ith a
rather defiant air. We understood it perfectly, and did not breathe a
syllable. If my pupil had even dared to open his mouth, he would have
deserved to be annihilated.

All the details of this illustration are far more important than they
appear. How many lessons are here combined in one! How many
mortifying effects does the first feeling of vanity bring upon us!
Young teachers, watch carefully its first manifestation. If you can
thus turn it into humiliation and disgrace, be assured that a second
lesson will not soon be necessary.

"What an amount of preparation!" you will say. True; and all to make
us a compass to use instead of a meridian line!

Having learned that a magnet acts through other bodies, we were all
impatience until we had made an apparatus like the one we had seen,--a
hollow table-top with a very shallow basin adjusted upon it and filled
with water, a duck rather more carefully made, and so on. Watching
this apparatus attentively and often, we finally observed that the
duck, when at rest, nearly always turned in the same direction.
Following up the experiment by examining this direction, we found it to
be from south to north. Nothing more was necessary; our compass was
invented, or might as well have been. We had begun to study physics.

Experimental Physics.

The earth has different climates, and these have different
temperatures. As we approach the poles the variation of seasons is
more perceptible,--all bodies contract with cold and expand with heat.
This effect is more readily measured in liquids, and is particularly
noticeable in spirituous liquors. This fact suggested the idea of the
thermometer. The wind strikes our faces; air is therefore a body, a
fluid; we feel it though we cannot see it. Turn a glass vessel upside
down in water, and the water will not fill it unless you leave a vent
for the air; therefore air is capable of resistance. Sink the glass
lower, and the water rises in the air-filled region of the glass,
although it does not entirely fill that space. Air is therefore to
some extent compressible. A ball filled with compressed air bounds
much better than when filled with anything else: air is therefore
elastic. When lying at full length in the bath, raise the arm
horizontally out of the water, and you feel it burdened by a great
weight; air is therefore heavy. Put air in equilibrium with other
bodies, and you can measure its weight. From these observat