Ulloa, more than a hundred years ago, expressed his astonishment that the Vulture of the Andes could soar at heights where the pressure of the atmosphere was less than fifteen inches.[JP] An opinion was at that time entertained, from the analogy of experiments made with the air-pump, that no animal could exist under this slight amount of atmospheric pressure. I have myself, as has already been mentioned, seen the barometer fall to 14·85 inches on the Chimborazo; and my friend, M. Gay-Lussac, breathed for a quarter of an hour an atmosphere in which the pressure was only 12·9 inches. It must be admitted that man, when wearied by muscular exertion, finds himself in a state of painful exhaustion at such elevations; but in the Condor, the respiratory process seems to be performed with equal facility under a pressure of 30 or of 13 inches. This bird probably raises itself _voluntarily_ to a greater height from the surface of our earth than any other living creature. I use the expression “voluntarily,” since small insects and siliceous-shelled infusoria are frequently borne to greater elevations by a rising current of air. It is probable that the Condor flies even higher than the above calculations would appear to show. I remember observing near the Cotopaxi, in the pumice plain of Suniguaicu, at an elevation of 14,471 feet above the level of the sea, this bird soaring at such a height above my head that it appeared like a black speck. But what is the smallest angle under which faintly illumined objects can be distinguished? Their form (linear extension) exercises a great influence on the minimum of this angle. The transparency of the mountain air is so great under the equator, that in the province of Quito, as I have elsewhere stated, the white cloak (_poncho_) of a horseman may be distinguished with the naked eye at a horizontal distance of 89,664 feet, and therefore under an angle of thirteen seconds. It was my friend Bonpland whom we observed, from the pleasant country-seat of the Marques de Selvalegre, moving along a black rocky precipice on the volcano of Pichincha. Lightning conductors, being thin elongated objects, are visible, as Arago has observed, from the greatest distances and under the smallest angles.