Theodolite Tangent

THEODOLITE TANGENT.

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

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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."