Instrument and method for measuring any distance
A level, two cross-staffs and stretched threads set as similar triangles to find the distance to a wall K
A large diagram lays out a method for measuring any distance to a far wall marked K. Two 'crosses' (cross-staffs) carrying white blowpipe-pellet nail-heads are made horizontal with the small level drawn at upper right, then aligned along a stretched thread of 50 to 100 braccia. Sighting a thread from nail t so that the line S t runs straight to K, the surveyor reads off the proportion by which S n enters S l, which by similar triangles gives the true span f l within l K. Each cross is graduated one braccio per line.
On this page
The level for setting the cross-staffs horizontal
A 'level' (livello), drawn as a small A-frame with a hanging plumb at upper right, is used to make horizontal the crosses shown below. Each extremity of a cross is levelled with it before sighting.
Cross-staffs graduated one braccio per line
The crosses set up at the foot of the sheet are graduated so that each line marks one braccio. On each extremity a nail is fixed whose head is the shape and size of a white blowpipe pellet, made white to be as evident as possible for sighting.
Finding the distance to wall K by aligned sights
Place the left cross about 2 braccia above the ground; stretch a thread of 50 or 100 braccia back along line f h and plant the right cross so that the white heads a, b, f, h fall in one line. Tie a thread to the foot of nail t and, holding end e, move toward nail a until S t is straight with K; then as much as S n enters S l, so f l enters l K, giving the true distance.