362 
POPULAR SCIENCE REVIETV. 
its being emphatically a foul-weather cloud adds interest to the 
investigation, for heavy rain and wind usually follow its ap- 
pearance. My own observations indicate, however, that rain is 
more certain to follow than wind. In Lancashire it is some- 
times termed ‘‘ Eain-ball.” 
The origin of these symmetrical cloud forms is very difficult to 
determine. Mr. Scott, in his paper above referred to, attaches 
considerable importance to the experiments of Sir J. Herschell 
and Mr. Jevons, by whom it has been shown that a similar ap- 
pearance may be produced by gradually mixing two fluids of 
slightly different specific gravities. 
The nature, however, of the forces which determine the 
peculiar symmetry of the forms in question seems almost equally 
obscure in the case of the experiments as in that of the atmo- 
spheric phenomenon they are used to illustrate. In the spherical 
forms observed by myself it is hard to imagine such an appear- 
ance being caused by a mere undulatory movement of the vapour 
arising from gravitation. We are rather reminded of the for- 
mation of bubbles ; but if that be an analogous case, the tension 
of the vapour must be something unusual. Let the reader, 
however, compare this phenomenon with the peculiar rounded 
prominences that occasionally appear on the sides of a large 
mass of the electric cumulus, and range themselves in regular 
lines proceeding from a common origin or vertex. Must we 
attribute these protuberances to an electrical decomposition of 
vapour ? 
