Exploding water cannons

, and looked at Percy. “Water cannons.”
“What?” Percy asked.
“The cannons on the walls,” Frank said, “they draw water from the aqueduct. There’s a pump system — heck, I don’t know how they work, but they’re under a lot of pressure. If you could control them, like you controlled the river — ”
“Frank!” Hazel beamed. “That’s brilliant!”

Percy,” Frank said, “now would be good.”
More kids came across the battlements to laugh at them. A few ran to the nearest water cannon and swung the barrel toward Frank.
Percy closed his eyes. He raised his hand.
Up on the wall, somebody yelled, “Open wide, losers!”
KA-BOOM!
The cannon exploded in a starburst of blue, green, and white. Defenders screamed as a watery shock wave flattened them against the battlements. Kids toppled over the walls but were snatched by giant eagles and carried to safety. Then the entire eastern wall shuddered as the explosion backed up through the pipelines. One after another, the water cannons on the battlements exploded. The scorpions’ fires were doused. Defenders scattered in confusion or were tossed through the air, giving the rescue eagles quite a workout. At the main gates, the Fifth Cohort forgot about their formation. Mystified, they lowered their shields and stared at the chaos.

SoN ch.12