What did Tolkien mean by "The Eagles are a dangerous 'machine'"?

Question

I'm very confused by this quote, found in Sean Crist's article Could the eagles have flown Frodo into Mordor?

"The Eagles are a dangerous 'machine'. I have used them sparingly, and that is the absolute limit of their credibility or usefulness. The alighting of a Great Eagle of the Misty Mountains in the Shire is absurd; it also makes the later capture of G. [Gandalf] by Saruman incredible, and spoils the account of his escape." (Letter 210)

Here's how I interpret it:

The Eagles are problematic, and because of this, I have not used them much in my stories.

I don't understand this:

and that is the absolute limit of their credibility or usefulness. The alighting of a Great Eagle of the Misty Mountains in the Shire is absurd; it also makes the later capture of G. [Gandalf] by Saruman incredible, and spoils the account of his escape."

What exactly is "the absolute limit of their credibility or usefulness"? What does that mean?

The alighting of a Great Eagle of the Misty Mountains in the Shire is absurd

Why is that? And does this simply mean that he means that it's "absurd" for a Great Eagle to descent somewhere in the Shire? How so? Why is it absurd?

it also makes the later capture of G. [Gandalf] by Saruman incredible, and spoils the account of his escape."

I understand each word, and I know how the story goes, but don't grasp at all what is meant by this.

Unfortunately, a lot of Tolkien quotes from letters make very little sense to me in the way they are written. It's not that he uses very difficult words, but they seem to make no sense to my brain.

Answer 1

The Eagles are a deus ex machina: a way to solve a difficult problem the foreground characters can't solve themselves. Naturally a good writer wants to minimize the use of such a technique, because it's more interesting if the characters struggle their way out of the problem on their own efforts.

It's absurd to have them come to the Shire from the Misty Mountains to do a rescue, because that's something like the distance between Vienna and, oh, Dublin. It's too far.

But using them to rescue the dwarves + Mr. Baggins from goblins near the Misty Mountains is less a stretch -- it's close to their home -- and rescuing Gandalf from Saruman is also less of a stretch, if we're to believe that Saruman's area, which is close to the Misty Mountains, is also close to a place where the eagles live. It's still a stretch because Gandalf is human-sized and too big for a real eagle to pick up. So these must be super-eagles.

As I recall, they also picked up Frodo at a place far from their home, but IIRC they were in that area because they had joined the war. And Tolkien is always emphasizing that hobbits are small.

Answer 2

The use of "machine" is an archaic sense -- it has a history of being used to mean "plot device," which I have run across only in old literary criticism.

This is closely related to their status as "deus ex machina" -- they come out of the blue and fix the problem the heroes are in without effort on their part.