The cause of the general lassitude he felt while climbing the stairs being purely mental, he became alarmed by its inexplicable character.

He hoped he was not sickening for anything.
He stopped on the dark landing to examine his sensations.

But a slight and continuous sound of snoring pervading the obscurity interfered with their clearness.

The sound came from his mother-in-law's room.

Another one to provide for, he thought--and on this thought walked into the bedroom.
Mrs Verloc had fallen asleep with the lamp (no gas was laid upstairs) turned up full on the table by the side of the bed.