As a metaphor, the cloud seems easy to grasp: our data is somewhere in the ether, floating, drifting and wireless, available wherever and whenever we need it. It carries hints of childhood wonder; the term is evocative because it is the opposite of the hard, material world of plugs and cables, disk drives and superhighways. But the thing about a cloud, Tung-Hui Hu reminds us in his mesmerizing new book, “A Prehistory of the Cloud,” is that you can only see it from a distance. How did we come to place our faith in a symbol that is so ephemeral—all vapor and crystal? “Like the inaudible hum of the electrical grid at 60 hertz, the cloud is silent, in the background, and almost unnoticeable.” What might we learn if we try to trace its mellow outline against the sky?