OF THE UNITED STATES. 305 



the others are completely indispensable for obtaining a sa- 

 tisl'actoiy result 



In ot)serving the celestial body, its transit is observed on 

 all the three vertical wires of tiie telescope ; and when the 

 sun is ol)served, which is the most advantageous of all for 

 aziniutli determinations, the contacts of both liml»s are taken. 

 By tliis mode of ob-erving, which is very easy, and even very 

 pleasant, with tliis instrument, the most essential clement of 

 tlic azimuths, the time, is observed six times (with the sun) 

 for each ol)servati()n, which is of course a great advania2;c as 

 to accuracy. I should even have wished that the telescope 

 had five vertical wires instead of three. 



The following is tlie order which I followed in such an 

 observation : — 



Having carefully verified all the adjustments of the instru- 

 ment, and the telescope being in the position which I have 

 called direct, I observed first the terrestrial object with which 

 the azimuth was to be taken, choosing for it the signal wliich 

 I»y its illumination at the time appeared the most distinct, 

 and in case of equality in tliis respect, that ?iearest to the 

 sun, and read oft' the three microscopes. Then I tiu'ned 

 the telescope horizontally, so as to receive the transit of both 

 limljs of the sun at all the three vertical wires, and having 

 clamped the telescope in this position. 1 ol)served these tran- 

 sits, the time of each being carefully oliserved by a chrono- 

 meter or clock to seconds and decimals, and written in the 

 day-book by the secretary. After this I read the three mi- 

 croscopes on the circle. 



Then I placed the telescope immediately in tlic reversed 

 position, and ol)servcd the transit of the sun again as before, 

 and after it the same terrestrial ol>ject again. 



By this method the two azimuths are oliscrved tlic near- 

 est possible to each other, and the terrestrial objects observ- 

 ed, correspond to each position of the transit. 



If the circumstances allow one more azimuth to l)e taken, 

 without however admittini? the chan2;e of the irislrument 

 upon its legs, and an assistant be at hand (o determine the 



