The Collaborative International Dictionary of English v.0.48:

Stadia hairs \Sta"di*a hairs\ or Stadia wires \Sta"di*a wires\
   (st[=a]"d[i^]*[.a] h[^a]rz). (Surv.)
   In a theodolite, etc., horizontal cross wires or hairs
   equidistant from the central horizontal cross wire.
   [Webster 1913 Suppl.]