The Cyanometer was invented by Horace-Bénédict de Saussure (Geneva 1740-1799) and is an instrument for measuring the blueness of the sky. He used Prussian blue to paint out a series of squares from light to dark which he held up to compare with the sky. He climbed Mont Blanc and used it to measure the sky form there.
Alexander von Humboldt a friend of Saussure took the instrument to Humboldt on his voyages and explorations in South America.
Here is my ‘Household’ version, I climbed Devils dyke to get my first reading – have a look at it on posts.