s are uniform movements, and are at regular intervals,
the vibratory action of the whistle, in case the trains were at rest,
would all be the same distance apart; but as the two trains were coming
together two things happened. At each moment your ear came nearer the
whistle, and the distance through which the sound had to travel
decreased. This made increasingly shorter waves, and not long, regular
waves, as when at rest. Short waves make a high pitch, and long waves
low pitch. After you passed the train the waves began to get longer, but
they increased in length more rapidly than when you were approaching
each other, so that if the whistle kept on blowing the waves would
finally get to be so long and so far below their original pitch that the
sound would cease.

"A little sketch will show this. (Figure 23.) The line A is the pitch of
the whistle; B its pitch when you first heard it; C shows the point
where you passed the whistle, and D shows how low the pitch was when it
died away."

CHAPTER IX

EXCITING EXPERIENCES WITH THE BOATS

During the nine months' life on the island all had the best of health.
The Professor grew strong, and he declared that his constitution was
more robust than it had been for years. They lived in the open much of
the time; their fare was plain and mostly devoid of sweets; the store of
honey which had been several times replenished, was the stock article in
the absence of sugar.

It was, therefore, a matter of surprise that Harry should complain of
having a tired and uncomfortable feeling, and would frequently lie down
during the day while in the workshop. The Professor was always at his
side during these periods, and while he had no instruments to enable him
to determine whether there was a high fever present, the flushed face of
his patient showed unmistakable symptoms.

"Do you think he has a very severe fever?" was George's inquiry, as the
Professor left Harry.

"It seems so, and in order to determine whether there is any change we
must at once set to work making a thermometer."

"We have neither mercury nor glass, and even if we had, how can we make
a tube for it?"

"That being the case, we must make a substitute for both."

"But we must have something which will expand."

"We can use iron for that. Get a piece of small steel bar, say two
inches long, and bend it in the form of a C. In the meantime I will make
a base to hold the thermometer."

"For your guidance I m