224 STRUCTURE AND MODE OF ACTION 



mena show the operation of subterranean forces, acting 

 either dynamically in earthquakes, in the tension and agita- 

 tion of the crust ; or in volcanos, in the production and 

 chemical alteration of substances. They also show that 

 these forces do not act superficially, in the thin outermost crust 

 of the globe, but from great depths in the interior of our 

 planet, through crevices or unfilled veins, affecting simul- 

 taneously widely distant points of the earth's surface. 



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 tliis 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 phi" 

 losophical study of nature endeavours, 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 observa- 

 tions carefully made and connected with determinate epochs, 

 certain fixed points which may afford a base for future 

 numerical comparisons. If we only possessed determina- 

 tions made once in each period of a thousand years, of the 

 mean temperature of the atmospheie 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 



