Photo courtesy American Stirling Company This engine can run using only the heat from your hand.

Photo courtesy of Saltfish.net.

Photo courtesy of the Weegee Collection.

Photo courtesy of Virginia Wing Above lower right China Kantner.

Photo courtesy of Steven G. de Polo.

Photo courtesy of the town of Gila Bend, Arizona.

Photo courtesy of Jahrundert Freemason initiation ceremony, 18th century.

Photo courtesy Caterpillar Hydraulic machines use fluid to transmit force.

Photo courtesy StrawBale.com A modern straw bale home before plaster is applied.

Photo courtesy of the Washington Post.