“Can’t we stop and watch?” asked Joyce.  “I should love to see it go off.”

I shook my head.  “Unless I’ve made a mistake,” I said, “it will be much healthier round the corner.  We’ll come back and see what’s happened afterwards.”

By the aid of some delicate manoeuvring I brought the Betty round, and then getting into the dinghy pulled myself ashore.

It was quite unnecessary for my experiment to make any complicated preparations.  All I had to do was to dig a hole in the bank with a trowel that I had brought for the purpose, empty my stuff into that, and tip in the gunpowder on top.  When I had finished I covered the whole thing over with earth, leaving a clear passage for the fuse, and then lighting the end of the latter, jumped back into the boat and pulled off rapidly for the Betty.

We didn’t waste any time dawdling about.  Joyce seized the painter as I climbed on board, and hurrying to the tiller I started off down the creek as fast as we could go, taking very particular pains not to run aground.

We had reached the mouth, and I was swinging her round into the main river, when a sudden rumbling roar disturbed the peacefulness of the dawn.  Joyce, who was staring out over the stern, gave a little startled cry, and glancing hastily back I was just in time to see a disintegrated-looking tree soaring gaily up into the air in the midst of a huge column of dust and smoke.  The next moment a rain of falling fragments of earth and wood came splashing down into the water—­a few stray pieces actually reaching the Betty, which rocked vigorously as a minature tidal wave swept after us up the creek.

I put down my helm and brought her round so as to face the stricken field.

“We seem to have done it, Joyce,” I observed with some contentment.

She gave a little gasping sort of laugh.  “It was splendid!” she said.  “But, oh, Neil, what appalling stuff it must be!  It’s blown up half Canvey Island!”

“Never mind,” I said cheerfully.  “There are plenty of other islands left.  Let’s get into the dinghy and see what the damage really amounts to.  I fancy it’s fairly useful.”

We anchored the Betty, and then pulled up the creek towards the scene of the explosion, where a gaping aperture in the bank was plainly visible.  As we drew near I saw that it extended, roughly speaking, in a half-circle of perhaps twenty yards diameter.  The whole of this, which had previously been a solid bank of grass and earth, was now nothing but a muddy pool.  Of the unfortunate tree which had marked the site there was not a vestige remaining.

I regarded it all from the boat with the complacent pride of a successful inventor.  “It’s even better than I expected, Joyce,” I said.  “If one can do this with three-quarters of a pound, just fancy the effect of a couple of hundredweight.  It would shift half London.”

Joyce nodded.  “They’ll be more anxious than ever to get hold of it, when they know,” she said.  “What are you going to do?  Write and tell McMurtrie that you’ve succeeded?”