Types of hygrometers; 18th century; to measure moisture content. Jean-Andre De Luc's whalebone hygrometer; and others by Horace-Benedict de Saussure; Benjamin Meggot Forster; Henry Kater and the Hungarian Machine. Copperplate engraving by Sidney Hall from Abraham Rees' Cyclopedia or Universal Dictionary of Arts; Sciences and Literature; Longman; Hurst; Rees; Orme and Brown; London; 1820.