THE CORRECTION OF SEXTANTS FOR ERRORS 



OF 



ECCENTRICITY AND GRADUATION. 



BY JOSEPH A. ROGERS. 



The sextant of reflection, in consequence of its portability and the ease 

 and rapidity whith which it affords results of considerable accuracy in 

 the hands of a capable observer, is so serviceable in navigation, survey- 

 ing, and in determining latitude and time for astronomical purposes, that 

 no reasonable proposition to increase the precision of observations made 

 with an instrument of such general usefulness can be devoid of interest. 

 One of the most obvious sources of avoidable error is a non-coincidence of 

 the axis of rotation of the index-bar with the axis of the graduated arc. 

 The necessary lightness of all the parts also entails a certain liability to 

 irregularities in the rotating motion, due to flexure of the slender axis 

 and its connections, or to wabbling when the axis is not properly supported 

 at both ends. With good workmanship, and proper attention to the con- 

 dition of the instrument when used, these irregularities may be kept within 

 narrow limits, but there is reason to believe that they are sometimes greater 

 than is commonly suspected. When the arc is extended into a complete 

 circle with opposite indices, the effect of eccentricity is eliminated, and 

 that of irregular rotation is much diminished, if not entirely compensated. 

 These advantages of the circle have been frequently urged, and are in- 

 disputable, but they cannot be secured without some sacrifice of other de- 

 sirable qualities. An instrument which is designed to be supported in 

 the hand when used, must be light and compact ; with equal bulk and 

 weight the limb of the sextant has a greater radius than that of the circle, 

 and consequently permits a closer reading of the angles measured, while 

 the uncompensated effects of irregular rotation in the former may be les- 

 sened by a somewhat more massive and substantial construction of the 

 axis and parts supporting it ; moreover, the sextant is the less costly of 

 the two, and is, perhaps, in shape, rather better adapted to convenience in 

 manipulation. The settlement of these conflicting claims by experience 

 has been upon the whole unfavorable to the circle, for its employment is 



