CHAP. I.] SEXTANT STAND 13 



extremely difficult to hold the instrument in the hand firmly 

 enough to prevent a little vibration of the images. 



Sextant stands should be lacquered, not bright, and should 

 have large heads to the foot screws, so as to be grasped easily 

 while observing. 



The bearing which carries the sextant should be accurately 

 fitted into the socket in the handle, and should be very sHghtly 

 conical. If too much so, it is liable to jam. 



The counterbalances are usually too heavy for an ordinary 

 sextant. They should be of such a weight as to balance the 

 sextant without the screws at the ends of the pivot being set 

 up too taut. Sometimes one weight is enough, or as much 

 lead can be taken out of each as is necessary to reduce the 

 weights to balance. The weights are now sometimes fitted 

 to slide in and out, thus allowing of adjustment. 



There is a great advantage in having the bearing which improved 

 carries the sextant cut square. The circular motion in this 1^^^^^^' 

 form of stand is given by means of a large disc which is con- 

 trolled by a clamp and tangent screw with a very coarse thread. 

 The tangent screw enables the sextant to be kept pointed 

 accurately to the Artificial Horizon as the sun or star changes 

 its altitude, without the necessity of actually touching the 

 sextant or moving it on its bearing by hand. This arrange- 

 ment avoids the liability to jam, and the consequent exertion 

 of force which may cause the sextant to move with a jerk, and 

 throw out of the field the sun or star, thus disturbing the 

 tranquillity of the observer and possibly losing one or two 

 observations in consequence. Sextant stands of this pattern 

 are made by Messrs. Gary and Porter. 



The threads of the foot screws of the sextant stand should 

 be of fine pitch. 



A small level may advantageously be fitted to the arm of 

 the sextant stand, so adjusted that when the arm is horizontal, 

 and consequently the plane of the sextant vertical, the bubble 

 is in the centre of its run. This adjustment is easily 

 made whilst observing the altitude of the sun in artificial 

 horizon, and making the suns cover during the process. 

 This level, used in conjunction with the other level fixed on 

 the Index bar of the sextant, ensures a star whose altitude 

 is known, being found in the field of the telescope, when 



