ENaG Chapter XIV: "Theodolite Tangent"

This is a short one, so I'll quote the whole section.

THEODOLITE TANGENT.

IF a spirit-level or a theodolite is "leveled," 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 leveled, 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."

So, despite having seen evidence of curvature himself, the author again dismisses it as being a result of what he calls "collimation". Theodolites would not be much use if they were as inaccurate as the author implies, and collimation is actually a part of ensuring that accuracy, not a cause of inaccuracy.

This is a short one, so I'll quote the whole section.

Quote

THEODOLITE TANGENT.

IF a spirit-level or a theodolite is "leveled," 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 leveled, 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."

So, despite having seen evidence of curvature himself, the author again dismisses it as being a result of what he calls "collimation". Theodolites would not be much use if they were as inaccurate as the author implies, and collimation is actually a part of ensuring that accuracy, not a cause of inaccuracy.

In fact, both your scientific methods are wrong.  I have done that experiment, too.  I used a 1 km length of polythene pipe that was to being used for irrigation works.  The height/altitude was the same for all three points on the pipe.