Instruments: described in the Saggi

Despite the chemical terminology and imagery evident on the title page of the Saggi, the experimental programme of the Accademia del Cimento was not primarily chemical in nature.  Nor, despite the consciousness of continuing the great Tuscan tradition of Galileo, was it focussed primarily on astronomy, cosmology, or physics.  Instead, it received its main impulse from one of the last of Galileo’s many direct disciples, Evangelista Torricelli, who also became his successor as mathematician to the Grand Duke of Tuscany.  More specifically, it derived much of its focus from Torricelli’s famous experiment of 1644, which proved, he claimed, that nature does not abhor a vacuum (as both Aristotle and Descartes maintained) and that air has weight. 

The Saggi demonstrate a similar orientation.  Their first part describes the instruments which dominate the experimental programme described in the body of the work.  Of the twelve sets of experiments they described in the Saggi, the first two measure air pressure with the use of mercury barometers, in the tradition of Torricelli, and investigate the existence of the vacuum and the weight of air in the footsteps of Boyle.  Then come three sets of experiments dealing with natural and artificial cooling and the effects of heat and cold. The sixth set investigates the compressibility, not of air, but of water, and the seventh uses a vacuum to challenge another Aristotelian idea: namely that fire naturally rises.

The original Italian Saggi di naturali esperienze is a lavishly illustrated book, with images of more apparatus than is found in the English translation.  Image 1 reproduces its first plate depicting six of the instruments described in its first section.

I-II. Thermometers. The first two are thermometers of a more or less familiar variety, the second of modest length (cf. Image 2), the first elongated.  Grand Duke Ferdinando II is himself credited with inventing sealed thermometers of this kind, to isolate the instrument from changes in humidity and barometric pressure.  The Florentine craftsmen’s ability to manufacture numerous, near-identical instruments, calibrated to the same standards, was the precondition for the Medici’s meteorological network, the first scientific community devoted to the systematic recording of meteorological information of a wide area.

II. Tall-stem thermometers, mid-17th century, height: 66-113 cm (Image 3). These instruments were divided into hundreds of degrees: black buttons indicate single degrees, white buttons every tenth degree, and the blue buttons every hundredth.  The Museo Galileo still possesses fifteen of these extremely delicate instruments, fundamental to many of the experiments of the Accademia del Cimento.

IV. Spiral thermometers, mid-17th century, height: 30-34 cm. (Image 4). Divided into as many as 420 units, these extraordinary instruments were designed to be less fragile than the tall-stemmed variety.  As the Saggi themselves reveal, they did not prove the best means of ‘determining correct and infallible proportions of heat and cold’, but were treasured instead ‘for the curiosity of seeing the liquid run the dozens of degrees, moved by the simple closeness of breath’  Four such instruments are preserved in the Museo Galileo.

V. Phial thermometer, mid-17th century, length: 65 mm (Image 5).  These instruments, allegedly invented by Grand Duke Ferdinand II himself, measure changes in temperature in a different way.  Each phial is filled with ‘acquarzente’ (i.e. alchohol) containing several small glass spheres of different densities. As rising temperature increases the volume and lowers the density of the acquarzente, first the less dense and then the more dense spheres rise within the phial. Compared to the highly attenuated tall-stem and spiral thermometers, this movement is very sluggish, giving this kind of thermometer the nickname ‘infingardo’, meaning slow or slothful.

VI. Condensation hygrometer, 19th century replica, height: 925 mm. (Image 6).  This reproduction corresponds closely to the one illustrated in the Saggi, and again probably devised by the Grand Duke. The Museo Galileo’s catalogue describes its operation as follows: ‘The truncated cone in the lower part was lined with glass, to make it impermeable, and filled with "snow or ice very finely ground." The humidity in the air, in contact with the iced glass, condensed, causing the formation of water drops that descended toward the apex of the cone, gathering in the graduated glass below. The higher the atmospheric humidity, the greater the condensation. The quantity of water gathered in the glass over a certain period of time was therefore a relative measure of the humidity.’  A video provides a still more detailed account.

Credits: Howards Hotson (November 2016).