silent sorrow in empty boats
Petra is an observant little girl by her very nature: she is the child of a mother who knows she is not preferred and a father who is preferred by everyone but does not care, except that she cares very, very much.
So Petra watches. She did not set the metronome in the study, but she is always hearing the metronome going, going, going.
Lydia walks by it, runs by it. Lydia pretends it is not there, but it is.
So Petra wants the metronome to stop, but is also afraid, quite afraid, of what will happen when it does.