ake a drawing (Figure 24), in which A is the base,
about five inches long, three inches at its widest end, and an inch wide
at the narrow end. This should be made of a thin piece of hard wood.
Bore a small hole in each end of the C-shaped piece. The next thing is
to make a pointer (B) nearly as long as the base, pointed at one end,
and provided with two holes at the other. The pointer is attached to the
base by a pin (D). One end of the C-shaped piece of metal is then hinged
to the other hole (E), and the other end of the C-shaped piece is
hinged, as at F, to the base. You will now see that if the ends of the
C-shaped piece spread apart the least bit the long end of the pointer
will swing over to the other side of the base."

[Illustration: _Fig. 24. THERMOMETER_]

"Do you intend to make the thermometer show the exact degrees of heat we
really have?"

"Yes; as nearly as possible."

"Why can't we make it exact!"

"For the reason that to make what is called the Fahrenheit scale we
should have freezing weather. The scale adopted by Fahrenheit was an
arbitrary one. He determined it in this way: The temperature of his body
was taken as one point in the scale. For zero he took the lowest
temperature observed by him in the year 1709. As the temperature of his
body was 86 he made a scale with 86 degrees on it, and then when he
observed ice melting in water he put his thermometer in and found it
registered at 32 degrees. It was not a very scientific way of doing it,
but it answered the purpose, as, of course, temperature is merely a
relative thing."

"But isn't there another scale to measure heat by?"

"The Centigrade method is on a more scientific basis. It is determined
by taking the freezing and the boiling points of water, and dividing the
scale into 100 degrees between those points."

It required the work of several hours to make the device as here shown,
so that the tension of the C-shaped piece would hold the point to one
side. The temperature of the atmosphere was about 65, as nearly as the
Professor could judge, but when the C-shaped piece was held in the palm
of the hand, the pointer moved to the lower edge of the base piece, and
a mark of 80 was put there as the starting point. As they had no
immediate use for a scale beyond 110 degrees, the end of the base was
marked off, as you see in regular divisions.

The C-shaped piece was put under the tongue or in the armpit, so that
the temperature of Harry could b