"A cannon as big around as it is long," said Ned.
"And shoots bomb-shells," said I.
"That's it," said Phaeton. "Now here, you see, is a big mortar up-town; only, instead of shooting a bomb-shell, it shoots a car. This car has no wheels, and has a big knob of India-rubber on the end for a buffer. When you get it full of people, you lock it up tight and touch off the mortar. This dotted mark represents what is called the line of flight. You see, it comes down into another sort of mortar, which has a big coiled spring inside, to stop it easy and prevent it from smashing. Then the depot-master puts up a long step-ladder and lets the people out."
Ned said he should like to be the one to touch off the mortar.
"And why wasn't that a good plan?" said I.
"There are some serious objections to it," said Phaeton, in a knowing way. "For instance, you can't aim such a thing very true when the wind is blowing hard, and people might not like to ride in it on a windy day. Besides, some people have a very strong prejudice, you know, against any sort of fire-arms."
"There wouldn't be much chance for a boy to catch a ride on it," said Ned, as if that were the most serious objection of all. "But tell us about the real invention."
"The real invention," said Phaeton, "is this," and he took up the little frame we had seen him making. Taking an India-rubber string from his pocket, he stretched it from one of the little posts to the other, and fastened it.
"Now," said he, "suppose there was a fly that lived up at this end, and had his office down at that end. He gets his breakfast, and takes his seat right here," and he laid his finger on the string, near one of the posts. "I call out, 'All aboard!' and then——"
Here Phaeton, who had his knife in his hand, cut the string in two behind the imaginary fly.