AMONG THE VOLCANOES. 85 



the elastic force becomes sufficient to lift the column of water. 

 The commencement of escape now diminishes pressure, and a 

 large volume of steam is instantly formed, which causes the 

 violent eruption. The heavy thumps sometimes heard before 

 and during the action are due to collapses of steam in contact 

 with the water, and are strictly the same in principle as the 

 sharp detonations frequently heard in the steam-pipes employed 

 for warming buildings. 



XV. AJVEONQ THE 



INDICATIONS OF INTEKNAL FIRES. 



VESUVIUS and ^Etna are the two volcanoes familiarly known 

 to classical antiquity. They offer for our observation nearly 

 all the characters with which the geological reader and student 

 should become acquainted in prosecuting his inquiries into the 

 present and past temperatures of the earth. Let us make the 

 ascent of Vesuvius, taking the usual route from Naples. 

 Driving a couple of hours down the coast to Resi'na, on the 

 site of the ancient Herculaneum, we begin the ascent either 

 on foot or on mule or horse back. For thirty minutes we fol- 

 low a rough road through vineyards. The road then turns to 

 the north and we enjoy a lovely view of the landscape and the 

 bay. Here stretch two dark streams of cold lava presenting 

 somewhat the aspect of enormous beds of enormous cinders 

 fibrous and twisted and waxy. They are the lava streams of 

 1858 and 1868. Next appears the huge lava wall of 1858. 

 In two hours from Resin a, we reach the Observatory, 2,218 

 feet above sea-level, erected in 1844 for meteorological and 

 seismic observations. Here Professor Palmieri, the celebrated 

 vulcanologist, is engaged in making the most minute studies 

 of the incidents in the history of the mountain. From this 

 station and the "Hermitage" just below, where the traveler 

 procures refreshments, the view over the black and herbless 

 lava slope is desolate beyond description. Soon the road be- 

 comes impracticable for quadrupeds. Crossing the lava flood 

 of 1871, we now reach the Atrio del Cavallo, at the foot of 



