Jamieson, Fausset & Brown

wax warm--rather, "At the time when." ("But they soon wax") [UMBREIT]. "they become narrower (flow in a narrower bed), they are silent (cease to flow noisily); in the heat (of the sun) they are consumed or vanish out of their place. First the stream flows more narrowly--then it becomes silent and still; at length every trace of water disappears by evaporation under the hot sun" [UMBREIT].