A composition of moderate length on any particular subject, or branch of a subject; originally implying want of finish, ‘an irregular undigested piece’ (Dr. Johnson), but now said of a composition more or less elaborate in style, though limited in range.
This definition of "essay" is taken from the Oxford English Dictionary. Beneath it the OED says: The use in this sense is apparently taken from Montaigne, whose Essais were first published in 1580.
A commentator on yesterday's post mentioned the name of Montaigne. That is what sent me to the definition above in order to verify my hunch: our English word "essay" does come from the French literary form developed by Montaigne. But Montaigne did not just invent the essay; he is The Essayist par excellence.
"Blog" is also in the OED: A frequently updated web site consisting of personal observations, excerpts from other sources, etc., typically run by a single person, and usually with hyperlinks to other sites; an online journal or diary. A fine definition, but more should be said.
The blog is a miniature essay. Too many blogs fit Dr. Johnson's definition: pieces of writing not reduced to order or harmony; not properly arranged or regulated; chaotic; confused.
In blogging we should aspire to the order and harmony of Montaigne's essays. He has shown us a way of writing that will help remove any blog's want of finish. Montaigne is a master of seeming spontaneous; he appears natural because of his immense art.
In whatever we post, let us learn from the master.