OF TIIU UNITED STATES. 381 



as to receive its reflection from the artificial horizon, whicli 

 is now placed in the situation indicated by the telescope, and 

 sheltered from vacillation, if necessary, from the wind, by 

 a glass cap. 



If the motion of the telescope is exactly vertical, the re- 

 flected star in the mercury will coincide with the same ver- 

 tical wire wliich was jiointed njion the star viewed dii'cctly : 

 if not, it must he corrected, half by movin>>; the plane of tlic 

 circle round upon the axis q, afrer unclampint!; the screws 

 /, /, and half by the tangent screw on the horizontal circle. 



If the circle is moved in tlie vertical, with the telescope 

 clumped and every \\\\n\x, arrans;efl as in the al)0ve oI)serva- 

 tion, the veiticality of its motion will be ascertained. If there- 

 fore there is any doul)t respectina; the parallelism of the circle 

 and tlie telescope, it will be I)est to begin first by making the 

 observation witli tlic circle, the telescope being clam|)cd, and 

 when this is adjusted, to adjust the wires accordingly. 



When these adjustments are made, and before any change 

 in the level of the instrument takes place, tlic level s on the 

 axis must I)e adjusted, and it will then serve to verify the vcr- 

 ticality of the circle, as long as this circle is ke|)t in its place. 

 1 therefore had the packing boxes so constructed, that the 

 instrument miglit be removed witliout taking it apart, as I 

 observed tliat the separate packing of the circle with tlic 

 great counter[joise subjected the instrument to injury. 



I think that some meclianical arrangeinent mizht be easily 

 a?id advantageously adopted, for tlie |)ur|)osc of giving to this 

 adjustment a still greater dcgn e of accuracy. 



The illumination of the readings must be carefully attend- 

 ed to, as at night tlv y are very dilTicuIt. and thereby become 

 uncertain. I applied here also, with the best success, paper 

 reflectors ; folding a quarto sheet exactly as dcscrilx-d for 

 the two-feet theodolite ; but instead of giviug it a foUl to 

 bend it at right angles, I cut from about threc-f|Uarters of 

 an inch behind the circular hole, which is here mad(r so as 

 to fit the tube of \\w magnifier, a slit to admit the arm of 

 this magnifier, about two inclics and a half loriir. and about 



