S2 Mr. R..W. Fox's Notice of an Instrument for ascertaining 



being overcome by a slight vibration of the concentric plate. 

 This vibration may be produced by gently tapping the plate 

 at the back, or the latter may be indented, so that the passing 

 of a brass rod once or twice rapidly over its surface may, by 

 the friction produced, effect the desired object. After a little 

 practice I have found that this plan causes the needle to 

 take its natural bearing in the most satisfactory manner, and 

 at least to equal in delicacy any other method of mounting 

 it. The perpendicular support of the box rests on a hori- 

 zontal plane or base, and can be turned round on its axis in 

 any direction, and the needle be made to face any point of 

 the compass without altering the base of the instrument, 

 which is of course furnished with levels and adjusting screws 

 in the usual manner. Besides the graduated circle on the 

 face of the box, I use another, coinciding with it, which is 

 placed immediately within the glass to direct the line of 

 sio-ht, and to enable the observer to ascertain with great mi- 

 nuteness the place of the needle. For example, if the outer 

 circle be twelve times more distant from the indicating mark 

 on the needle than the latter is from the inner circle, it is 

 evident that any visual lines crossing at the mark, will be 

 separated from each other at the circles in the ratio of the re- 

 lative distances of the latter from the mark ; so that a degree 

 on the outer circle will in this way represent one twelfth, or 

 five minutes of a degree on the inner one. This mode of 

 division need only be limited by our means of observation. 



The plane of the magnetic meridian may be easily found with- 

 out a horizontal compass, by moving the face of the instrument 

 round till the needle becomes vertical (after vibration), which 

 will be its position when it is exactly opposite the magnetic 

 north or south, so that the needle will be in the plane required 

 when turned 90° from that point. If the base of the instru- 

 ment be furnished with a graduated scale round the circular 

 edge of the support or axis on which the box revolves, and 

 the axis itself with a vernier, or vice versa, it will enable us to 

 ascertain the variation of the magnetic from the true meridian 

 with great precision. If from defects in the construction 

 of the needle (to which all are liable,) its magnetic axis and 

 centre of gravity should be so untrue as to prevent its being 

 vertical when placed at right angles to the plane of the mag- 

 netic meridian, the error will be shown, and its extent de- 

 tected, by repeating the observations with the face of the in- 

 strument turned towards the opposite quarter; for in this case, 

 if the azimuth distance, when the needle again becomes ver- 

 tical, should exceed or fall short of 180°, the difference will 

 show the correction that must be applied. This property of 



