Origin of Mount Etna. 393 



case, the arguments adduced against the possibility of a souleve- 

 Hient in volcanic rocks. But as difficulties such as I allude to 

 cannot prevail against direct proofs, I shall terminate this ana- 

 lysis, which is already too long, by simply pointing cut the dis- 

 cussion of which I have spoken to the attention of the " com- 

 missaires,*" to whom I feel desirous that the Academy should re- 

 fer the detailed examination of my memoir. 



Questions for Solution relating to Meteorologij, Hydrography ^ 

 and the Art of Navigation. By M. Arago. 



I HAVE somewhere read, that an individual was once lamenting, 

 in presence of D'Alembert, that the Encyclopaedia had acquired 

 such a vast extent. You would have had much more reason for 

 complaint, replied the philosopher, if we had drawn up a nega- 

 tive Encyclopaedia (meaning thereby an Encyclopaedia contain- 

 ing a mere indication of things, with which we are unacquaint- 

 ed) ; for in that case a hundred folio volumes would not have 

 been sufficient \i 



This reply, I must admit, has hitherto appeared to me to 

 have more point than justice. It is true that the progress of 

 human knowledge shews us daily how far our predecessors were 

 ignorant, and how far we in our turn will appear so to those 

 coming after us; but the greater number of important dis- 

 coveries have taken place spontaneously, without having been 

 foreseen or suspected by any one. Thus, to cite only two or 

 three examples, D'Alembtrt's negative Encyclopaedia could not 

 have contained the most remote allusion to that important and 

 prolific branch of modern physics, now known under the name 

 of Galvanism, or, as it is more properly called. Voltaic Electricity. 

 The multiplicity of phenomena, likewise, which are produced 

 by the polarization of light, when viewed in relation to its re- 

 flection, its ordinary refraction as well as that depending on the 

 action of crystallized plates, would not even be indicated ; and 

 the same thing may be said of the theory of luminous interfi- 

 rences^ in which the singularity of the results is not less re- 

 markable than their infinite variety. 



It must be admitted, however, that apart from those import- 

 ant and rare discoveries, which are made from time to time all 



