Slope board (if clinometer is not available).
"Manual of Military Training" by James A. Moss
This angle can be observed with a simple clinometer.
"Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 7" by Various
I met Captain Stace, R.E., to whom I lent a clinometer while he worked at this invaluable construction.
"The Secrets of a Kuttite" by Edward O. Mousley