This is a standard theodolite as made by Troughton & Simms in the second quarter of the
19th century. The horizontal circle is silvered, graduated to 20 minutes, and read by two verniers
with magnifiers to 20 seconds. The vertical arc is graduated and read in a similar manner. The
scale "Diff. of Hypo. & Base" that appears on the back of the vertical arc is used to correlate the
angle of elevation or depression with horizontal distances when surveying sloping ground. The
"No. 1" was probably added by a former owner.