DISTRIBUTION OF RAIN. 261 
Through the entire line it has been sought to harmonize the results, as a survey of consecu- 
tive differences, with those obtained by new points of reference to the sea-level, and by refer- 
ence of distant camps to the initial point, and to one another. To reconcile these absolutely on 
a line of such extent, and nearly across the continent, could not be anticipated, as there are 
sources of great error in non-periodic variations of pressure, and in the variable value of obser- 
vations at the several camps and stations. Every check possible to be applied to the results 
has been applied, and the principal points are believed to be very near to accuracy. 
All the constants of correction have been fully applied. A variable error from variation of 
pressure among the months would give a slight reduction of the elevations at Albuquerque and 
eastward to the sources of the Canadian. West of Albuquerque it would slightly add to the 
elevations. " 
CLIMATOLOGY. 
The accompanying chart is prepared to givea comparison of the distribution of rain over the 
portion of the continent traversed by this line of survey. It is copied from a general chart of 
mean annual fall of rain for all parts of the continent as determined from the entire amount of 
American observation of this character. Most of the amounts are from positive measurements. 
Those for the interior, in positions not occupied by military posts at which observations have 
been made, are estimated from the climatological descriptions of chiefs of surveys and recon- 
noissances, and from general climatological laws. : 
Thus, a more abundant fall of rain in the tract bordering the Canadian than either north or 
south of it, is inferred from the less elevation of the mountains westward and southwestward. 
Rains are not wholly arrested by these mountains as by the greater elevations northward, and 
atthe south the border of the region of summer rains is not far off. The Estacado of Texas, 
the Rio Grande valley, and the Plains of the Gila, are in desert latitudes, or between the tropi- 
cal and temperate climates. Few measurements have been taken of amount of rain on the 
Canadian, but the comparative abundance of rain in this belt is fully shown in the naratives of 
the survey. 
West of the Rio Grande, the Zuñi plateaux and the adjacent mountains and districts are 
known to be comparatively well watered. The still more elevated mountains at the sources of 
the Rio Grande arrest an immense precipitation, and the extension westward of this profusion 
of rains, at certain seasons at least, is farthest at the 35th parallel, and on the San Francisco 
mountains and the adjacent plateaux. The Sierra Madre and coast ranges of mountains have 
also an abundant precipitation, though mainly confined to certain seasons, and not equally dis- 
tributed through the year. ` 
Generally the districts near the 35th parallel on the western part of this continent are at the 
southern border of the regions of equally distributed rains. Those bordering the 30th parallel 
are the most extremely arid—the normal desert climates, though locally relieved in many cases. 
Southward, or at the 25th parallel, the summer rainy season and the alternations of tropical 
climates are fully instituted. 
The temperature distribution of this portion of the continent is unimportant, except that its 
elevation moderates its extreme heats and gives it a general resemblance to that of the 40th 
parallel in the eastern United States. The valley of the Colorado of California is an exception, 
however, having very high temperatures, but a very dry, elastic atmosphere. The cool invigor- 
ating climates of the immediate coast of the Pacific are also a striking and valuable feature of 
this general climatology. 
