in Thoreau’s Journal:

Stood by the riverside early this morning. The water has been rising during the night. The sun has been shining on it half an hour. It is quite placid. The village smokes are seen against the long hill. And now I see the river also is awakening, a slight ripple beginning to appear on its surface. It wakens like the village.

It proves a beautiful day, and I see that glimmering or motion in the air just above the fields, which we associate with heat.