REMARKS ON FIELD OBSERVATIONS.—DEPARTURE FROM ALBUQUERQUE. 
57 
appear coincident, tlie signal “time” will be given to the recorder, who will he on the watch, 
and notice the instant required. Opposite to this record of the chronometer time, should he 
noted the double altitude or reading of the sextant. Fifteen such observations upon a star 
north, and a like number upon a star south, of nearly the same altitude, both taken within ten 
minutes of the meridian, should give the latitude within 10" of arc. Another set of stars 
nearly equal in altitude, one east and the other west, will give a result for time; and hence, 
knowing the error and rate of chronometer at any other station, we obtain the difference of 
longitudes. The greater the number of pairs of stars observed upon, the more accurate will he 
the result. Index error should he observed each day or night. The diameter of the sun may 
he measured by bringing its reflected image tangent first upon one side, then upon the other; 
and half the difference of the readings will give the index error. Or the reflected image of a 
star may be brought in contact with that seen through the telescope; and the zero of the ver¬ 
nier will then indicate the point which should he the zero of the arc. Hence, the reading will 
denote the index error, to he added to the altitudes observed when the reading is off the arc, 
and vice versa. 
If observations he made for latitude upon one star north, and upon another south of the 
zenith, of very nearly equal altitudes, and those observations, corrected for index error, he com¬ 
puted, the difference in the results should he due to eccentricity of the instrument. One half 
this difference may he adopted as the eccentricity for that angle, and may be applied to future 
observations. 
GENERAL REMARKS. 
Polaris, being near the pole, may he observed for latitude at any hour of the night when visible. 
All instruments should be handled lightly and delicately. Ho clamp screws should he tightly 
pressed. Every part of an instrument not absolutely necessary should he dispensed with; as, 
for instance, clamps on the vertical circle of a meridian transit, or bars to hold its axis in posi¬ 
tion. Sufficient care in the manipulation supersedes the necessity of such appliances. 
Lists of occupations will he frequently examined ; and if the moon is above the horizon at 
the time when the occurrence of an occupation is predicted, the large telescope, four feet focal 
length, will he mounted, and the moment of the disappearance of the star behind the moon’s 
limb will be noticed. During the moon’s first quarter, and until full moon, this occupation 
will, take place behind the dark limb. Afterwards the star will be occulted by the bright limb, 
and the time of emersion from the dark side should then be marked. 
The error of chronometer being determined at one station, and its rate approximately known, 
for the succeeding date and station apply the rate, and the estimated or measured distance due 
east or west, reckoning plus or minus four seconds of time to the mile. In this latitude, this will 
generally give the error of chronometer with sufficient accuracy to set the transit in the meri¬ 
dian by a circumpolar star. 
Formuhe and methods of computing the results of the foregoing observations may be found 
in the “Tables” prepared by Capt. Thomas J. Lee, Topographical Engineers, for the use of 
the corps. This admirable and comprehensive work contains almost all that is necessary for 
field and office computations, and is now an indispensable companion of the topographical 
engineer. 
November 8.—Our party has to-day been divided; one portion of it continuing the survey. 
It is conducted by Lieutenant Ives; Mr. Albert Campbell and Dr. Ivennerly accompanying him. 
They will proceed by the way of Isleta, so as to bring that point into the main survey, and 
thence, passing the ridge westward, determine the practicability of crossing the valley of Kio 
Puerco in that direction. 
November 10.—Taking leave of our hospitable friends at Albuquerque, with the rear division 
of the party we recommenced the journey westward. Fording the river, we passed through 
the village of Atrisco, and encamped in the valley at a rancho four miles from Albuquerque. 
8 h 
