4 BAROMETRIC OBSERVATIONS. 
os 8. 
1857, Dec. 27. Transit € L, longitude of i orae | eee cee, 7 39 19.9 
Oe ire ec ce Ce RE eee ee eis ce eee caer 7 39 20.6 
mT. oho ee eS Gas a bee dee ek od 2 8k oS Boe Kine ws 120 2a 
27. Immersion of Anon. *, No. 4 of Bessel... ---+-- +++. esse eens 1. 30: Jack 
Adopted longitude of Camp 1 «+--+ eee ee cece ee cece eee T 39 25.0 
1858, Mar. 24. Transit ( L, longitude of Camp 61 .....- +++. esse ee eee eee T 8T 06.4 
eee ace Reese Ge ee eee eee ee a a 
Adopted longitude of Camp 61 «+--+. eee ees cee eee eeeee tas. 300 
The determination for Camp 1 must be very exact. The situation of the stars in the moon’s 
path was favorable ; the places of the stars are well known ; and the corrections of the lunar 
ephemeris in right ascension and declination were derived from our observations at Cambridge. 
I presume that as yet the position of few stations on the west coast has been better determined. 
Besides the above, the following has been used as a zero point for the chronometric longitudes: 
amp 12. Longitude, Th. 38m. 21s., 
assuming the junction of Gila and Colorado to be in longitude Th. 38m. 25s. 
Before concluding I would again call your attention to the importance of the transit obser- 
vations. Their value and precision you seem to have greatly underrated, to judge from the 
remarks in your letter of October 21. 
Respectfully yours, 
* % 
G. P. BOND. 
Lieutenant J. C. Ives 
United States Topographical Engineers. 
*. 
2. REMARKS UPON THE BAROMETRIC OBSERVATIONS. 
The barometric observations were computed in pursuance of the system described by Lieu- 
tenant Abbot, topographical engineers, in Vol. VI, Pacific Railroad Reports. 
These readings of the barometers were first reduced to what they would have been at 32° 
Fahrenheit, and then corrected for instrumental error. The series of hourly observations 
taken at Camp 1, after being corrected as above, were used to form a table for the correction 
of the horary oscillation. The observations for each day were plotted in a curve, and by these 
curves one or two manifest errors of observation were corrected. The mean of all the obser- 
vations for each hour was then found, thus forming a mean day, which was also plotted. The 
mean of all the hourly readings of this mean day was then found, and the difference between 
it and each of the hourly readings of the mean day taken; affecting this difference with the 
sign + when the grand mean was the greater, and with the sign — when this mean was the 
less. The corrections for the respective hours, thus found, were applied to all the observa- 
_ tions used. 
The series of three hourly observations taken at Fort Yuma were used to form a table of 
corrections for abnormal oscillations, the observations being all plotted inacurve. The interval 
between January 5 and January 12, during which there were no observations, was filled up by 
a drawing the curve as correctly as possible from the general direction of the curve preceding 
__ and following the interval. The record for this period was then made up from the curve as 
thus interpolated. 
