The Collaborative International Dictionary
spring of an arch
Springing \Spring"ing\, n.
The act or process of one who, or that which, springs.
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Growth; increase; also, that which springs up; a shoot; a plant.
Thou blessest the springing thereof.
--Ps. lxv. 10.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.