V8 Description of a Compass [Aug. 



of the boxes, f, opening with hinges for this purpose, and the 

 bottom of the feet, I:, I, have notches to rest upon the pivots ; 

 if the axis does not prove level, one of the Y's must be elevated 

 or depressed, by a screw in the farther pillar, N, until the level 

 stands horizontally, and, reversing it end for end, proves every 

 thing to be correct. 



Now set the vernier, D, at zero, and put an additional object 

 glass, over the glass, P, of the telescope ; and in this state the 

 marks near A A, against which the needle reads, can be seen 

 through it when directed to them ; this proves the zero of the 

 division to be exactly in the plane of the telescope motion ; a 

 small screw near Q will rectify it, if necessary, by moving the 

 Y a small quantity in the box,_/*; the transit may be reversed in 

 its Y's to rectify every thing. 



The telescope is now to be adjusted to the meridian by the 

 transit of the stars in the usual manner, the screws, W, X, being 

 used to turn the whole instrument round when requisite ; but 

 after these observations have been once made in a satisfactory 

 manner, distant marks should be set up, both north and south, 

 and these will give the means of adjusting it at once, and in the 

 day time. 



When the instrument is properly placed, the taking of the 

 observation is extremely simple. The needle being suffered to 

 settle, the box is turned about on its centre till the mark comes 

 very near the point of the needle ; the clamp screw, L, is fhen 

 fastened, and the screw, k, is employed to make the coincidence 

 exact, in which the microscope, M, greatly assists the eye. The 

 vernier, D, now shows the angle of variation. It is proper, 

 after the needle has been once observed, to attract it with a 

 piece of iron, and cause it to make a slight vibration. When it 

 settles again, it will not in all cases come to the same point, 

 because of the friction of the suspending point ; it is therefore 

 advisable to make this trial three or four times, and take the 

 mean of the whole. 



The instrument is placed on a pillar, in the open air, free 

 from any iron, and is adjusted before every observation ; the 

 vernier is divided to half minutes, and if the meridian be not 

 at a considerable distance, an advantage will be gained by dimi- 

 nishing the aperture of the object-glass by a pasteboard cap. I 

 have found the needles should not exceed the weight mentioned^ 

 (iaJ- grains. 



i It may be worth while to remark, that this instrument will 

 serve as a portable transit instrument for naval officers to examine 

 the rate of their chronometers while on shore. 



