OF VOLCANOS. 383 



The greater the variety of structure in volcanos, or in the eleva- 

 tions which surround the channel through which the molten masses 

 of the interior of the earth reach its surface, the greater the im- 

 portance 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 phe- 

 nomena, 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 com- 

 parison 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, atmo- 

 spheric 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 Volcan de Toluca, the Popo- 

 catepetl, 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 want- 

 ing, accurate angles .of altitude, taken at points which are exactly 



