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Blackboard.

A diagram board used in schools and lecture-rooms for the public demonstration of problems, the exhibition of examples, and the illustration of propositions in natural philosophy, etc. They are prepared by closely joining together wellseasoned boards planed smooth, and painting them with several coats of black paint mixed with finely pulverized pumice-stone or similar material, so as to impart a slight roughness to the face, that the chalk employed in writing may leave distinct marks on the board and yet rub off freely.

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