Who wrote the episode called "mercury falling"?
CREATE TABLE "table1_13740746_1" (
  "ep_no" real,
  "title" text,
  "director" text,
  "written_by" text,
  "original_air_date" text,
  "production_no" real
);
SELECT "written_by" FROM "table1_13740746_1" WHERE "title"='\"Mercury Falling\"';