G. Poidett Scrope — The Mechanism of Stromholl. 531 



Height of the lip of the Crater (presumably Mr. Mallet's point C in Diagram). 

 HoflFmann (by Barometrical measurement) 600 feet below 



the summit = 2,253 feet above the sea. 



Abioh (from repeated measurements) 2,178 German feet ... ^2,239 English feet. 

 (Mallet, by aneroid measurement and estimate 1,200 feet !) 



Taking then 2,245 feet (the mean of MM. Hoffmann and Abich's 

 figures) as the true height of C in Mr. Mallet's Diagram, and ad- 

 mitting his own estimate of 600 feet as the vertical difference between 

 C and D, it is seen that the height of the latter point cannot be less 

 than 1,660 feet, instead of 600, which Mr. Mallet makes it. While, 

 if we accept the measurement of the several Government Charts for 

 the summit of the mountain, 215 feet must be added to 1,650, making 

 about 1,900 as the true height of D from the sea. 



The relative heights of the summit and lowest crater rim (D in 

 Diagram) are seen in the drawings attached to the English and 

 French Admiralty Charts, in that of Abich (copied in Woodcut, 

 Fig. 2), that of Sir W. Hamilton, of myself, and in every other pub- 

 lished drawing of the island, as well as by several taken by my friend 

 Mr. J. W. Judd, F.G.S., in the spring of this year, to be in proportion 

 of about 3 to 2. 



Indeed it is difficult to understand how Mr. Mallet himself can 

 have seen the island from the sea at any distance on the north side, 

 without perceiving this. 



These gross errors of height-measurements make the following 

 passage from Mr. Mallet's paper more remarkable (p. 502) : 



"The statements which have been" (hitherto) "made as to the 

 relative heights of the different jDoints of the island appear to be 

 only derived from guess, and are greatly in error." 



I might add, that Mr. Mallet seems incapable even of reading 

 correctly the measurements given by others ; since he expressly says 

 in reference to the depth of water immediately beneath the crater- 

 slope (p. 509) : 



" The Admiralty Charts indicate that for some miles in the offing 

 here, the Mediterranean does not exceed 100 fathoms in depth." 



I have, however, the Admiralty Chart before me, which gives 

 soundings at more than one point within half a mile of the base of 

 the crater-slope of 258 fathoms ; which the French Admiralty Charts 

 confirm, giving 480 metres, at a little distance from the same base. 



Mr. Mallet visited Stromboli in 1864. He was unfortunate, it 

 seems, in the state of the atmosphere on the occasion of his visit, 

 and was consequently unable to see to the bottom of the crater, 

 through the clouds of vapour that filled it. He witnessed, however, 

 as every other visitor had done, a series of intermittent explosions, 

 throwing up red-hot scoriae, bombs, and fragments of lava, together 

 with steam, from what was evidently the crater of a volcano. And 

 now, after ten years of mature deliberation, he has brought forward 

 a theory to account for these explosions, which has at least the merit 

 of novelty. He seems, indeed, to have formed his opinion, without 

 the slightest reference to the modus operandi of other active volcanos, 

 and without inquiring whether in any, or if so, in what respects the 

 phenomena of Stromboli differ from those of an ordinary volcanic vent. 



