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whirlwind is evidently the originating force, but whether the 
direction of the currents is originally upwards or downwards I 
must leave to other observers to determine. Perhaps we may 
find the true explanation of some of the more striking cloud 
forms in the action of two vapour currents of different densities 
meeting each other ; but in the cases we have just considered, 
where a rapid motion is observed while the atmosphere all 
around is still, I think we may find a solution by applying the 
principle of correlation of forces, i.e. in the generation of elec- 
tricity by heat or evaporation. The 44 festoon ” cloud (fig. 2) 
which is commonly seen before squally weather and with a 
falling temperature, may, however, be due to ascending cur- 
rents of vapour meeting colder currents of air descending 
towards the earth. 
This may, perhaps, be also the origin of the next form of 
Fig. 3. 
DROPPING UMULUS. 
a. Ordinary Forms. 6. Accidental varieties in the shape of a dumb-bell. 
cumulus which we take for examination, viz., the 44 dropping ” 
cumulus (fig. 3). I am not aware that previous writers have 
considered this a distinct species ; but it deserves to be so re- 
cognized for two reasons, viz., its value as a weather prophet 
(indicating wind and rain); and the curious inquiries which 
it suggests concerning the coherence of cloud masses, and the 
relation which the forces that determine this coherence bear to 
that of gravitation. A very ancient writer* has pointed out the 
scientific problems involved in this adjustment of mechanical 
laws, as yet far from solution. 
There is a close affinity between this form and the 44 festoon ” 
* Book of Job, chap, xxxvii. v. 16. 
