earth one thousand five hundred and seventy-eight French feet above
the surrounding plains of Mexico. The position of these singular
mountains in longitude and latitude was ascertained by astronomical
observations. We took the heights of the different parts by the aid
of the barometer, and determined the dip of the needle and the
intensity of the magnetic forces. Our collections contain the
plants which are spread over the flanks of these volcanoes, and
specimens of different rocks which, superposed one upon another,
constitute their external coat. We are enabled to indicate, by
measures sufficiently exact, the height above the level of the
ocean, at which we found each group of plants, and each volcanic
rock. Our journals furnish us with a series of observations on the
humidity, the temperature, the electricity, and the degree of
transparency of the air on the brinks of the craters of Pichincha
and Jorullo; they also contain topographical plans and geological
profiles of these mountains, founded in part on the measure of
vertical bases, and on angles of altitude. Each observation has
been calculated according to the tables and the methods which are
considered most exact in the present state of our knowledge; and in
order to judge of the degree of confidence which the results may
claim, we have preserved the whole detail of our partial
operations.

It would have been possible to blend these different materials in a
work devoted wholly to the description of the volcanoes of Peru and
New Spain. Had I given the physical description of a single
province, I could have treated separately everything relating to
its geography, mineralogy, and botany; but how could I interrupt
the narrative of a journey, a disquisition on the manners of a
people, or the great phenomena of nature, by an enumeration of the