SQL CONSTRAINTS - Constraining time values from two attributes on a table.

Hi,
I was looking for a solution on how to write a constraint into a ' create
table ' expression that would ensure that one ' TIME ' attribute value
called arrival_time (declared as TIME) is allways constrained to have a
value that ensures it is allways after another attribute value called
departure_time (declared as TIME). 
Any know how this constraint would be written??

Any help on this would be much appreciated,
Thanks,
Chris.