The springers are special arch stones. They are the first row of tapered stones that the rest of the voussoirs spring from
Their bottoms are level. Their tops (in a segmented arch) angle towards the centre point of the circle below the form.
Hopefully lot of them can be 'hand-dressed' from appropriate shaped rocks found in the tons of random material needed in the building of a dry stone bridge.
Some of them might need to be sawn.
This is done by finding a likely candidate stone for a springer and sawing straight lines at the right depth and correct angle, and then chiselling in the strips of material away. In the photo above you can faintly see the remaining saw lines after the material has been removed. A template was used to trace the angle on this big springer stone.
It is the angle and shape shown in this diagram.
It is best if the springers made from the template have some thickness at the nose rather than have the stone come to a point, so that they are stronger and less likely to break .
This nose will become part of the intrado which is the name for the underside of the arch.