is liberty has been to them the greates of calamities, the heaviest of curses a metaphor/simile?
It's definitely not a simile. Do you know why?
Could be metaphor, but ... judge for yourself:
http://library.thinkquest.org/J0112392/metaphor.html
http://volweb.utk.edu/school/bedford/harrisms/hyperbole.htm