Presently the Adjutant came up and, stopping near, glanced along the rippling line that marked the curve of the bridge.
“These center pontoons look rather prominent, as if they’d been pushed upstream a foot or two,” he remarked. “Was that done by Captain Maitland’s order?”
“No, sir,” Dick answered with some awkwardness. “For one thing, I found they’d lie steadier out of the eddy.”
“They do, but I don’t know that it’s much of an advantage. Had you any other reason for modifying the construction plans?”
Dick felt embarrassed. He gave the Adjutant a quick glance; but the man’s face was inscrutable. Captain Hallam was a disciplinarian where discipline was needed, but he knew the value of what he called initiative.
“Well,” Dick tried to explain, “if you notice how the wash of the head-rapid sweeps down the middle of the pool——”
“I have noticed it,” said the Adjutant dryly. “That’s why the bridge makes a slight sweep. But go on.”
“We found a heavy drag on the center that flattened the curve. Of course, if we could have pushed it up farther, we’d have got a stronger form.”
“Why?”
“It’s obvious, sir. If we disregard the moorings, a straight bridge would tend to curve downstream and open out under a shearing strain. As we get nearer the arch form it naturally gets stiffer, because the strain becomes compressive. After making the bridge strong enough for traffic, the problem is to resist the pressure of the current.”