82 
vapour and grow more rapidly in size. They would therefore 
descend and leave other particles to form the top of the 
cloud. In this way we should have in embryo a continuous 
succession of drops. 
Eddies in the cloud also form another possible cause of 
the origin of drops and stones. Whenever the direction of 
motion of a portion of the cloud is not straight the suspended 
particles will have more or less motion through the air. And 
if, as in an eddy, the motion of the cloud varies from point to 
point both in direction and magnitude, then the motion of 
the particles through the air will also vary, and they may 
overtake one another, and combining form larger particles or 
drops in embryo. 
Whatever may be the cause of the 
variation in the size of the particles 
which form the cloud, we may know 
from observations on fogs that such 
variations do exist. In fogs we have 
particles of all sizes, from those which 
are too fine to be seen even by the aid 
of a microscope, and which will remain 
suspended for hours without any ap* 
preciable descent up to such a size that 
they can be easily detected with the 
naked eye, and descend with a very 
appreciable velocity so as to form a 
drizzle. When a coarse mist, such as 
this is superimposed over a fine mist> 
then rain must ensue if the particles 
are water, and hail if they are ice. 
IMITATIONS IN PLASTER OF PARIS. 
