48 
PRECISE LEVELING IN THE UNITED STATES, 1903-1907. 
Hods T and U were used during part of March and April, 1904. The value used 
in the computation for the mean length was 3.00115 meters, or an excess of 0.38 
millimeter per meter. 
These rods were determined by the Bureau of Standards as follows: 
Date. Rod T. j Rod U. 
m. m. 
January, 1904 3.0010 3.0009 
January, 1905 3.0015 3.0014 
The value used is therefore very nearly the mean of these two determinations. 
As in 1902, the methods of observation, limit of error, etc., were the same as in 
Coast and Geodetic Survey leveling, with one exception. In the latter part of 
1903, on the line Warwick to Ben wood, and on the line Cumberland to Ben wood, 
a change was made by always reading the back rod first. Mr. Carr stated that this 
change was made solely to save time. The Coast and Geodetic Survey practice 
is to read the back rod first at one instrument station and the fore rod first at 
the next, and so on. See paragraph 14 of the general instructions, page 12. The 
purpose of this provision is to eliminate the effect of any rising or settling of the 
instrument. In these two lines, therefore, of the Baltimore and Ohio Railroad 
leveling, this systematic error has not been eliminated. 
The accumulated discrepancy between the forward and back lines on all the 
leveling except the first 43 miles of Cumberland to Benwood had a plus sign and 
showed a steadily increasing value. 
WASHINGTON, D. C., TO BALTIMORE, MD. 
At Washington one bench mark established the previous year was recovered. 
In Baltimore the leveling was connected with the city leveling on two bench marks. 
Connection was made with sea level at Baltimore by the Coast and Geodetic Survey 
in 1905 (see p. 28). The line was continued from Baltimore to Philadelphia, but 
this portion, which would be a spur from the level net and which is without a check, 
was not included in this publication. 
The accumulated discrepancy between the forward, and backward line (B — F) 
at Baltimore was +0.0724 foot ( + 22.1 millimeters) on 40.7 miles (65.5 kilome 
ters), or +0.34 millimeter per kilometer. The accumulated discrepancy at 
Philadelphia, the end of the line, was +0.1462 foot ( + 44.6 millimeters) on 136.4 
miles (219.5 kilometers), or +0.62 millimeter per kilometer. 
RELAY TO WASHINGTON JUNCTION, MD. 
At Relay one bench mark established early in the same season was recovered. 
At Washington Junction three bench marks established the previous year were 
recovered and found undisturbed. This line closed a small circuit of Baltimore 
and Ohio Railroad leveling, namely, Washington-Relay-Washington Junction- 
Washington. The closing error was 0.0723 foot, or 22.0 millimeters, and the length 
of the circuit was 131.5 miles, or 213 kilometers.