vapour at the time and place of its formation ; the stillness 
and electrical condition of the air ; the number and the size 
of the drops it meets with in its course ; the amount of vapour 
in the air through which it passes, etc. ; all of which are 
fortuitous, or at least immeasurable conditions. 
With such drops we have here nothing to do, but only 
with those which are formed under fixed circumstances. On 
the other hand we shall have t) consider upward moving 
drops.* Drops of the latter kind are so se ldom seen, that while 
an upward moving drop appears an incongruous expression, 
yet no distinctive name has been given ;o them. 
Without attempting an exhaustive 'definition, it will be 
sufficient to define a drop as a mass of liquid collected and 
held together only by the attraction of i ;s parts, and separated 
from other matter by the attraction of gravitation. This 
definition will exclude such drops as those of mist or rain, and 
will include the upward moving drops referred to above. 
Such being the definition of a drop, it follows at once that 
the size of a drop may depend upon and be influenced by 
variation in : — 
1. The self-attraction and cohesion of the drop -generating 
liquid ; 
2 . Its adhesion to the matter upon which the drop is 
formed ; 
3. The shape of the matter from which the elrop moves ; 
4. The physical relation between the medium through 
which the drop moves, on the one hand to the liquid of which 
the drop is formed, and on the other to the matter on which 
it is formed ; 
5. The attraction of the earth or gravitation upon the drop- 
* Owing to the numerical preponderance of downward moving drops, 
we are prone to associate the ideas of drop and down. How far Imay be 
justified philologically in using the expression “ drop up ’’ must depend 
upon the relative primativeness of the noun and verb drop. Once for 
all I beg permission to use the term in the extended sense. 
Of course, in the absence of positive levity, an upward drop can only 
be caused by the downward motion of the medium in which the drop 
moves. 
