Hijdro-Electrical Currents. 



93 



extremities of each of its arms there is fixed a star, each star composed 

 of an equal number of sewing-needles, about 26 ; these needles are mag- 

 netized to saturation. The arrangement is made to float on a fine steel 

 point, by which the agate centre is supported, and the whole covered by 

 a thin glass shade to protect it from currents of air. A slow rotation 

 of the stars round their centre of support, is produced by the rays of the 

 bright sun, which continues so long as the air surrounding the needle 

 receives heat from them. The needles in the stars are in planes vertical 

 to the plane of the horizon. On turning them through 90°, so as to be 

 parallel to it, the rotation entirely ceases. 



Two slips of watch-spring, 3 inches long, of the same weight, and 

 magnetized equally, were fixed on the ends of a light piece of wood with 

 agate centre, similar to that used for Mr Watt's sun needle. 

 A A. A light horizontal needle of Fig. 5. 



wood. 

 B. Agate centre. 

 C C. Pieces of metal of equal 

 weight. 

 D. Support with fine steel point. 

 EE. Stand. Weight of needle and 

 agate 5 grains, exclusive 

 of the pieces of metal C C. 

 The planes of C G are inclined 

 about 5° to the vertical plane, the 

 one dipping to the right side of the 

 arms A A, the other to the left, so 

 as to balance on B the centre. 



Under a thin glass shade exposed to the solar rays, these two pieces 

 of watch-spring rotated as well as Mr Watt's needle ; and as this took 

 place without reference tO' whether the poles pointing to the earth 

 were like or unlike, I was led to infer that magnetism had nothing to do 

 with the rotation: which was soon confirmed by changing the magnets 

 for pieces of unmagnetired watch-spring ; then for slips of copper ; and 

 next for pieces of bismuth, the most imperfect conductor of heat among 

 the common metals ; with all of these the rotations were easily obtained. 

 To succeed with magnetised pieces of steel is more uncertain, for if the 

 arrangement is not very nearly astatic, the needle will only oscillate to 

 and from its point of repose, or change its position in azimuth as the sun 

 passes through tlie sky. 



The sun, in this country, is not powerful enough to produce rotations 

 when inferior conductors of heat are substituted for the metallic slips CC; 

 wood, cork, charcoal, were tried in June without success. As these 

 organic substances exposed as much surface to the action of currents, 

 which the sun's rays may be expected to give rise to within the shade, 

 their remaining at rest shews, that the other rotations with copper or 

 bismuth were not produced by those currents. 



