double hung sash window, any way to prevent the top from being moved down?
Here's what the previous owner of my house did to prevent the top sash from moving: Painted it liberally.  They will be sealed and painted shut.  You could also use caulking.  They make clear caulking too so it wouldn't be noticeable, or in case you have bare wood still that has survived unpainted for the past 98 years. As an added bonus that would keep the drafts down a little too!