340 SUUVHY OF Tilt COAST 



same manner as with the repeating circle, by a succession of 

 the above alternating motions, until the desired series is ob- 

 tained, ending at last witli the same motion with which the 

 series was begun, as here by a pointing with the tangent 

 screw at X. 



7. All the times of such a series, except the last, will be- 

 long to the series of Z and Y, and all times, excepting the 

 first, to the series of the reading, W, X. 



If several series of the same zenith distance of a terrestrial 

 object should be desired, for the sake of great accuracy, it 

 would be advisable to take these series in the two different 

 manners which the instrument admits, and to use the indis- 

 criminate mean in preference to either. 



The manner of taking out the results of such a series of 

 observations, and of calculating them, is of course exactly 

 the same as in the repeating circle with two telescopes ; and 

 the mode of registering the observations and the results in 

 the day-books and journals may therefore be omitted here, 

 as sufficiently obvious. 



It is evident that the method of observing time described 

 in the repeating circle, applies also to this instrument with 

 equal advantage and ease. 



Besides this, the ease with which the light vertical circle, 

 or the telescope alone, moves upon the transit axis, affords, 

 in the present case, another method, — viz. to measure dou- 

 ble altitudes by reflection on a mercury horizon, the level 

 showing the stability of the instrument, which for this kind 

 of observation needs no inversion or movement in azimuth, 

 except for the small progress of the celestial body during the 

 observation. 



By alternating the motions of the transit, with and witliout 

 the circle, between the direct object and its image in the 

 mercury horizon, a series of observations of double altitudes 

 may be obtained with great celerity ; and if the sun should 

 be the object, the limbs may be alternated, as in another ob- 

 servation. The times of the two series will follow exactly 

 as in the series for the zenith distances ; and all other diree- 



