OF ECCENTRICITY AND GRADUATION. 6 



The apparatus constructed at the Hyclrographic Office is represented 

 by Fig. 1, in which ^4 is a stout walnut plank 4} feet long, 2 feet wide, 

 and nearly 2 inches thick, supporting the other parts, and mounted upon 

 legs at a convenient height above the floor. The horizontal collimator B, 

 having an aperture of 2 inches, and a focal length of 19 inches, is 

 firmly secured to the base-board at an elevation nearly equal to that of 

 the telescope of a sextant in the position it occupies while undergoing 

 examination. In the focal plane is a thin metallic i)Iate with a minute 

 circular hole drilled through it, which appears as a sharply defined bright 

 disc a little less than 2' in diameter, when illuminated from behind and 



J-i^r.J. 



viewed with a telescope. At C is a graduated circle revolving upon a 

 vertical axis, and supported by a tripod base after the fashion of a the- 

 odolite. One foot of the tripod, resting on its foot-screw, is directed toward 

 the collimator as shown at D. The axis of the circle must be kept per- 

 pendicular to the line of collimation ; this adjustment is made by means 

 of the foot-screw D, and has usually' been tested by placing an alidade 

 with plane sights on the face of the cii'cle, as extreme precision is not es- 

 sential ; but a reversible collimator, having collars of equal size supported 

 by a stand laid on the table, would be a better device. The circle is 

 divided to 5', and reads to 3" by four equidistant verniers ; its diameter 

 is about 11} inches. Three arms radiate from the top of the vertical 

 axis and bear leveling screws at their extremities supporting the brass 

 table E, which has adjustable clamps upon its upper surface to 

 receive the legs of a sextant, and prevent them from movi :g in any 

 direction. A small circular mark in the center of the table indi- 

 cates the location of the vertical axis. A sextant, F, laid upon the 

 table, with the axes of the graduated arc and circle approximately 

 coincident, can be secured in that position by the clamps, and the 



