2S0 Baron Humboldt on the Structure and Action of 



of phenomena, to connect the present with the past. To esta- 

 bhsh a periodical return, or to iix the laws of progressive and 

 variable plienomena, it is necessary to have some well determined 

 points of departure, observations made with care, and which, 

 being connected with determined epochs, may furnish numeri- 

 cal comparisons. Had only the mean temperature of the atmo^ 

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

 rature of the barometer on the edge of the sea, been determined 

 from one century to another, we should have known in what pro- 

 portion the heat of climates has increased or diminished, and 

 whether or not the height of the atmosphere has viedergone 

 changes. These points of comparison are required for the decli- 

 nation and inclination of the magnetic needle, as well as for the 

 intensity of the electro-magnetic forces. If it be a praiseworthy 

 occupation for societies to follow, with assiduity, the cosmic vicis- 

 situdes of heat, of the pressure of the air, and of tlie magnetic 

 direction and intensity ; it is, on the other hand, the duty of the 

 geologist, in determining the inequalities of the earth's surface, to 

 take into consideration the change of height of volcanoes. What 

 I attempted at the time, in the mountains of Mexico, at To- 

 luca, Nauhamputeptel and Jorullo, and in the Andes of Quito at 

 Pichincha, I have had an opportunity, since my return to Eu- 

 rope, of repeating several times at Vesuvius. 

 • In 177B, Saussure measured that mountain at a period when 

 the two edges of the crater, the north-west and south-west, ap- 

 peared to him of equal height. He found their elevation 609 

 toises above the level of the sea. The eruption of 1794 occa- 

 sioned a falling in of the southern part, and an inequality of the 

 edges of the crater which the most inexperienced eye distin- 

 guishes at a considerable distance. In 1805, M. von Buch, M. 

 Gay Lussae and myself, measured Vesuvius three times. The 

 result of our operations was, that the height of the north 

 edge, the Rocca del Palo, which is opposite the Somma, agreed 

 with Saussure's measurement, but that the south edge was 75 

 toises lower than in 1773. The total elevation of the volcano, 

 towards the Torre del Greco, the side towards which the fire 

 had principally directed its action for thirty years, had diminish- 

 ed an eighth part. The cone of ashes is, to the total height of 

 the mountain, on Vesuvius, as one to ten ; on the Peak of Te- 



