In the drawing it is represented in an inclined position, being not quite drawn up, that the parts might more easily be seen.

At _d_ there is a small projection of the tin upward, which touches the clapper of the bell suspended above every time the plate passes up or down, and thus gives notice of its motions.
[Illustration] Of course the construction may be varied very much, and it may be more or less expensive, according to the wishes of the teacher.

In the first apparatus of this kind which I used, the plate was simply a card of pasteboard, from which the machine took its name.

This was cut out with a penknife, and, after being covered with marble-paper, a strip of white paper was pasted along the middle with the inscription upon it.

The wire _c c_, and a similar one at the top of the plate, were passed through a perforation in the pasteboard, and then passed into the board.