From the fourteenth century onward, many instruments were invented for the purpose of measuring atmospheric humidity, that is, the quantity of water vapor in the air. The first hygrometers, known as hygroscopic hygrometers or hygroscopes, exploited the properties of hygroscopic substances that absorb and release the water in the air with slight changes in their dimensions and weight. Nicholas of Cusa suggested weighing wool on a scale. The weight of the wool, which varied with atmospheric humidity, would have given an indication of the air's hygrometric state. Leon Battista Alberti proposed the use of a sponge rather than wool; his idea was revived by Leonardo da Vinci. In the early seventeenth century, Santorio described several hygrometers: one of them measured the variation in the weight of a hygroscopic salt; another computed the elongation of a taut string carrying a weight at its center; a third used a coiled string connected to an index. Francesco Folli and Vincenzo Viviani used a string or strip of paper whose changing lengths indicated the variations in atmospheric humidity. The hygrometer later devised by Robert Hooke exploited the fact that the awn of a grain of oats will coil and uncoil according to the humidity of the air. Some instruments used whalebones, hair, ivory strips, swim-bladders, etc. In the second half of the eighteenth century, Horace-Bénédict de Saussure designed a hair hygrometer that became widely used. A popular model in the nineteenth century was the condensation hygrometer, which used the relationship between the temperature of the "dew point" and atmospheric humidity. The first instrument of this kind was invented by Grand Duke Ferdinand II de' Medici in the mid-seventeenth century. Among the condensation instruments most commonly used in the nineteenth century, we should note the one developed by John Frederic Daniell in 1820, and the one perfected by Henri-Victor Régnault in 1845. Another hygrometric instrument is the psychrometer, which consists of two identical thermometers: one with a dry bulb, the other with a bulb continuously wetted with a moist cloth. The drier the air, the greater the evaporation of water from the cloth, causing the so-called "wet" bulb to cool.