The First Thermometer

And if you want to know the hotness of the air and the relation between [its hotness from] one day to another, prepare for yourself one of these two instruments, begins the passage shown at right in a seventeenth- century Hebrew text. A philosophy professor from the University of Arkansas, Jacob Adler, discovered that this description of a liquid-in- glass thermometer predates the earliest known by at least 20 years. The thermometer is essentially the same as those used today, except that it was filled with brandy rather than mercury. The illustration is from the book Ma’yan Ganim (A Fountain of Gardens), published in 1629 and written by the physician and rabbi Joseph Solomon Delmedigo.  The two engravings beside the Hebrew text show an air thermometer--an open-ended tube partly filled with water or alcohol--and a sealed liquid thermometer, at near right. The air thermometer was already well known at the time, but historians had previously credited the liquid thermometer to Ferdinand II, Grand Duke of Tuscany, in 1654. The small lettering to the right of the liquid thermometer says oleh, ascending, the direction of the brandy when the bulb gets warm.  Delmedigo does not claim to have invented the device, says Adler. In fact, he appears to have been more of a journalist than a scientist. In an age when scientific journals were unknown, he spread news of discoveries across Europe through his many letters to such eminent contemporaries as the astronomer Johannes Kepler. Adler suspects that Galileo, or perhaps a physician named Santorio Santorio, invented the device. Both taught at the University of Padua, where Delmedigo studied medicine.  Adler stumbled on Delmedigo’s text while working on a book about the philosopher Benedict de Spinoza, who apparently owned a copy of Ma’yan Ganim. History-of-science people mostly do not read Hebrew and wouldn’t think of a Jewish book to find the first publication of something of this sort, says Adler.  Even after parts of Ma’yan Ganim were translated into English by Isak Heilbronn in 1913, historians continued to overlook Delmedigo’s thermometer because a mistranslation suggested that the instrument had an open top, making it an imprecise and impractical instrument needing frequent refills to replace evaporated alcohol. Delmedigo’s thermometer was in fact accurate enough, says Adler, that you could measure the change in temperature from day to day.