IF a spirit-level or a theodolite is "levelled," and a given point be read on a graduated staff at the distance of say 100 chains, this point will have an altitude slightly in excess of the altitude of the cross-hair of the theodolite; and if the theodolite be removed to the position of the graduated staff, again levelled, and a back sight taken of 100 chains, another excess of altitude will be observed; and this excess will go on increasing as often as the back and fore sight observations are repeated. From this
it is argued that the line of sight from the theodolite is a tangent, and, therefore, the surface of the earth is spherical. The author has made experiments similar to the above, and found it to be as stated; but the cause is not that the line of sight is a tangent, but the same "collimation" as that referred to in the section on "Spherical Excess."
264:1 "Marine Advertiser," Sept. 19th 1871.