Page:Popular Science Monthly Volume 12.djvu/741

Rh made. It remains to obtain from these impressions the aerial vibrations which made them. Nothing is simpler. The plate A, with its point, P, is moved away from the cylinder by pulling toward you the lever H G. Then the motion of the cylinder is reversed till you have brought opposite to the point P the beginning of the series of impressions which it has made on the foil. Now bring the point up



to the cylinder; place against the vulcanite plate, B B, a large cone of paper or tin to reenforce the sounds, and then steadily turn the crank D. The elevations and depressions which have been made by the point P now pass under this point, and in so doing they cause it and the thin iron plate to make over again the precise vibrations which animated them when they made these impressions under the action of the voice. The consequence of this is, that the iron plate gives out the vibrations which previously fell upon it, and it talks back to you what you said to it.

By the following method we have just obtained several magnified traces on smoked glass of the contour, or profile, of the elevations and depressions made in the foil by the sonorous vibrations. On the under side of the shorter arm of a delicate lever is a point, made as nearly as possible like the point P under the thin iron plate A. Cemented to