On the river by Hubbard's meadow.

June 17.

A small thunder-shower comes up in the south-west. The thunder sounds like moving a pile of boards in the attic. We see the increasing outline of the slate-colored falling rain from the black cloud. It passes mainly to the south. We feel only the wind of it at first, but after it appears to back up and we get some rain.  

In the damp, warm evening after the rain, the fireflies appear to be more numerous than ever.

H. D. Thoreau, Journal, June 17, 1852

The fireflies appear to be more numerous than ever. Compare July 20, 1852 ("The stars are few and distant; the fireflies fewer still.”)