A little over a year ago, I presented a paper at Mythcon 37 in which I attempted to trace the most likely sources for Tolkien’s well-known, but usually overlooked, literary collocation “Circles of the World.” The three I discussed were the Old Norse Heimskringla, the Biblia Vulgata, and the famous mappa mundi at Hereford Cathedral in the West Midlands. The two philological analogues here are the Old Norse kringla heimsins “circle of the world” and the Latin orbis terrarum “circle of the worlds”, and though I think I was successful in plausibly connecting both to Tolkien, I didn’t have a good transition from the one to the other — apart from the rather obvious fact that they mean the same thing. Well, thanks to serendipity — and to a diet of abstruse reading :) — I’ve now discovered a single source, one that Tolkien definitely knew, that connects these two precedents (as well as other conspicuously Tolkienian concepts) in the space of just a few pages. Curious?
I found it in Volume II of Jacob Grimm’s Deutsche Mythologie (1883–1888). This is in the James Steven Stallybrass translation, which he retitled Teutonic Mythology. I was reading Chapter XXV Time and World, when I came across this (with apologies for the choppy editing):
The ON. heimr is mundus, domus, and akin to himinn, himil (p. 698), as mundus also is applied to world and sky; heimskrîngla, orbis terrarum. Ulphilas renders οικουμένη [...] by [Gothic] midjungards; to this correspond the AS. middangeard [...]; the OHG. mittingart [...]; the OS. middilgard; the ON. miðgarðr [...]; and even a Swed. folksong [...] has retained medjegård. This is quite something, to see my two philological sources separated by no more than a comma! And compellingly followed by a laundry list of precursors for “Middle-earth”, one of which (the Greek οικουμένη) Tolkien mentioned explicitly in a letter .
As Grimm continues, more tantalizingly Tolkienian references appear. These include linking orbis terrarum to the idea of the “sea-girt world,” with cognates and sources aplenty, most notably the Old Norse miðgarðs ormr, the “serpent [...] coiled round the earth’s circumference, [...] evidently the ocean.” This, too, finds a point of connection with my paper, in an argument suggesting that the Hereford mappa mundi, also depicted as girded round by water, may find an echo in Tolkien’s Ekkaia, the Encircling Sea in his fictive geography.
Then, at the bottom of the page, Grimm surprisingly produces the Finnish ilma “air, sky”, which many of you will recognize for Tolkien’s use of the same word in Quenya, with an obviously related meaning (“starlight”); as well as in Ilmarin (“Mansion of the High Airs”), the palace of Manwë and Varda atop Taniquetil; and even Ilmarë, the handmaid to Varda. Tolkien, we’re pretty sure, would have gotten this word from the Finnish Kalevala, but he evidently saw it here too.
And even more, as Grimm continues over the next two pages, we see mention of the Völuspá, a self-acknowledged source for Tolkien; Yggdrasil, the World Ash, connected by many scholars to Tolkien’s own metaphoric and mythopoeic use of the Tree imago; and specific mention of the Old Norse dwarves “Dâinn [sic] and Dvalinn,” both names memorably recycled by Tolkien. All of this, from start to finish, in just three consecutive pages of a text Tolkien knew — and knew well. It’s therefore no great stretch to see Deutsche Mythologie as having gone into the same leaf-mould that produced Tolkien’s “Circles of the World.”
I love that! Now, to incorporate this into a revision of my paper!
 Grimm, Jacob. Teutonic Mythology, Volume II. Trans. James Steven Stallybrass. Dover Phoenix Edition. Mineola (NY): Dover, 2004, p. 794.
 Tolkien, J.R.R. The Letters of J.R.R. Tolkien. Ed. Humphrey Carpenter. Boston: Houghton Mifflin, 1981, p.239 (#183).