in Thoreau’s Journal:

The heat to-day (as yesterday) is furnace-like. It produces a thickness almost amounting to vapor in the near horizon. The railroad men cannot work in the Deep Cut, but have come out on to the causeway, where there is a circulation of air. They tell with a shudder of the heat reflected from the rails. Yet a breezy wind, as it were born of the heat, rustles all leaves. Those drifting piles of clouds in the north, assuming interesting forms, of unmeasured rocky mountains or unfathomed precipices, light-colored and even downy above, but with watery bases, portend a thundershower before night.

Well, I can take shelter in some haven or under a bridge. It shall not spoil my afternoon.