Inertia Demonstrator
   We still do this demonstration today. 
A stiff file card is placed on a pillar and a marble is balanced atop the card. A quick blow with the hand on the edge of the card knocks it out of the way, and the marble drops into a small depression in the top of the tower. 

   Since the hand or finger is unreliable, here are four solutions to the problem of snapping the card. At the right is a device on display at the National Museum of American History of the Smithsonian Institution, and on the left is an inertia demonstrator at Middlebury College. 

   At the left below is a device at Grinnell College in Iowa, and at the right is one from Union College in Schenectady, New York.

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