50 Perret — Representation of Volcanic Phenomena. 



Before taking up the consideration of the various phenom- 

 ena it may be well to realize the scope and limitations of the 

 method. The volcanic forces and manifestations not being 

 generally measurable in exact terms, it is obvious that the dia- 

 gram will not be of mathematical precision. It is not intended 

 to indicate, for example, the explosive pressure in any given 

 outbreak nor to compare this rigorously with that of another 

 volcano, but the object is rather to present the various phe- 

 nomena of an eruptive phase in their relative proportions, and 

 thus to reveal its character. The maximum values once adjudi- 

 cated, it is easy to estimate the others in relation thereto, and 

 this is far simpler than might, at first, be supposed. At Etna, 

 for instance, in 1910, the great stream of lava, issuing at tor- 

 rent velocity and extending nearly ten kilometers, easily 

 formed the distinguishing characteristic and, constituting as it 

 did, even for this volcano, a very notable outflow, it is indi- 

 cated on the diagram with the full ten points. At Teneriffe, 

 instead, although the lava-flow constituted the principal 

 external manifestation, its magnitude was mediocre, not merely 

 pei' se but also more in comparison with former outflows in the 

 same general locality, and the value assigned to it on the dia- 

 gram is seven points. 



It will be evident, also, that the difference of a point either 

 way will not, as a rule, materially affect the integrity of the 

 diagram. At Stromboli, 1912, the electrical phenomena are 

 indicated by a radiant of six points. A second observer might 

 have argued against a higher value than five on the ground 

 of the brief duration of the phenomena, while a third might 

 have assigned seven points in order to give prominence to a 

 manifestation extremely rare at this volcano — all would have 

 been agreed, however, as to a characteristic of very notable, 

 but not paramount, importance and thus, in any case, the radi- 

 ant would have a value conveying this impression. 



In the case of certain outbreaks of brief duration — and espe- 

 cially those in which the character of the action does not 

 greatly change during the course of the eruption — a single dia- 

 gram may be made to represent the entire event. This has 

 here been done with Teneriffe 1909, Stromboli 1907 and 1912, 

 and, somewhat reluctantly, with Etna 1910. But it is evi- 

 dently preferable that the diagram should show the eruptive 

 conditions at a given time, different phases being represented 

 on separate charts and forming a series of documents reveal- 

 ing the progression of the eruption, and it is here that the 

 beauty and value of this graphic method is most clearly seen. 

 At the great Vesuvian outbreak of 1906 the character of the 

 eruptive phenomena swung through a succession of inter- 

 merging but widely differing phases, the more salient of which 



