er by a third pipe
in which there is a third tap. I will first blow one bubble and shut it
off with the tap 1 (Fig. 22), and then the other, and shut it off with
the tap 2. They are now nearly equal in size, but the air cannot yet
pass from one to the other because the tap 3 is turned off. Now if the
pressure in the largest one is greatest it will blow air into the other
when I open this tap, until they are equal in size; if, on the other
hand, the pressure in the small one is greatest, it will blow air into
the large one, and will itself get smaller until it has quite
disappeared. We will now try the experiment. You see immediately that I
open the tap 3 the small bubble shuts up and blows out the large one,
thus showing that there is a greater pressure in a small than in a large
bubble. The directions in which the air and the bubble move is indicated
in the figure by arrows. I want you particularly to notice and remember
this, because this is an experiment on which a great deal depends. To
impress this upon your memory I shall show the same thing in another
way. There is in front of the lantern a little tube shaped like a U half
filled with water. One end of the U is joined to a pipe on which a
bubble can be blown (Fig. 23). You will now be able to see how the
pressure changes as the bubble increases in size, because the water will
be displaced more when the pressure is more, and less when it is less.
Now that there is a very small bubble, the pressure as measured by the
water is about one quarter of an inch on the scale. The bubble is
growing and the pressure indicated by the water in the gauge is
falling, until, when the bubble is double its former size, the pressure
is only half what it was; and this is always true, the smaller the
bubble the greater the pressure. As the film is always stretched with
the same force, whatever size the bubble is, it is clear that the
pressure inside can only depend upon the curvature of a bubble. In the
case of lines, our ordinary language tells us, that the larger a circle
is the less is its curvature; a piece of a small circle is said to be a
quick or a sharp curve, while a piece of a great circle is only slightly
curved; and if you take a piece of a very large circle indeed, then you
cannot tell it from a straight line, and you say it is not curved at
all. With a part of the surface of a ball it is just the same--the
larger the ball the less it is curved; and if the ball is very larg