5 X 4 
On Water and Air . 
[August, 
ledture, he compared the particles of air to little springs 
pressing against each other. When the air was pressed 
these springs yielded, and the air was forced into a smaller 
space. When the pressure was relaxed these little spiral 
springs, as it were, expanded, and caused the air to expand. 
In this way he figured the mechanical adtion of the air, and 
he wrote some celebrated papers on what he called the 
weight and the spring of the air. There are one or two 
illustrations of this spring of air which I think will impress 
it upon your mind. One which I should like to mention, to 
you occurs to me just at the moment. . Sometimes in going 
up a mountain you notice a curious pain in the ear ; some- 
times, at intervals, you feel as if a kind of bubble of gas 
got into your ear : some persons are subject to this feeling, 
and some are not. The cause of this effect is this : — Here 
we have the passage of the ear, through which all sound, or 
the great body of sound, passes. That passage is stopped 
at a certain place by a membrane called the tympanic mem- 
brane, or, to use the common term, the drum of the ear. 
Beyond that there is another membrane. When the sound 
vibrations come into the ear they strike upon the drum of 
the ear, and the vibrations are transmitted to the membrane 
on the other side, and sent on to the auditory nerve, and 
from the auditory nerve to the brain, where they are trans- 
lated into sound. It is necessary that the same pressure 
should exist within the drum of the ear and without it. If 
the pressure were greater within than without it would bulge 
the tympanic membrane or drum-head outwards, and cause 
pain. If the pressure within were less than without, the 
excess of the pressure of the atmosphere outside would force 
the drum inwards, and cause pain ; and this is very fre- 
quently the case. I do not experience the pain myself, but 
I have often walked beside others who have experienced it. 
How is this pain to be avoided ? There is, among the won- 
derful adaptations that we see here, a tube from the mouth 
to the drum of the ear : that is the only passage to the ear. 
This tube is called the Eustachian tube. When you are 
climbing a mountain, and feel this inconvenience, simply 
go through the process of swallowing. The adt of swallow- 
ing opens the Eustachian tube, and allows the air to get in 
or to get out, as the case may be, and so establishes an 
equilibrium between the air within and the air without, and 
thus you have all the inconvenience removed. This is due 
to what Boyle called the spring of the air. 
There is another instance of the same spring which I 
dare say you are all well acquainted with. It is called the 
