INSTRUMENT FOR MEASURING MAGNETIC ATTRACTIONS, &C. 245 



is to be determined. These bars are readily fixed to the plate, 

 by being slipped through a circular aperture in the end of a 

 spring 6, which, perforating the moveable plate, and acting 

 downward, firmly embraces any substance laid along the 

 groove. The angular position of the moveable plate is mark- 

 ed by a circle FF, screwed upon the side of the table. It is 

 graduated so as to mark the angle between the moveable plate 

 and the horizontal plane, whether above or below it. To in- 

 sure accuracy in this angle, the true horizontal position of the 

 table, and with it the horizontal line on the circle, is determin- 

 ed by means of A spirit-level G ; and that the movements of 

 the plane may be accurate, and the angle marked true, the pin 

 which passes through the hinges also forms the centre of the 

 wheel D, and terminates exactly in the centre of the graduated 

 circle F. H is a moveable flat plate of brass, divided into 

 rhumbs and degrees : it is furnished with a magnetic needle, 

 having an agate cap traversing on a brass or steel point. The 

 needle can be changed according to the nature of the circum- 

 stances J a very light and strongly magnetized one being used 

 in delicate experiments. The compass or plate carrying the 

 needle, being moveable, its distance from the bar resting on 

 the limb C, can be varied at pleasure. The centre of the 

 hinges is one-tenth of an inch above the level of the table ; 

 the magnetized needle stands at the same elevation ; and the • 

 bars in use being one-fourth of an inch diameter, are sunk in ' 

 the groove of the moveable plate to such a depth, that their 

 axis, or centre, precisely corresponds with the centre of the 

 hinges ; hence the middle of the extremity of each bar is at the 

 same elevation, and at the same distance from the needle in 

 every position of the moveable limb. To give firmness to the 

 instrument in making experiments, the table is fixed by the 

 feet to a mass of lead I, of seven or eight pounds weight* By 



H h 2 means 



