OP THE UNITED STATES, 847 



series would be obtained, giving again directly the double 

 altitude corresponding to tlie mean of two last times, if the 

 observation is a vertical one on a mercury horizon. 



12. To continue tlie series farther, the next operation will 

 be the movit)g of the alhidade A, or the mirror alone, as in 

 No. 0, and the alternation may be continued as far as de- 

 sired ; the last observation being always one of the same kind 

 as tliat first made, in order to give to both series the same 

 number of angles. 



13. The times belonging to the first series A ^' B will be 

 all the times observed, except t!ie last; and the times of the 

 scries C ^ D, all the times except the first, as in tlie repeating 

 circle with two telescopes, and the calculation of each series 

 will be separate. 



If the angle observed should increase during the observa- 

 tion, as in observing time for instance, it will be proper to 

 attend occasionally to the stops, that they may not be too 

 near, so as to occasion them to be touclied by the alhidade, 

 and disturb the readings. In observing the sun, these stops 

 are best removed to the end of tlie circle. 



In keeping fast the clamp of the alhidade C, which holds 

 the circle, and moving only the mirror, the instrument will 

 perform exactly the same functions as Mr. Troughton's cir- 

 cle, by the single cross observation ; and in determining the 

 point of parallelism of the mirrors, the same observations 

 may be made as with a sextant ; but in these the instrument 

 loses not only its peculiar advantages, but even would not 

 serve so well as a sextant, which is lighter, and its parts [)ur- 

 poscly calculated for solidity in this kind of observations. 



If in terrestrial angles, the two objects observed sliould be 

 equally well illuminated, so that the equalisation of light 

 could be made constant, the alternation of the angles can 

 be effected without inverting the instrument, by changing al- 

 ternately the object to be viewed directly ; i)y whicli tlic other 

 will be brought to alternate equally with the position of the 

 mirror, to receive the reflection ; and the inconvenient posi 

 tion of the hand, or the change of handles, is avoided. 



