The Wizard's Well

Colin and Susan encounter the Wizard's Well during their exploration of the Edge in the first chapter:

'...the children came upon a stone trough into which water was dripping from an overhanging cliff, and high in the rock was carved the face of a bearded man, and underneath was engraved:
DRINK OF THIS AND TAKE THY FILL FOR THE WATER FALLS BY THE WIZHARD'S WILL'