The following table of comparison between altitudes, determined by the level and by barometric observations, shows how much has been gained in accuracy, since the time of Humboldt, in the use of the barometer:
| STATIONS. | MEAN READING OF BAROMETER. | MEAN TEMP. | BAROMETRIC HEIGHT. | HEIGHT BY LEVEL. | DIFFERENCE. | ||
|---|---|---|---|---|---|---|---|
| INCHES. | DEG. | FEET. | FEET. | FEET. | |||
| Sea coast | 29.874 | 80. | ... | ... | ... | ||
| First camp on Truando | 29.817 | 75.4 | 58.39 | 44.57 | + 13.82 | ||
| Tocame | 29.805 | 76.8 | 69.6 | 57.39 | 12.21 | ||
| Foot of Saltos | 29.759 | 76.1 | 122.65 | 97.5 | 25.15 | ||
| Observatory Hill | 29.663 | 76.6 | 207.45 | 204.95 | 2.5 | ||
| Head Salto Grande | 29.741 | 75.9 | 132.3 | 138.79 | - 6.49 | ||
| Head of Saltos | 29.737 | 75.9 | 138.1 | 183.47 | 45.37 | ||
| Junction of Rivers Nercua and Truando | 29.674 | 77. | 192.5 | 192.6 | + 0.44 | ||
| Tambo | 29.607 | 77. | 260.92 | 264.4 | - 3.48 | ||
| First Ridge west of Rio Nercua | 28.815 | 75.2 | 1,046.45 | ... | ... | ||
| [9] Log Crossing on |  | No. 1362 | 29.053 | 75.2 | 809.42 | 791.23 | + 18.19 |
| Rio Hingador | ... | ... | Mean 879.9 | ... | ... | ||
| No. 1363 | 28.912 | 75.2 | 949.94 | ... | ... | ||
| Camp on Hingador | 29.074 | 75.2 | 788.6 | 814.32 | - 25.72 | ||
| Dividing Ridge | 28.913 | 75.2 | 948.5 | 947.44 | + 1.06 | ||
| Rio Chupepe | 29.631 | 75.2 | 240.24 | 241.35 | - 1.11 | ||
| Rio Totumia, below Dos Bocas | 29.837 | 75.2 | 40.6 | 45.3 | 5.24 | ||
These hypsometric determinations differ from the true levels at the points of observation from two to forty-five feet. These figures fall considerably within the limit of error considered as probable by Baron Humboldt. This distinguished observer states that the barometer may be trusted to determine heights to within from seventy-five or ninety feet of the truth.
CHAPTER VII.
Physico-Geographical Features—Deficiency of Information—Barometer—Colonel Williamson—Lieutenants Gibbon, Herndon and Maury—Señor Moro—Popagayos—Influence of the Andes—Climate—Rainy Season—Colonel Hughes —Statistics—Population—Indians—Vegetation—Building Materials—Woods—Geology.
The present chapter includes certain physico-geographical features subsidiary to the duties of the engineer and explorer. The object of this paper excludes all matter, not possessing practical value for this purpose, and admits of little more than mere mention.
The previous chapters indicate a deficiency in information in regard to the following routes:
1. Nicaragua—The practicability of a route between Monkey Point and the Lake Nicaragua, or San Juan River.