This device at Smith’s Tavern is a spring scale, which measured mass by reporting the distance that a spring deflects under a load. Spring scales measure force, which is the tension force of constraint acting on an object, opposing the local force of gravity. They are usually calibrated so that measured force translates to mass at earth's gravity. The object to be weighed can be simply hung from the spring or set on a pivot and bearing platform.
In a spring scale, the spring either stretches (as in a hanging scale in the produce department of a grocery store) or compresses (as in a simple bathroom scale). By Hooke's law, every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Weighing scales use a spring with a known spring constant and measure the displacement of the spring by any variety of mechanisms to produce an estimate of the gravitational force applied by the object. Rack and pinion mechanisms are often used to convert the linear spring motion to a dial reading.
There are two types of Mancur Scales: The sector, or V-spring Scale and the C-Spring Scale. The scale at Smith’s Tavern is a C-Spring Scale. The scale is made by forging an elliptical steel spring stripe, suspension rings, load hooks, and a brass dial plate with graduations. A C type scale can one hook or two hooks and to different graduation scales.