OF VOLCANOS. 



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 ex- 

 cellent 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 mag- 

 netic direction and intensity, so it is the duty of the geolo- 

 gical 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 Nauhcampatepet), 

 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 com- 

 parison 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 



VOL. II. Q 



