All 3 Uses of

periphery
in
Atlas Shrugged

The present generating equipment in our laboratory is capable of producing rays to cover-through the outlets which you may observe under the dome-the entire countryside within a radius of a hundred miles, a circle with a periphery extending from the shore of the Mississippi, roughly from the bridge of the Taggart Transcontinental Railroad, to Des Moines and Fort Dodge, Iowa, to Austin, Minnesota, to Woodman, Wisconsin, to Rock Island, Illinois.†
Chpt 3.3

Within the circle of a radius of a hundred miles, enclosing parts of four states, telegraph poles fell like matchsticks, farmhouses collapsed into chips, city buildings went down as if slashed and minced by a single second's blow, with no time for a sound to be heard by the twisted bodies of the victims-and, on the circle's periphery, halfway across the Mississippi, the engine and the first six cars of a passenger train flew as a shower of metal into the water of the river, along with the western spans of the Taggart Bridge, cut in half.†
Chpt 3.9

He was struggling not to allow into his mind the thought that was pressing against its periphery: What were the chances and how long would it take-according to the mathematical theory of probability-for primitive men, working by rule-of-thumb, to hit the right combination of parts and re-create the motor of this engine?†
Chpt 3.10 *

Definition:

the outside boundary or surface of something;

or more rarely: outside the area of main activity