Fig. 35.—PEA AND PIPE.
845. Why does a pea, into which a pin has been stuck, dance in suspension upon a jet of air blown through a pipe?
Because the jet of air, being slightly compressed under the convex form of the pea, by the weight of the pin, forms a concave cup of air, in which the pea rests.
In the case put, it is supposed that the pin is passed through the pea until its head comes in contact with it. The pin is dropped into the hole of the pipe, and the breath is then applied, the pipe being held upright. The pea will rise in the air, and be suspended upon the jet, while the point of the pin will rotate around the stem of the pipe. There are other methods of fixing the pin which alter the result, and require a different explanation to that given above.
LESSON XL.
846. Why does a mouse, painted upon one side of a card, and a trap upon the other, represent to the eye a mouse in a trap when the card is rapidly twirled upon a string?
Because the image of the mouse is brought to the retina of the eye before the image of the trap has passed away. The two impressions, therefore, unite upon the retina, and produce the image of a mouse in a trap.