353 



I will describe them as foliovvs, viz: — 



1. A sextant of 7^ inches radius, made by Simms, of London, read- 

 ing by aid of the vernier to 10 seconds of arc. 



2. An artificial horizon of quicltsilver. 



3. A sidereal chronometer No. 2557, by Parkinson and Frodsham, 



of London; beats half-seconds. 



4. A mean solar chronometer No. 141, by Isaiah Lukens, of Phila- 

 delphia; beats half-seconds. This chronometer runs eight days 

 without winding. It was made by Mr. Lukens about the year 

 1830 or 1831, while on a visit to London. It is oneof the earliest 

 chronometers, I know of, made by an American. It is now an 

 excellent time-keeper. 



The latitudes, as will be seen, are derived from observed circum- 

 meridian altitudes of stars arranged in pairs, one of each pair passing 

 the meridian to the north and the other to the south of the zenith. 

 When it could be done, they were selected of such declinations as to 

 cause them to pass the meridian at altitudes varying only a few de- 

 grees, say 2° to 3°. But this last mentioned advantage for a close 

 elimination of errors could not always be secured on the occasions 

 here presented. There is, however, an approximate elimination from 

 having one of the stars of each pair to pass the meridian to the north 

 and the other to the south of the zenith. 



The time stars were selected also in pairs, the one being observed 

 eastward and the other westward of the meridian, and conformable, 

 as nearly as was practicable under the circumstances attending each 

 case, to the principle stated in my communication of the 29th ultimo. 



The longitudes are all derived from chronometrical comparisons 

 with the meridian of Chicago. They rest, for accuracy, on the cor- 

 rectness of my determination, in the year 1842, of the longitude of 

 the citadel of Quebec, west of Greenwich, already alluded to in my 

 previous communications, and on the sextant observations for the time 

 at Chicago and at the several places whose longitudes are sought. 



All the comparisons of time with the meridian of Chicago, — now 

 assumed as a primary for my operations in our western country, — 

 were by means of electric signals transmitted forth and back along the 

 telegraphic wires. 



The system adopted was as follows: — The night before visiting a 



place whose geographical position was to be ascertained, observations 



were made for the time at Chicago, with the sextant, the artificial 



horizon of quicksilver, and the sidereal chronometer. Both before 



VOL. VI. — 3 c 



