Perret — Representation of Volcanic Phenomena. 49 



of expression is needed — a method of graphically setting forth 

 all the phenomena in their just proportions and thus to reveal, 

 in a composite figure, the true character of the volcanic activ- 

 ity. The complete presentation of any volcanic event will 

 then be of a threefold nature, consisting of the written descrip- 

 tion, the photographic illustrations and the diagrammatic 

 interpretation of the eruptive characteristics. 



Long convinced of this need, the writer has devoted himself 

 to the problem of devising a method which should be simple 

 and thoroughly practical and, at the same time, so comprehen- 

 sive as to fully attain the scope of the undertaking, and this is 

 believed to have been finally achieved in the form herewith 

 presented. 



The Diagram Chart. 



This is furnished in the form of a simple postcard which is 

 easily handled, readily carried in the pocket and mailable at 

 lowest rates. One side is spaced for the address and for notes, 

 while the other is devoted to the diagram proper (see Plates 

 I-IY, giving twelve examples, each reduced one-third) which is 

 constructed as follows : 



Eight radiants diverge from a center and terminate in a cor- 

 respondingly divided circle which, with its extensions, forms 

 the hey of the diagram. The circle divisions are marked to 

 represent the various phenomena, which are here expressed in 

 the most general terms — explosive, effusive, seismic, etc. — and 

 are then developed and extended into subdivisions indicated by 

 letters and numbers. 



Each radiant is a line of ten small, blank circles destined to 

 be blackened or filled in progressively from the center out- 

 ward, according to the value assigned to the phenomena which 

 it represents. Tie lines are then to be drawn between the 

 radiants intercepting these at the marked extremities, thus 

 enclosing a space and constituting a figure, to the form and 

 angles of which the eye is peculiarly sensitive. 



This construction of the radiants has been adopted, not only 

 as forming a convenient decimal scale of values, but because it 

 is easier to blacken a small circle than to draw a straight line, 

 and also for the reason that a more salient appearance is thus 

 given to the radiant proper — which indicates the real value 

 assigned to the characteristic — than to the rest of the figure 

 as formed by the intercepts. 



The subdivision is to be indicated in the diagram by its 

 letter and number, and in the case of there being more than 

 one — and of lesser value — its circle may be left blank, thus 

 contrasting with the others and marking the value of that par- 

 ticular detail, all of which is clearly shown in the examples 

 accompanying this article. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVII, No. 217.— January, 1914. 



4 



