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 honourable 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 endeavours made by me for this object in the Mexican mountains, in respect to the Volcan de Toluca, the Popocatepetl, the Cofre de Perote or Nauhcampatepetl, and the Jorullo, 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 barometric measurements are wanting, accurate angles of altitude, taken at points which are exactly determined, may be substituted for them; and for a comparison of determinations made at different epochs, angles of altitude so measured may even be often preferable to the complication of circumstances which more complete operations may involve.
Saussure had measured Mount Vesuvius, in 1773, when the two margins of the crater, the north-western and the south-eastern, appeared to him be of equal height. He found their
