OF THE UNITED STATES. ^23 



cle, and read its four verniers D, E. F, G, takiria; a mean be- 



tween the readings within and without llieii' nominal, wlioii 

 any difference appears ; or I read tiicni in their accidental 

 position, which may be that of the last preceding obser- 

 vation. 



'riicse readings are written in the third, fourth. Hftli, and 

 sixth columns of the Day-Book. Iti the second colunm the 

 lirst letter of the series of readings is written, and the first 

 column is left for the times. 



Ill a night observation, the stops e. e, are placed to their 

 j)ropei zenith distance, allowing some free space for the quar- 

 ter zenitli distance of the star out of the meridian, and to 

 avoid touching in pointing the telescope. The alhidade with 

 the levels is moved so as to bring them near the projecting 

 g of the front telescope, taking care to avoid actual contact, 

 'i'hc circle is now turned so as to bring the level into an ho- 

 rizontal position. 



By the horizontal revolution of the circle, the star will 

 now api)ear in the field of the telescope, and when found, 

 the horizontal stop 7', fitting to the side on which the circle 

 is, is placed in contact with the alliidade A. 



Tht>n the observer will make tiie accurate pointing u|)on 

 the star or ol)ject, by the motion of tlic screw m, which guides 

 the whole circle, while the assistant observer will adjust the 

 level l)y its proper screw at the arm h. When l)oth these 

 are riglit together, the time is marked in seconds and deci- 

 mals from a time piece l)y a second assistant, acting as a se- 

 cretary. The time is written in the first column of the Day- 

 Book Iicfore the readings of tlie front telescope, and forms 

 the first tinie for the scries of atigles of the front telescope. 



By the |)lacing of the level, the verniers \V. X. Y, Zcome 

 into their first position for the series of the level or back te- 

 lescope. These four verniers arc therefore now read off. 

 and their readings written below the others in the same or- 

 der, l)cing marked in the second cohmm l)y the letter W. 



The circle being now brouirht on the other side of the co- 

 lumn bv a half revolution on the vertical axis, the observer 



