Complex sentence: Cesspits were often placed under cellar floors, or in the yard of a house, some of which had wooden chutes to carry the excrement from the upper floors into the cesspit, sometimes flushed by rainwater.
Split into simpler sentences: Cesspits were often placed under cellar floors or in the yard of a house. Some had wooden chutes to carry excrement from the upper floors to the cesspit, sometimes flushed by rainwater.