Mr. Tatum on Electro-Magnetism. 243 



of the dissimilar effects produced on the opposite ends of the 

 needle when applied to the electro-magnetic apparatus. I also 

 observed, that I considered a similar difference was produced 

 by the wire connected with the copper end of a Voltaic ap- 

 paratus, to what was produced by that which was united with 

 the zinc end. I have since thought it might not be amiss to 

 introduce a simple figure of the apparatus by which I ascer- 

 tained these opposite effects. 



Let ABCDEFGH (fig. 13) represent the wire connecting 

 the two poles of the apparatus ; the part A connected with the 

 copper, and the part H with the zinc : — this is not one con- 

 tinued wire, for I wished to have the opportunity of rotating 

 the perpendicular part E F, which is represented with a small 

 cup at E, and terminating in a point F, which dips into an- 

 other small cup, both of which are filled with mercury, which 

 forms a complete connexion. While the apparatus is thus 

 situated, it will be found that the very opposite effects are pro- 

 duced from H G F to E, to what are produced from ABC 

 to D : as for instance, if one end of a needle be brought under 

 the horizontal part of the wire C D, it will be attracted; but 

 if brought under the horizontal part G H, it will be repelled : 

 and if brought to the right side of the perpendicular part A B, 

 it will be attracted; but if removed to the right side of the 

 jjerpendicular part E F, it will be repelled. If the equator 

 needle be carried nearer to the wire, the reverse of the above 

 effects is produced, as was shown in my former paper. From 

 which I deduce, that a contrary circulation of the fluid is 

 produced by the positive side of the apparatus, to what is 

 exhibited by the negative side. 



I scarcely need observe, that these effects will be applied by 

 every one to the rotatory copper and zinc cylinders of Ampere, 

 in which it is well known that the copper rotates in an oppo- 

 site direction to that of the zinc. J. T. 



P.S. — I hope in the course of a few weeks to have a little 

 more leisure, when perhaps I shall be enabled to resume this 

 interesting subject. 



LIII. True apparent Right Ascension of Dr. Maskelyne's 

 36 Stars for evei-jj Day in the Year 1 823, at the Time of 

 passitig the Mcridia7i oj Grcewmch. 



[Continued from page 199.] 



N. B. On tliosc (lays where an Asterisk is prefixed tiic Star passes twice; 

 the A\ given is that at tiie first passage. 



1823. 



