A downloadable computer simulation which showcases a series of villages that surround an active volcano. The volcano is modeled after Mount Vesuvius and is getting close to erupting. The goal of the exercise is to preserve as many people...

This site gives a very clear discussion on the topic of closure of sets over an operation with many examples.

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A map exercise from 1872 for drawing North America. The map shows the general outline of the coasts, major waters and rivers, and a vertical profile cross-section of the continent from the San Francisco Bay area to Cape Hatteras....