Mournful means something sad
Tread means to trample on something to damage
The poem saying what happened to Lincoln (captain) and the USA (ship) is very bad and sad not good.
For the exact definition for mournful clik: http://dic http://dictionary.msn.com/find/entry.asp?search=mournful tionary.msn.com/find/entry.asp?search=mournful
For the exact information for tread clik: http:/ http://dictionary.msn.com/find/entry.asp?search=tread /dictionary.msn.com/find/entry.asp?search=tread