Vintage hydrometers pair of

Vintage hydrometers pair of marked fisher scientific co Brief History
Chester G. Fisher founded the Scientific Materials Company on May 6, 1902, in Pittsburgh, Pennsylvania, when he was only 20 years old. As a recent engineering graduate, he saw the need for a company that would supply the scientific tools for Pittsburgh’s many industries, including the burgeoning steel business. Shortly after, in 1904, the company published its first catalog. The business was officially renamed the Fisher Scientific Company in 1925 to set it apart from the many new companies springing up with generic names. The Fisher Scientific catalog would become a staple in labs all over the world. 
History. The hydrometer uses Archimedes' principle that a solid suspended in a fluid is buoyed by an equal force to the weight of the fluid displaced by the submerged part of the solid. It is assumed Archimedes built and used a hydrometer as early as the 3rd century BC.
32 cm long in original boxes we believe they date from 1950s but could be earlier would make  lovely gift