Locking column, but not row

Hi all,
What I am trying to do is copy a formula by row and have the following changes.

When I copy the formula across the next formula should be.

=(I34-((E15-408)*1.138))*0.1

and the one after should be

=(I35-((F15-408)*1.138))*0.1

and so on and so forth - so the "i" bit should increase in number i34 - becomes i35

and the "15" bit should increase in letter so "E" becomes "F"