446 Mr. Barloio on the Magnetii Effects induced in Iro7i [June, 



" A B C D is a strong wooden frame, resembling that of a 

 common electrical machine, the shell S being hung in the same 

 manner as the cylinder ; the axis is made in two parts of gun 

 metal, and very strong ; s s are two strong screw bolts and nuts, 

 which were used for fixing the frame firmly to the top of the 

 table, the bolt passing through from below. EGF is a substan- 

 tial table with its feet sunk into the ground, and the floor of the 

 room cut away where they passed through, in order to prevent 

 any effect of shaking on the stand carrying the compass. 



" The stand consisted of an upright pedestal filled with sand, 

 to render it steady, and to this was fixed the table M L, with a 

 semicircular hole cut in it, so that it might be placed near the 

 shell. This table might be elevated or depressed at pleasure, 

 and it was divided into the points, quarter-points, &c. of the 

 compass. 



" By means of different holes bored in the top of the table, 

 the machine might be placed N and S, E and W, &c, at pleasure, 

 and the motion of the shell be inverted by turning the handle to 

 the right or left. The large wheel is six times the diameter of 

 the small one ; and as it might easily be turned twice in a 

 second, the number of revolutions of the shell were gradually 

 about 720 per minute. The little apparatus seen above the shell 

 is a small stand and shding wire, carrying a common lamp-glass, 

 in which a very small dipping-needle was suspended by silk ; 

 and when the lamp-glass was out of the ring, the latter served 

 for setting the horizontal needle on, so as to bring it over any 

 required point of the shell. It should be observed that the 

 pedestal was moveable, and might therefore be placed on either 

 side of the machine. The stand and upright figure 2, is one of 

 two large magnets ultimately employed for neutralizing the 

 needle. 



"The machine being thus prepared, I screwed it down ; first 

 with its axis in the magnetic meridian, and then placed the 

 compass successively at the several points on the table all round, 

 and registered the deviation produced at each, with the motion 

 of the shell direct and reversed. I then removed it, and placed 

 the axis east and west, and again registered in the same manner; 

 but the results were very irregular with respect to quantity. 

 Although 1 obtained some unilbrmity regarding direction onlif, 

 viz. in both cases I found four points of change at about 30° 

 from each extremity of the axis, or four points of non-action. 

 For example, when the axis was in the meridian from N30° Eto 

 N 30° W, the motion of the needle arising from the rotation 

 was made to the right. From N 30° W to S 30° W to the left. 

 From S 30° W to S 30° E to the right. From S 30° E to N 

 30° E to the left ; the direction of motion in the shell being the 

 same; with the direction of motion reversed, the deviation was 

 reversed also. While at these four points themselves, the needle 



