An Early Victorian Mason’s Hygrometer or Wet & Dry Thermometer engraved to Bennett Watchmaker, 65 Cheapside, Maker to The Royal Observatory.
The instrument is comprised of two thermometers affixed by four brass clasps to a silvered U shaped scale plate. Both using a Fahrenheit scale, the left (dry bulb) registers 20 to 115 degrees whilst the right (wet bulb) measures from 15 – 115 degrees. The maker’s name is engraved to the top as follows, “Bennett, Watchmaker 65 Cheapside, Maker to The Royal Observatory”.
This rare and unusual thermometer set is contained within an immaculate red morocco leather case with original red velvet and silk lined interior. The hinged case is secured by two hook and eye catches which are both still present. As a testament to the craftsmanship of this piece, both catches are set upon brass washers to ensure that the movement of the catch does not rub the leather whilst being operated. This was a top quality instrument during its time and the care and attention is very evident.
Meteorological instruments such as these have a number of associated names, hygrometer meaning the measurement of moisture or water, the name psychrometer (cold meter) was used by the German inventor Ernst Ferdinand August (1795-1870) and during the same period in 1836, the Surgeon, John Abraham Mason was presenting his new, Mason’s Hygrometer. An article in the Record of General Science describes a “new hygrometer illustrated by experiments and a comparison of its results with Sir John Leslie’s”. Leslie had in 1805 developed an air thermometer based upon Galileo’s thermoscope. Sadly, Mason’s name is no longer commonly associated with his invention but its continued manufacture into the 1840’ and 50’s is testament to its superior accuracy.
It is now more commonly dubbed, the wet and dry thermometer relating to the comparison of two thermometers, one with a wet bulb and the other dry. This is essentially the same as Mason’s pattern where one thermometer is kept wet by means of a water soaked sock covering one bulb and the other kept dry. The evaporation of water from the sock has the effect of lowering the temperature reading on the wet thermometer and the difference against the dry thermometer is used to calculate relative humidity of the air.
This example by Bennett is a small portable version of the Mason’s hygrometer being incorporated into its own carrying case, a larger version with a stand is contained in the collection of The Museum of the History of Science in Oxford.