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Show OF VOLCANOS. 383 The greater the variety of structure in volcanos, or in the elevations which surround the channel through which the molten masses of the interior of the earth reach its surface, the greater the importance of submitting this structure to strict investigation and measurement. The interest attaching to these measurements, which formed a particular object of my researches in another quarter of the globe, is enhanced by the consideration, that at many points the magnitude to be measured is found to be a variable quantity. The philosophical study of nature endeavors, in the vicissitudes of phenomena, to connect the present with the past. If we desire to investigate either the fact of a periodical return, or the law of progressive variations or changes in phenomena, it is essential to obtain, by means of observations carefully made and connected with determinate epochs, certain fixed points which may afford a base for future numerical comparisons. If we only possessed determinations made once in each period of a thousand years, of the mean temperature of the atmosphere and of the earth in different latitudes, or of the mean height of the barometer at the level of the sea, we should know whether, and in what ratio, the temperature of different climates had increased or decreased, or whether the height of the atmosphere had undergone changes. Such points of comparison are also needed for the inclination and declination of the magnetic needle, as well as for the intensity of the magneto-electric forces, on which, within the circle of this Academy, two excellent physicists, Seebeck and Erman, have thrown so much light. As it is an honorable object for the exertions of scientific societies to trace out perseveringly the cosmical variations of temperature, atmospheric pressure, and magnetic direction and intensity, so it is the duty of the geological traveller, in determining the inequalities of the earth's surface, to attend more particularly to the variable height of volcanos. The endeavors made by me for this object in the Mexican mountains, in respect to the V olean de Toluca, the Popocatepetl, the Cofre de Perote or N auhcampatepetl, and the J orullo, and also the volcano of Pichincha in the Andes of Quito, have been continued since my return to Europe at different epochs on Vesuvius. Where complete trigonometric or baromet.ric measurements are wanting, accurate angles of altitude, taken at points which are exactly |