springing line of an arch

from The Collaborative International Dictionary of English v.0.48

Springing \Spring"ing\, n.
   1. The act or process of one who, or that which, springs.
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   2. Growth; increase; also, that which springs up; a shoot; a
      plant.
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            Thou blessest the springing thereof.  --Ps. lxv. 10.
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   {Springing line of an arch} (Arch.), the horizontal line
      drawn through the junction of the vertical face of the
      impost with the curve of the intrados; -- called also
      {spring of an arch}.
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