Note:  Do not rely on this information. It is very old.

Jolly Balance

Jolly Balance, a special form of sensitive spring-balance named after its inventor. A long, fine spiral spring is supported on a vertical stand, and bears a small scale-pan at its lower extremity. The stand carries a long, vertical strip of mirror glass, on which a scale is engraved. The amount of extension produced by any load in the scale-pan is read off on the scale, the mirror helping to avoid parallax (q.v.), and enabling the observer to take an exact reading. This balance is generally used in estimations of the density of solids or liquids.

where it is unnecessary to know the exact equivalent in weight-units of the extensions produced.