STRUCTURE AND ACTION OF TOLCANOS. 363 



of Quito at Pichincha, I have had opportunities since my 

 return to Europe of repeating, at different periods, on Mount 

 Vesuvius. Where complete trigonometric or barometric mea- 

 surements are wanting, their place may be supplied by angles 

 of altitude laid down with precision, and taken at points accu- 

 rately determined. The comparison of such determinations, 

 made at different periods of time, may sometimes be even 

 preferable to the complication of more complete operations. 



Saussure measured Vesuvius in 1773, and at that time 

 both the north-western and south-eastern margins of the 

 crater appeared to him to be equal in height. He found 

 their elevation above the level of the sea to be 3894 feet. 

 The eruption of 1794 occasioned a falling in towards the 

 south, and an inequality in the margins of the crater, which 

 may be distinguished from a considerable distance even by 

 the most unpractised eye. Leopold von Buch, Gay Lussac, 

 and myself, measured Mount Vesuvius three times in the year 

 1805, and found that the elevation of the northern margin, la 

 Rocca del Palo, opposite the Somma, was exactly as it had 

 been given by Saussure, while the southern margin was 479 

 feet lower than it had been in 1773. The elevation of the 

 volcano itself towards Torre del Greco (the side towards which , 

 for thirty years, the volcanic action has been principally 

 directed) had, at that time, decreased one-eighth. The cone 

 of cinders bears to the total height of Vesuvius the relation 

 of 1 : 3; in Pichincha, the ratio is as 1 : 10, and at the Peak 

 of Teneriffe, as 1 : 22. Of these three volcanic mountains, 

 Vesuvius has, therefore, comparatively, the highest cone of 

 cinders ; probably because, being a volcano of inconsiderable 

 height, it has chiefly acted through its summit. 



A few months ago, in the year 1822, I succeeded not only 

 in repeating my earlier barometric measurements of Mount 

 Vesuvius, but also in determining more completely all the 

 margins of the crater (1) during three ascents of the mouu- 

 tain. 



