378 Observations on an. 
be considered: the levity of foam, compared 
with the superior weight of water, may lead 
some persons to suspect, that this light sub- 
stance will not mix with water, but will float 
‘on the surface of the reservoir, in which it 
is formed. Supposing this suspicion to be 
well-founded for the sake of argument, we 
must allow the foregoing theory of recipro- | 
cating vessels to be defective in a very essen- 
tial point; because if foam cannot sink, the 
air, that proceeds from it, cannot find its 
way into the tubes or siphons, which convey 
the water from such vessels. Being unwil- 
ling to leave this objection unanswered, I 
resolved to put the truth of this principle to 
the test of direct experiment; which was 
done in the following simple manner. A 
small bell glass, being first filled with water, 
was inverted in six quarts of the same fluid, 
contained ina small tub. Things being thus 
prepared, the contents of the open vessel 
were agitated briskly; and the air which 
entered the water, found its way into the 
inverted glass, the upper part of which it 
occupied. The water of the tub was agitated 
by the motion of a whisk, or a bundle of 
slender twigs; it was sometimes taken up 
in a pitcher, and returned into the vessel 
quickly, from the height of a foot or more: 
