1) "Projects",
http://www.bloodhoundssc.com/, bloodhound
SSC, accessed April 24 2017
2) "Marble",
https://en.wikipedia.org/wiki/Marble_(toy),
Public, April 24 2017 Web. April 24 2017
3) "Sizing
Game Marbles",
https://www.moonmarble.com/t-marbleinfo.aspx,
MoonMarbles.com, 2017. Web April
24 2017
4)"Densities
of Solids"
http://www.engineeringtoolbox.com/density-solids-d_1265.html
Web April 24 2017