparison of 

 such sketches, and of notes that might accompany them, 

 would give us most important data, and might lead to 

 the determination of the locality of the discharge 



Simultaneous observations, at widely different stations, 

 of the disturbances of terrestrial magnetism that always 

 accompanied the aurora might, if compared, give us useful 

 information as to the direction and velocity of the electric 

 discharge ; and would probably at least help us to decide 

 whether it is to the discharges themselves, or to earth- 

 currents, or to both combined, that these disturbances 

 are due. 



I wish to describe an instrument planned by Sir 

 William Thomson, which may be easily constructed, and 

 with which the variations of the horizontal component of 

 terrestrial magnetism can be determined with great accu- 

 racy. 



A fl.1t wooden support, seven or 

 eight inches high, is fixed on a con- 

 venient foot furnished with levelling 

 scrc.vs, and in the face of it a groove, 

 rather more than four inches long and 

 about -j'j; of an inch decj', is cut. 

 From 1x point at the top of this 

 groove, a v-ery light mirror with 

 magnets attached — such as is used 

 in Thomson's reflecting galvano- 

 meter — is suspended by a single silk 

 fibre about four inches long; and 

 ii II V in front of the groove there is 



fastened, if the mirror be concave, 

 a slip of plate glass to keep off 

 currents of air : or, if it be a plane 

 mirror, a lens is fastened in front of it, and the remainder 

 of the groove is covered up with a slip of glass or in some 

 other way. A lamp is placed in front of the mirror, and 

 the reflected image of it is received on a scale. The 

 motions of the reflected light upon the scale indicate the 

 deflections of the magnet. 



Suitable mirrors and lenses arc constructed by Mr. 

 \yhite, instrument maker, Glasgow. In making the 

 mirrors, a large number of the lightest circular glasses 

 used for covering objects on slides for the microscope are 

 silvered ; and from these those which give an image 

 perfectly free from di.stortion are selected by trial. Many 

 of the mirrors formed are much twisted and quite unfit 

 for US' ; but mirrors are obtained by this plan of selection 



rT\ 



by trial far superior in lightness and in freedom from 

 distortion to any that can be made by expending 

 extreme care in the glass-work. To the Ijack of each 

 mirror four small magnets are attached ; an arrangement 

 which has been found by trial to give the best result. 

 The object is to m.ake the mirror with its magnets suffi- 

 ciently light, and to give it at the same time the greatest 

 possible magnetic moment. The mirror is three-eighths 

 of an inch in diameter, and weighs not more than one- 

 third of a grain. 



Plane mirrors arc generally used in Glasgow, and the 

 lens is of such power that a lamp placed at a distance of 

 one metre (about 40 inches) gives an im.age at the same 

 distance from the mirror. The lamp is placed behind a 

 screen, and in the screen an oval hole is cut and a vertical 

 wire * is stretched across it. The image of this wire is 

 received upon a scale. The scale may be set at a distance 

 of 40 inches (one metre) from the mirror ; that is to say it 



may be attached to the screen between the mirror and the 

 lamp ; or it may be put much farther away, at, say two or 

 three times that distance. The lamp and screen, with its 

 slit and wire, must then be brought near enough to the 

 mirror to throw back the conjugate focus sufficiently. 

 This arrangement gives of course increased sensibility. 

 We use for if t paraffin oil lamp, of which the reservoir 

 is a verj' shallow rectangular vessel. The slit in the screen 

 is slightly above the horizontal plane through the centre 

 of the mirror, and the scale slightly below that [plane. 

 The reflected ray passes below the reservoir of the lamp 

 to the scale beyond. 



Our scales, which are also obtainable from Mr. White, 

 are divided into fortieths of an inch, and are generally 

 attached to a piece of wood, cut out so that its curvature 

 corresponds to that of a circle described with the distance 

 of the mirror as radius. Thus, by dividing the number 

 of scale divisions by the distance of the mirror in fortieths 

 of an inch from the scale, the angle is obtained to which 

 that number of scale divisions correspond. At a distance 

 of 5o inches we can easily read the position of the image 

 of the wire on the scale to less than half a scale division, 

 which, since the angle turned through by the reflected 

 beam of light is twice that turned through by the mirror, 

 corresponds to an angular deflection of about 20". 



The great advantage which the arrangement that I 

 have just described possesses over any that are ordinarily 

 used for observing rapid variation in magnetic declination 

 lies in the lightness of the mass moved. The heavy 

 declinometers employed in observatories are unable, 

 through their great inertia, to follow accurately the sudden 

 variations that occur during a magnetic storm. 



James Thomson Botto.mley 



The College, Glasgow 



* A simple vertical slit was formerly used, but the vertical wire in the 

 middle of the slit, a suegestion of Prof. Tait, is a great improvement, as it 

 eaatles us to use plenty of light, while it'givcs increased precision to the 

 reading on the scale. 



