THE OBJECTION had been so often urged by illogical critics that tests upon inland waters were not satisfactory, that it was decided that observations be made upon the Gulf itself, the conformity of which to the contour of the earth no sane mind will question. Against the results of such observations, no subterfuge can be brought to bear. For this reason also, the air line was surveyed upon the Gulf coast.
Six and one half miles lie between the points of the mainland extending into the Gulf at Gordon's and Doctor's Passes. These points are long sand-bars, the
elevation of which is equal to the high tide of the Gulf. On the point at Doctor's Pass a large target 3½ feet square was fixed upon supports; the top of the target was just 5 feet above low tide.
On March 2, the mounted telescope was taken to Gordon's Pass, and the visual axis of the instrument was fixed at an elevation of 3½ feet above low tide. At this altitude above the low tide level, all of the surface of the target was visible, and the white line of the sandy beach lying beneath it was distinct. No convexity was observable at this elevation.
On the morning of March 3, at a time when the Gulf was calm, the observation was repeated. With the telescope fixed 2 feet above the water level, the target was still visible; the same at 18 inches, and finally by reclining at the water's edge, with the axis of the instrument 12 inches above the water's surface, the target was still in view.
Under the conditions of the last observation, if the water were convex the horizon would be only 1¼ miles distant, leaving 5¼ miles of the surface of the Gulf to decline downward,--amounting to 18 1-3 feet. As the target was only 5 feet above the low tide water level, it would be 13 1-3 feet below the line of vision. After deducting nearly one seventh of this declination to make up for the usual allowance for refraction, 11 1-3 feet would remain as the amount of depression of the target below a line extending through the visual axis of the telescope over the horizon, to the distance of 5½ miles beyond the horizon. All of these observations were repeated in the afternoon, with the same results.
A straight reach of 4½ miles was found upon the smooth waters of Naples Bay. At the most southern extremity a target of white cloth 29x30 inches was fixed upon an upright with cross-arms; the top of the target stood 2 feet above the high-tide mark, leaving a space of 4 or 5 inches to the water's surface.
On March 5, at time of high tide, the Staff sailed to the farthest point northeast from which the target could be seen with the telescope. To the naked eye, the target was entirely invisible. The horizon seemed to occult the lower limbs of the belt of mangrove trees constituting the background of the view.
Over the water at the point of observation, the telescope was fixed at an altitude of 30 inches above the water, and through it the target stood out in bold relief. The instrument was then lowered to within 18 inches, with the same observed results. Afterward, at the height of 10 inches above the water, the entire surface of the target was still visible.
Very careful observations were made and repeated with the telescope at this altitude. The target was clear cut and well defined, and even the space between the bottom of the target and the water was observable. Then, to make the test absolutely satisfactory and conclusive, the telescope was fixed upon the water's surface; with the instrument almost touching the water--indeed, it could not be placed closer without wetting the lenses--long and careful observations were made. There could be no mistake; the entire surface of the target could be seen, with a small dark line of the background appearing beneath it.
The terrestrial eyepiece was then exchanged for
the astronomical eyepiece of greater power. The target was increased in size, and the relations of the target and the water's surface and the background came out still more noticeably. The object glass is 3 inches in diameter; the axis of the telescope was 2 inches above the water. On the basis of convexity, the horizon would be but one half mile away--for the declination for one half mile is considered to be 2 inches--leaving 4 miles of surface to decline from the horizon point, amounting to 10¾ feet. The target would have to be higher than 10¾ feet above the water in order to be seen; as it was at an altitude of only two feet, it would be 8¾ feet below the line of sight.
These are the most satisfactory observations thus far made by the Geodetic Staff, because the tests were more crucial. The results were conclusive, as they afforded an ocular demonstration of the earth's concavity. A stake 2 feet in height was placed midway between the Observing Station and the target, with cross-bar at top of stake.
With the telescope at the same altitude, the cross-bar was observed to be a little below the top of the tar-get, with the target foreshortened by perspective to a breadth equal to about one half the length of the stake. With the visual axis of the telescope 2 inches above the water, the cross-bar was seen to be in line with the top of the target.
Besides this observation, an absolutely satisfactory view was had of the water surface itself. With the telescope placed absolutely level, the water appeared to slope gradually upward to the center of
the telescopic field. With the objective end of the telescope placed a little upward from the true level, and with the water still visible near the objective end of the instrument, the actual concavity of the water--a mid-way depression--was clearly observable.
This midway depression was at the point of the stake with cross-bar midway between the point of observation and the target, from which midway depression there was a gradual slope upward to the target. This view was obtained by the long, terrestrial eye-piece, and also by the astronomical eyepiece, the concavity through the latter being the more marked. There could be no mistake as to the concave arc; the. water was seen to be not convex; it did not appear to be a plane, but concave!