The errors of the barometer have led to singular inferences, and the errors of observers to many more. Humboldt, La Condamine, Boussingault, give a decreasing pressure along the Andes; and Orton, taking this statement for granted, asks, “Are the Andes sinking?” The evidence of geological and historical periods is, that the Andes and sea coast are rising. The exceptions to this rule are local, and perhaps only in appearance. These objections to the use of this instrument only apply to situations where the spirit-level can not be used. To determine heights inaccessible to any other instrument, or for simultaneous observation of the meteorological condition of an extensive area of country, the portability of the barometer render it invaluable. The height of the barometric column, on the Pacific slope of the Andes, according to Orton, is 29.930. He gives two values for the Atlantic side, 29.997 and 29.932. Michler gives the Atlantic coast of the Atrato 29.874. CLIMATE. A well-defined rainy season prevails for the most part throughout the Isthmus, and permits the selection of suitable weather for the operations of the engineer. Rain varies with proximity to the mountains, etc., but the interval from December to May may be regarded as the dry season. The seasons are sometimes reversed, as in Costa Rica. There the dry season prevails upon the Pacific coast from November to April, but on the Atlantic the contrary prevails. Fall of rain in Honduras from May to October is 90.89 inches. The tierras templadas, or elevated table-lands, are universally healthy, and the climate in those regions possesses a charm which belongs exclusively to the tropics. The unhealthy influences of the marshes and sea-coast is much exaggerated, and may be said to cease during the winter or dry season. Col. Hughes, who visited the most insalubrious part of the Isthmus, remarks that travelers, “who live like civilized beings,” have little to fear from the climate. The writer spent six months, chiefly near the sea-coast of Columbia, during part of the time compelled to sleep among the swamps of the delta of the Magdelina, and although exposed to the sun during the day, and sleeping in the open air at night, not one case of febrile sickness occurred in the party of which he was a member, nor were more than two cases of fever observed among the natives during the period of residence. The temperature varies with the elevation above the sea. Thermometric records are of small value without the monthly and daily means of localities. The following table may give some general notion of their range: