182 Prof. Encke 0}i Hadley's ScxtatU. 



sextant it is only necessary to know it to the nearest mi- 

 nute, which may be easily attained by a common level, or 

 some other method. In fact, the equally high sight vanes, 

 or the trial telescope which has been proposed for this pur- 

 pose, come under the description of levels. Besides the level, 

 all that is absolutely required will be a detached telescope fur- 

 nished with cross wires, whose magnifying power need not 

 be greater than that of the telescope of the sextant, but which 

 ouglit to have a large apei'ture; a sweeper will best answer 

 this purpose. The telescopes of sextants have commonly two 

 wires, between which the contact is to be observed. For a 

 more accurate determination of the line of collimation, cross 

 wires are besides placed nearly in the middle between the two 

 former wires. One may at first ascertain the distance of each 

 wire from the intersection of the cross vvires. For this pur- 

 pose they are to be placed, as nearly as the eye can judge, per- 

 l)endicular to the plane of the sextant; then the direct image 

 of a clear and distinct terrestrial object is to be placed in the 

 intersection of the cross wires, while the image of the same 

 object by double reflexion is bisected by one of the lateral 

 wires. Let the angle read off after this operation be called s, 

 which is to be taken as negative if it is on the arc of excess. 

 The two images are next made to change places, so that the 

 direct image is now on the lateral wire, and the doubly re- 

 flected one on the intersection of the cross wires, and let the 

 angle then be = s'. It will appear from fig. 1 . (See above, p. 85.) 

 that under these circumstances, calling the distance of the la- 

 teral wire m (positive if to the right of the cross wire), we have 



s — Co = WJ — ^ sin 2/3 and 5^—0^= — ?«— ^ sin 2 (|S — w) 



whence m = \{s—s') + =j- sin m . cos (2j3—7n) 



Co = H^ + *') + "4" cos 7)1 . sin (2 /3 — ?«) 



The sign of m will without any uncertainty decide the po- 

 sition of the lateral wire, if, agreeably to the rule constantly to 

 be observed, we consider as negative such s's as fall on the arc 

 of excess. These determinations serve for having in the field 

 of vision an estimate of the errors still remaining. 



Let the wires now be placed parallel to the plane of the 

 sextant, and let this plane be put into an exactly horizontal 

 position while the telescope points to a clear terrestrial object 

 liaving some distinct points on it. It will now be convenient 

 to take away the small mirror which is in the way of seeing 

 the object, as also the coloured screens, if they should be found 

 to interfere with this operation. 



The 



