Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Removed, Understated, and Slightly Cynical
The narrator of the story is not the emotional type. Rather, he's pretty removed and uninvolved from what is going on, just telling it as "objectively" as possible (we'll call the narrator a "he," since the author is male). To the extent he has any "air," it's of being thoroughly unsurprised at what's happening. Check out the very first sentence:
"It's a remarkable piece of apparatus," said the officer to the explorer and surveyed with a certain air of admiration the apparatus which was after all quite familiar to him. (1)
It's all in the "after all." It's the very first sentence of the story, we have no idea who the officer and the explorer are or what the "apparatus" is, and yet the narrator's "after all" indicates that he himself finds it quite obvious. Of course the officer is familiar with the apparatus. This language has the effect of pushing aside the reader's curiosity only to increase it.
Here's another example from just a little later in the paragraph:
In any case, the condemned man looked so like a submissive dog that one might have thought he could be left to run free on the surrounding hills and would only need to be whistled for when the execution was due to begin. (1)
The narrator has in a stroke dehumanized a character for the reader and made us think of him as an animal, and yet the "in any case" makes this seem utterly unimportant. It's like the narrator is saying, "Yes, the condemned man's more dog than man. What of it? What did you expect?" The comment is so casual it's rather humorous.
As the story grows more troubling, the narrator's understatement remains:
The explorer considered the Harrow with a frown. The explanation of the judicial procedure had not satisfied him. (14)
The explorer has just learned that the condemned man is to be put to death in some bizarre machine without any trial or defense, without even knowing what he's guilty of, and it's said he's "not satisfied." The distance between the seriousness of the situation and the narrator's lack of reaction to it is so great that it's almost funny. (The narrator is so non-reactive that it's difficult to tell at times if he's really describing all that's going on in the explorer's mind). As the story continues to become more grotesque and disturbing, the narrator's understatement also has the effect of giving it a certain appearance of being normal. Which makes it seem all the more nightmarish.