The inability of iron battle-ships to withstand this improvement in artillery had already set the naval architects of the world upon the work of constructing warships which would not attract the magnetic shell--which was effective even when laid on the bottoms of harbors--and Roland Clewe had been engaged in making plans and experiments for the construction of a paper man-of-war, which he believed would meet the requirements of the situation.
When Clewe determined to follow Margaret Raleigh's advice and give up for a time his work with the Artesian ray, his thoughts naturally turned to his automatic shell.

Work upon this invention was now almost completed, but the great difficulty which its inventor expected to meet with was that of inducing his government to make a trial of it.

Such a trial would be extremely expensive, involving probably the destruction of the shell, and he did not feel able or willing to experiment with it without governmental aid.
The shell was intended for use on land as well as at sea, against cities and great fortified structures, and Clewe believed that the automatic shell might be brought within fifty miles of a city, set up with its trough and ram, and projected in a level line towards its object, to which it would impel itself with irresistible power and velocity, through forests, hills, buildings, and everything, gaining strength from every opposition which stood in the direct line of its progress.
Attacking fortifications from the sea, the vessel carrying this great projectile could operate at a distance beyond the reach of the magnetic shell.
Now that the automatic shell itself was finished, and nothing remained to be done but to complete the great steel trough in which it would lie, Roland Clewe found himself confronted with a business which was very hard and very distasteful to him.

He must induce other people to do what he was not able to do himself.

Unless his shell was put to a practical trial, it could be of no value to the world or to himself.
In one of the many conversations on the subject; Margaret had suggested something which rapidly grew and developed in Roland's mind.
"It would be an admirable thing to tunnel mountains with," said she.