and Remarks on the Theory of Magnetism. 23 



One circle of needles, affixed to a small bar of wood, appears 

 to be the best for making experiments with ; but the most pre- 

 ferable form of the instrument for shewing the influence of the 

 sun simply, is to affix two circles of needles to the bar of wood, 

 one at each end, in a perpendicular position *. As the circles 

 always turn their edges to the sun, the bar of wood in this form 

 will be directed to the sun, or to the angle of incidence described 

 by its rays. 



In constructing the solar compass, the wood should be of the 

 lightest kind, as willow or British fir. An agate or ruby cap, 

 and a fine steel point, are requisite to facilitate the traversing 

 of the instrument ; and fine long shaped needles should be 

 used. 



Magnetism is easily communicated to the needles, by two 

 magnetic bars, in the usual way. A paper-full of needles, con- 

 tidning twenty or fifty needles, may be used, as the magne- 

 tism can readily be conveyed to the needles at once through the 

 paper : this can be performed by one holding down the pa- 

 per with the needles, by placing a knife or thin slip of wood up- 

 on the centre of the paper, and drawing the bars along it about 

 twenty or thirty times. If they are rendered sufficiently strong, 

 they will spring asunder as soon as the paper is opened, similar 

 poles being in contact. It is necessary that the needles should 

 be fully impregnated with the magnetic fluid. 



A piece of clear amber, formed into a convex lens, if fixed in- 

 to a circle of cork, and suspended, by any flne hair or filament, 

 under a glass cover, will also be so arrested by the incidence of 

 the solar rays, that it will continue to present its surface to the 

 sun, if unclouded, as long as he is visible above the horizon. 

 And if, in addition to the lens, there is added a few small bars 

 of amber, attached horizontally to the edges of the circle, it ra- 

 ther increases its aptitude to exhibit the solar influence. The 

 amber I exposed to the beams of the sun, in this form, never 

 ceased to vacillate a little the whole of the day, the sun's posi- 

 tion, however, being opposite always to the centre of the arc of 

 vibration. 



The power of the solar compass does not seem diminished, 



* Represented by Fig. 3. PI. I. 



