1826.] arising from it$ Rotation. 29 



B ^, a short hollow cylinder slides freely, having a circular rim 

 raised '6 inch from it, to support the iron plate C c at right 

 angles to the axis of the cylinder. Over the plate of iron is a 

 wooden washer D d, which is pressed on it by the screw h work-, 

 ing on the short cylinder. The cyhnder with the plate is fixed 

 in any position on the arm by the clamp M m. In the part A a 

 of the arm are two openings o, o\ on the chamfered edges of 

 which are indexes in a line with the axis of the cylinder B by so 

 that when each points to the same arc on the semicircles S^E N, 

 5 (c n, the axis of the cylinder B ^ is directed towards their 

 centre, and every point in the edge of the plate is at the same 

 distance from that centre. As the weight of the plate was a 

 considerable strain on the instrument, a scale to contain a coun- 

 ter-weight was suspended from the ceiling of the room, and the 

 line from it passed through a moveable pulley, attached to the 

 arm B h, so that the weight might easily be adjusted to relieve 

 nearly altogether the strain of the plate on the arm in any position. 

 The arm was also occasionally supported, and kept steady in its 

 position, by a sliding rod resting on the table T t. The compass 

 consists of a circular box, containing a circle 6 inches in diameter, 

 very accurately divided into degrees, and again into thirds of a 

 degree ; and a very light needle, having an agate in its centre, 

 and its point of suspension only '07 inch above the surface of 

 the needle. The extremities of the needle are brought to very 

 fine points; so that by a little practice, with the assistance of a 

 convex lens, I could read off the deviations very correctly to two 



minutes, being the tenth of the divisions on the circle 



In the experiments which I had previously made, and in those 

 which I proposed making with this apparatus, I conceived a 

 sphere to be described about the centre of the needle, referring 

 the situation of the iron to a plane, in which, according to the 

 hypothesis I had adopted, it should equally affect the north and 

 south ends of the needle. The hne in which the needle would 

 place itself, if freely suspended by its centre of gravity, I consi- 

 dered as the magnetic axis ; the points where this axis cuts the 

 sphere, the poles, the upper being the south, and the lower the 

 north pole ; and the great circle at right angles to the axis, the 

 equator, being the plane above mentioned. The position of 

 the iron was thus determined by its latitude and longitude ; the 

 longitude being always measured from the eastern intersection 

 of the equator with the horizon. The angle which the axis 

 makes with the horizon I considered to be, according to the 

 most accurate observations, very nearly 70° 30'." 



" After making a very few sets of experiments with this instru- 

 ment, I found that it was necessary to attend very particularly 

 to the situation of certain points on the iron plate with respect 

 to the Hmb, since, with one point coinciding with it, the devia- 

 tion of the needle, when the centre of the plate was on the raeri- 



