Lirael took a deep breath. Charter marks flowered in her mind, and she let them flow into her lungs and throat, circling there. Then she whistled, and the pure notes leapt up into the sky.
Answering the whistle, the river behind them darkened. Ripples and white peaks sprang up across the water and streaked across towards Finder and her waiting sail.
A few seconds later, the wind hit. The boat heeled over and picked up speed, the rigging adding its own whistle at the sudden pressure.

Lirael, Ch. 40, pg. 409