LETTER TO TIIE SECRETARY OF WAR. 
Washington, D. C., June 30, 1855. 
Sir : In conformity with your instructions, the accompanying report of explorations for “ a 
railway route from the Mississippi river to the Pacific ocean” is hereby submitted to the depart¬ 
ment. As in many respects it differs from my previous communication upon this subject, I beg 
leave to make the following brief explanation. Upon my return from field duty, at the end of 
May, 1854, Congress, then in session, desired to obtain the principal results of the survey ; and 
I was directed to furnish them before its adjournment. The mass of material collected was too 
great to he carefully looked over within the time specified, and it became necessary to make up 
a trace of the route by a superficial inspection of the notes and field computations. The remarks 
accompanying it were sketched in the same hasty manner. As for some months afterward no 
further information was called for, the assistants were discharged, and the work partially sus¬ 
pended. At the succeeding session of Congress, the Pacific railroad reports were ordered to he 
reprinted ; and it was then suggested as proper to make a revision of the material, provided it 
might he done without delaying its publication. The astronomical positions, therefore, were 
revised, and the general maps reconstructed. Accurate profiles being of primary importance, 
the barometric observations, upon which they depend, were re-examined, and the altitudes of 
numerous additional points determined. These have been united with the field topography, 
making, for a portion of the route, a series of sketches, upon a scale of ; thus delineating 
the proposed location for a railway, and giving approximately the natural inclination of the sur¬ 
face upon that line. Profiles have been constructed upon the same horizontal scale, showing 
the grades, and, roughly, the cutting and filling required to obtain them. These exhibit, upon 
certain sections, fewer difficulties than superficial examinations had led me to expect. The 
length of the line, determined by this trace, is less than that measured by the odometer, which 
followed many sinuosities and large deflections that, for a railroad, would be unnecessary. A 
barometric profile has been constructed through Campbell’s Pass of the Sierra Madre, by which 
that mountain range may he crossed without a tunnel or deep excavation, with a maximum 
grade of fifty feet per mile, the summit level being 6,952 feet above the sea. This avoids the 
sharp crest where a tunnel was proposed, upon the direct route to Zuni by the way of the 
Camino del Obispo. 
The summit of Aztec Pass is found to be only 6,058 feet above the sea. With moderate exca¬ 
vation and embankment it can easily be surmounted. 
The connexion with Lieutenant Williamson’s survey, from Tah-ee-chay-pah Pass to Rio 
Mojave, shortens the length of the route to San Francisco, and avoids the Cajon Pass, with the 
expensive tunnel which it would require. 
The above are some of the considerations which have led to a modification of nearly all of the 
approximate results previously submitted. 
The material gathered upon the survey has required a diffusive report. It has, therefore, 
been divided into distinct parts, in order that the various subjects it embraces—each having its 
obvious and appropriate bearing upon the main purpose of the expedition—may be directly 
referred to. 
