OP THE UNITED STATES. 341 



tions given for these apply equally to the series of double 

 altitudes. An observer, with a little experience, will be able 

 to make such an observation without fiirtlier direction. All 

 that is desirable for it is a large mercury horizon, in order 

 to have no need of moving it during the observation. 



It is easy to adapt to this instrument a stopping arrange- 

 ment for fmding stars by night. These may be very ligiit, 

 and re moveable, when not needed. On this occasion, I 

 may remark that it is proper to make the touches of these 

 stops liglit springs, and not solid purtjt, that in case they 

 sliould come to touch, the telescope or other part of the in- 

 strument stopped, may not be atlected by it. 



Description of the Repeating Circle of Reflection. 



The application of the principle of reflection from plane 

 n)irrors has produced tlie instiument whicli has most con- 

 tributed to the advancement of nautical astronomy and geo- 

 graphy. 



When tlie mirrors are perfect, the accuracy which maybe 

 obtained in the measurement of vertical angles observed by 

 means of the mercurial horizon, is certainly far superior to 

 that from any other instrument of equal size, in which the 

 level or plumb line is used, the circumstances in all other 

 respects being the same. 



The use of a circle, instead of a sextant or octant, intro- 

 duced by Tobias Meyer, has in this, as in all other instru- 

 ments, freed the results from the influence of eccentricity ; 

 and the improvements of it hy Horda have furnished the 

 means of correcting t|je errors of the glasses and adding the 

 property of repetition. 



In multiplying instruments, the constant parallelism of the 



VOL. II. — \ s 



