As I did so, a low, rumbling sound, like chariot-wheels rolling afar off, broke suddenly on our ears.
"Thunder," remarked Mr.Challoner quietly.

"I thought we should have it.

It has been unnaturally warm all day.

A good storm will clear the air." In my brief backward look at Heliobas, I noted that when that far-distant thunder sounded, he grew very pale.

Why?
He was certainly not one to have any dread of a storm--he was absolutely destitute of fear.