subtracting values of sequential rows

Once a minute I collect a particular statistic, the total count of jobs completed. I can report on the last 5 minutes' progress with the query below.

I would like to add on the the report the count of the jobs completed in that minute, which is of course the current minute's value of NCOMPLETED minus the previous minute's value of NCOMPLETED.

Can I do this with a single SQL query? i.e., can I reference the value of NCOMPLETED for "rownum - 1"?

select * from (

    select snaptime,ncompleted from op_thumbcounts     order by snaptime desc
) where rownum <= 5

SNAPTIME                  NCOMPLETED              (What I would like)
------------------------- ---------------------- 
19-AUG-07 00:17:00        2841878                 140
19-AUG-07 00:16:00        2841738                 169
19-AUG-07 00:15:00        2841569                 159
19-AUG-07 00:14:00        2841410                
19-AUG-07 00:13:00        2841220                

Many TIA!
Mark

-- 
Mark Harrison
Pixar Animation Studios