Decreasingly

from The Collaborative International Dictionary of English v.0.48

Decreasing \De*creas"ing\, a.
   Becoming less and less; diminishing. -- {De*creas"ing*ly},
   adv.
   [1913 Webster]

   {Decreasing series} (Math.), a series in which each term is
      numerically smaller than the preceding term.
      [1913 Webster]