penthemeric

 
After reading Richard Hamblyn’s book The Invention of Clouds, which describes the life and times of the man who invented the cloud-classifying terms cirrus, stratus, cumulus and nimbus, I popped over to Google Books to look at some old weather-geek magazines.

In Symons’s Monthly Meteorological Magazine, February 1885, I found on page 8 a letter from J.H. Hill of Yorkshire announcing: “I have invented a Table of Rain which I call a Penthemeric Table, a term which explains itself.”

In a penthemeric table, rainfall amounts for the first five days of the month are added together and the sum is published as a single number. Then there is another number representing the next five days of the month, and so forth. This sort of table is more compact than a table listing the 30 or 31 days of the month separately, but gives more detail than a single number representing the whole month.

The word penthemeric seems to have vanished from the face of the earth after this one appearance in print. But I couldn’t help thinking there must be another word in English meaning “an interval of five days.” My first guess, quinquediurnal, only produced two Google hits.

And then by googling for definition ‘period of five days’ I came across the word pentad, which is widely used in connection with rainfall data.

So it seems that J.H. Hill’s idea of publishing rainfall amounts in five-day sums became popular, or perhaps it was independently re-invented elsewhere, but the word he/she coined failed to survive. How sad.