Why did the narrator think that the barometer was useless?

The narrator believed the barometer to be useless because it was never accurate.

The weather is a thing that is beyond me altogether. I never can understand it. The barometer is useless: it is as misleading as the newspaper forecast.

There was one hanging up in a hotel at Oxford at which I was staying last spring, and, when I got there, it was pointing to “set fair.” It was simply pouring with rain outside, and had been all day; and I couldn’t quite make matters out. I tapped the barometer, and it jumped up and pointed to “very dry.” The Boots stopped as he was passing, and said he expected it meant to-morrow. I fancied that maybe it was thinking of the week before last, but Boots said, No, he thought not.

I tapped it again the next morning, and it went up still higher, and the rain came down faster than ever. On Wednesday I went and hit it again, and the pointer went round towards “set fair,” “very dry,” and “much heat,” until it was stopped by the peg, and couldn’t go any further. It tried its best, but the instrument was built so that it couldn’t prophesy fine weather any harder than it did without breaking itself. It evidently wanted to go on, and prognosticate drought, and water famine, and sunstroke, and simooms, and such things, but the peg prevented it, and it had to be content with pointing to the mere commonplace “very dry.”