In the 'Valaquenta' I recently discovered an interesting passage.
"When Manwe ascends his throne and looks forth, if Varda is
beside him, he sees further than all other eyes, through mist, and through
darkness, and over the leagues of the sea. And if Manwe is with her,
Varda hears more clearly than all other ears the sound of voices that call from
east to west, from the hills and valleys, and from the dark places that Melkor
has made upon Earth."
It occurs to me that this principle is at work in the magic
thrones that are on the hills of Amon Lahw and Amon Hen. It would appear
that;
a) Perhaps these hills are in some sense sacred to Manwe
and Varda, or perhaps they once exercised their power from this place to the
extent that it lingered. We can then see the Dunedain discovering a
'natural' feature and building the chairs to exploit it.
b) Perhaps the Dunedain, when new-come from Numenor,
retained the wisdom and/or skill to invoke the blessing/powers of Manwe and
Varda, fixing it in this spot. Your option as to whether this would be
considered a work of magic or the granting of a miracle.
c) The Dunedain, by their own magical prowess, were
able to fabricate an imitation of powers of Manwe and Varda.
d) It's an accidental coincidence.
I choose to make option A my utterly unsupported theory of
the day. Option B represents an unlikely intervention by the powers into
the affairs of men, while option C begs the question of why, with the skill on
hand, such things were not done more often (or in more convienient
locations. Option D, of course, is strictly for the
small-minded.