Measuring a span with a piece of string

You can learn most of what matters about an arch with a ball of string, a stone and a notebook.

Tie the stone to the string. Stand at one springing, lower the stone to the water and mark the string. Walk to the other springing under the arch (or along the parapet, if the river disagrees) and measure the horizontal distance the same way. Span, rise, and the ratio between them.

A rise of about half the span means a semicircular arch and usually an early date or a builder in a hurry. A rise around a third of the span is the comfortable segmental shape most country bridges settled on. Less than a quarter and someone was showing off.

Write down the weather too. You will not remember it, and it changes what the stone looked like.