in Thoreau’s Journal:

No one to my knowledge, has observed the minute differences in the seasons. Hardly two nights are alike. The rocks do not feel warm to-night, for the air is warmest; nor does the sand particularly. A book of the seasons, each page of which should be written out-of-doors, or in its own locality wherever it may be.

[Later the same June, Thoreau used his book A Week on the Concord and Merrimack Rivers to explain what he meant by a book written “out-of-doors:”]

June 29, 1851

I thought that one peculiarity of my “Week” was its hypaethral  character, to use an epithet applied to those Egyptian temples which are open to the heavens above, under the ether.  I thought that it had little of the atmosphere of the house about it, but might have been written wholly, as in fact it was to a great extent out of doors.  I was only at a late period in writing it, as it happened that I used any phrases implying that I lived in a house or led a domestic life.  I trust it does not smell so much of the study and library, even of the poet’s attic, as of the fields and woods, that it is a hypaethral or unroofed book, lying open under the ether, and permeated by it, open to all weathers, not easy to be kept on a shelf.