'Presently they were shrieking with laughter.'

In the novel Lord of the Flies by William Golding, there is the below sentence in bold that confuses me. What is the purpose of the word 'presently'?

“I got the conch,” said Piggy bleakly. He turned to Ralph. “I got the conch, ain’t I Ralph?”

Unwillingly Ralph turned away from the splendid, awful sight

“What’s that?”

“The conch. I got a right to speak.”

The twins giggled together.

“We wanted smoke-”

“Now look-!”

A pall stretched for miles away from the island. All the boys except Piggy started to giggle; presently they were shrieking with laughter.