October 2017, Davis, California. Forests are burning not far away, and it is still hot outside, almost 90 degrees. Wait a minute, ninety degrees? No wonder forests are burning. True, this is just another anecdote about vintage unit systems, also known as the “U.S. customary system”, but there is a lot more to discover. Did you ever hear about the unit called “slug” (not the mollusc)? I didn’t, until Greg handed me a spreadsheet template one scorching day in October 2017. A slug is defined as the mass that accelerates by 1 foot per second squared when a force of one pound is applied, so it comes out to roughly 14.59-something kilogram because one foot is 0.3048 meters and one pound (lbs) is 0.45359… kilograms.

The term “slug” was introduced by British physicist Arthur Mason Worthington before 1900, allegedly from the phrase “sodlock of metal”. There is even an inch-based counterpart to the slug, sometimes called a “blob” or “slinch”. And suddenly it makes so much sense that California legalized weed products. I imagine these units for sluggishly slow application speed (because of the many conversion bumps) might still be relevant when interpreting historic (somewhat scientific) literature – and for postdocs in the US.

For anyone curious, 90 degrees Fahrenheit is about 32°C. A British ton is 2,240 lbs, while a metric ton is 2,205 lbs (=264.174 gallons of water), and the US short ton corresponds to 2,000 lbs.

It is definitely time to introduce special ecohydraulic units: tuna fin strokes per second vs. trout fin strokes per second – who needs Hertz – 1/s – anyway? Or let’s define a measure of aquatic habitat availability, hack some quality weights into it, and call it WUA. OK, the WUA concept is going to need its own blog post.

• Cardarelli, F., 2003. Encyclopaedia of scientific units, weights and measures: their SI equivalences and origins. Springer, London, UK & New York, NY, USA.
• Klinkenberg, A., 1969. The American Engineering System Of Units And Its Dimensional Constant Gc. Ind. Eng. Chem. 61, 53-59. https://doi.org/10.1021/ie50712a010

Author: Sebastian Schwindt

Lives and works in Stuttgart, Germany - find out more at https://sebastian-schwindt.org and check out https://hydro-informatics.com