ragged-bottom in a multi-score book

[Adrien]

version 2.8.3

Hello,
I'm making a book wich contains a lot of different scores :

--- the "book" file : ---

\include "firstscore.ly"
\include "secondscore.ly"
...

\book {
   \header {}

   \score {
      \firstmusic
      \header {
         breakbefore = ##t
      }
   }

   \score {
      \secondmusic
      \header {
         breakbefore = ##t
      }
   }

...

}

--- end of "book" file ---


each of the piece is a file with this structure :

--- firstscore.ly ---

firstmusic = {
   %the notes
}

--- end of "firstscore.ly" ---


My problem is :
How can I tell Lily that I want to use the "ragged-bottom" function fore some 
of the scores and not for the others ?¨

Thanks

Adrien