134 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



gles, being applicable to geodesical as well as astronomical 

 purposes. In the hands of the surveyor, it becomes a the- 

 odolite of rather large dimensions, measuring with great 

 accuracy both vertical and horizontal angles. It does not 

 ^possess the power of repetition ; but the effect of any error 

 of division on the azimuthal circle, may be reduced or de- 

 stroyed, by measuring the same angle upon different parts 

 of the arc ; thus, after each observation, turn the whole in- 

 strument a small quantity on its stand, and adjusting it, 

 again measure the required angle. A fresh set of divisions 

 is thus brought into use at every observation, and the same 

 operation being repeated many times, where great accuracy 

 is required, the mean result may be considered as free from 

 any error that may exist in the graduation. A repeating 

 stand has, of late years, been frequently added to this in- 

 strument, and is a most powerful and convenient appendage, 

 when great accuracy is required in the measurement of 

 azimuthal angles. The two opposite micrometers being read 

 off at each observation, will always remove the effect of any 

 error in the centring. The vertical angles shou]/ 1 in all 

 cases, be taken twice, reversing the instrument before taking 

 the second observation, when, as before observed, one of the 

 readings will be an altitude, and the other a zenith distance ; 

 the sum of the two readings, therefore, if the observation be 

 made with accuracy, and no error exists in the adjustments 

 of the instrument, will be exactly 90 ; and whatever the 

 sum differs from this quantity is double the error of the in- 

 strument in altitude, and half this double error is the correc- 

 tion to be applied -f- or to either of the separate observa- 

 tions, to obtain the true altitude or zenith distance, + when 

 the sum of the two readings is less than 90, and when 

 greater. 



In applying the instrument to astronomical purposes, it 

 was formerly the custom to clamp it in the direction of the 

 meridian, and after taking an observation, or series of ob- 

 servations, with the face of the instrument one way, to wait 

 till the next night, or till opportunity permitted, and then 

 take a corresponding series of observations of the same 

 objects, with the face of the instrument in a reversed posi- 

 tion. But this method being attended both with uncer- 

 tainty and inconvenience, it is now usual to complete at once 

 the set of observations, by taking the altitudes in both posi- 

 tions of the instrument as soon as possible after each other. 



