562 VIEWS OF XATL'RE. 



The more varied the structure of volcanos. that is to say, 

 of elevations inclosing a channel through which the molten 

 masses of the interior of the earth reach the surface, the 

 more important it is to form a correct idea of these structures 

 by careful measurement. The interest derived from mea- 

 surements of this kind, which I made a special subject of 

 inquiry in the western hemisphere, is increased by the consi- 

 deration, that the objects to be measured vary in magnitude 

 at different points. A philosophical study of nature seeks, in 

 considering the changes of phenomena, to connect the present 

 with the past. 



In order to ascertain the periodic recurrence, or the laws 

 of the progressive changes in nature, we require certain fixed 

 points, and carefully conducted observations, which, by their 

 connection with definite epochs, may serve as a basis for 

 numerical comparisons. If the mean temperature of the atmo- 

 sphere and of the earth in different latitudes, or the mean 

 height of the barometer at the sea level, had been deter- 

 mined only once in every thousand years, we should know to 

 what extent the heat of climates has increased or diminished, 

 and whether any changes have taken place in the height of the 

 atmosphere. Such points of comparison are especially required 

 to determine the inclination and declination of the magnetic 

 needle, and the intensity of those electro-magnetic forces on 

 which Seebeckand Erman, two admirable physicists belonging 

 to this Acadcmv, have thrown so much lisrht. If it be a meri- 

 torious undertaking on the part of learned societies to investi- 

 gate with perseverance the cosmical changes in the heat and 

 pressure of the atmosphere, and particularly the magnetic 

 direction and intensity, it is no less the duty of the travelling 

 geologist to direct attention to the varying height of volcanos 

 in determining the inequalities of the earth's surface. The 

 observations which I formerly made in the Mexican mountains, 

 at the volcano of Toluca, at Popocatepetl, at the Cofre do 

 Perote, or Nauhcampatepetl, and Xorullo, and in the Andes 



