Traditionally, history of mathematics was more interested in the production of abstract concepts and major theories than in the one of methods and tools of calculation. As a result, it has often neglected to consider the mathematical practices of engineers. I want to sketch here a sample of these practices through the example of nomography in the second half of 19th century and first half of the 20th century, precisely at the time when the professional community of engineers structures itself and deepens its mathematical culture.