G. K. Gilbert—Special Processes of Research. 459 
instant; if similar charts were prepared for all other instants of 
the storm’s life; and if these were superposed on each other so 
as to bring common features together, the result would be the 
same as by the process just described, and this result has there- 
fore been compared by Davis with Galton’s photographic over- 
printing and aptly called a “composite portrait” of the storm. 
This composite (of which my diagram is a mere abstract) was 
explained and illustrated to the Society at its Boston meeting, 
and you will recall what an orderly presentation it gave of the 
eomplex congeries of phenomena. Its inspection afforded at 
once to the investigator a large body of generalizations con- 
cerning the individual storm and of hypotheses concerning 
thunderstorms in general. 
If now we examine these three processes of classification in 
their relations to each other and in relation to the purposes of 
the investigation, I think we shall be able to perceive the con- 
siderations which led to theiremployment. One of the primary 
Fig. 2.—Composite portrait of thunderstorm; arrows and the number of their 
feathers show direction and force of wind. TT, T, observation of loudest thunder. 
Straight lines mark observations on the width of rain area. 
objects of the research was the determination of the mechanism 
of the thunderstorm, and this object manifestly required the 
study of the simultaneous phenomena of the storm. The first 
effort in classification was therefore the isolation of the indi- 
vidual storm. A natural second step would have been the 
selection of some moment of the storm’s life for special exam- 
ination, but if such examination was attempted, it met with an 
obstacle arising from the fact that the simultaneous observations 
at any one moment were so few as to be quite inadequate to 
the purpose. The analogy of other investigations suggested 
that the desired result might be accomplished by combining the 
observations on the individual storm without reference to time 
or place, but with reference to some central feature or features 
