PUBLIC RECORD OFFICE
Reference :-
885
BEPLERIC.O.
20 PUBLIC RECORD OFFICE, LONDON
ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO
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ranges, and names of various hills where ascertainable, telescopic sketches being added of any points likely to be difficult of recognition. Future points with alterna- tives could then be selected.
Magnetic bearings, and notes as to position and aspect of hill, best approach, nearest water, location and details of beacon, &c., were then entered in note-book. The above could be carried on during the beaconing even in bad weather.
cover.
Resection of position and plotting had usually to be done afterwards under
Beacons. After preliminary trials, the form of beacon decided upon was the quadripod, about 12 feet high, as described in the Topo. Text-Book, but of round timber, with top portion thatched, without vanes, and with only a short flag-pole where needed. The poles were about 4 inches in mean diameter, as straight as could be found, and of "Yasi" wood, which best resists rot and white ants. They were spiked to a short, thick, vertical piece at the top-stiffened by cross-pieces, and stout pickets driven in and fixed to each leg. Owing to the difficulty of getting perfectly straight poles the top portion is often not quite symmetrical, but this can be made up in the thatching.
These beacons have proved most satisfactory even for short rays, and in spite of gales, no move has hitherto been detected. The timber can be obtained and the beacon constructed by a party of eight men in one day.
Marks. Much labour, is saved with the above beacons by fixing the mark after the framework of the beacon has been erected and made rigid.
Rock appears to be seldom found at the surface of the hill-tops, so that mark- stones have to be buried. Both stones were heavy, and only roughly dressed near the mark; the lower one was embedded about 3 feet below, and the upper one flush with, the surface of the ground. They were adjusted so that each mark came under the point of the plumb-bob suspended from the centre of the top pole of the beacon.
The mark consisted of an inverted copper rivet, 0.75 inch long and 0.1 inch in diameter, cemented into a hole in the stone with point flush, and was protected by a few stones. After the point had been observed from with the theodolite, a cairn of stones was built above it.
These marks are easy to put in, and have been found quite efficient for their purpose. Guide marks for locating the point have not been found necessary.
Protection. The question of protection of beacons by law was raised in November, 1903, and it was decided that it would be sufficient to bring these under the existing Ordinance by granting me a Commission as a Land Surveyor of the Colony. No case has hitherto occurred of tampering with beacons.
(C.) THEODOLITE OBSERVATIONS.
Theodolite. A 6-inch two-microscope theodolite by Troughton and Sims was brought out from England. The telescope had a 1.9-inch object-glass, with a focal length of 13 inches, and was fitted with an extra direct eyepiece giving a power of 35, and a special diagonal hinged eyepiece for astronomical work. The hinge allows of change of face without removing the eyepiece. The spare fittings included two spare levels, an etched glass diaphragm, front reflector for night work, and star sights. The circles were divided to 10 min., and the micrometer heads to 5 seconds, which could be read by estimation to 0.5 second. The instrument boxes were carried in padded leather sling cases.
The instrument was thoroughly gone over in January and as accurately adjusted as possible. The usual adjustments were afterwards made daily before commencing work.
The microscopes allowed of no bodily motion, and inconveniently large index errors were found. This error amounted to nearly 2 feet in the case of the vertical microscopes, and could not be got rid of, whilst in order to make the readings of the horizontal microscopes correspond the comb had to be moved so that its index was near the limit of distinct vision. Some trouble was also caused by the horizontal microscopes not being fixed at right angles to the plate-graduation, which had to be remedied in one case by the insertion of a metal slip between the plate and the microscope-bracket. The reflectors were carried on the end of the object-glass cell by a screw-thread; this arrangement is not a good one, as when rotation is at all stiff, it is apt to involve rotation of the object-glass cell, and has caused considerable inconvenience.
A defect in these micrometers appeared to be the difference in hardness between the male screw of the micrometer, which is of steel, and the female screw, which is of comparatively soft brass. An examination of the screw shows the collection
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of brass dust on its thread and end. This must not only shorten the life of the apparatus, but is, I think, one of the main causes of variation in readings.
In intersections by the tangent-screws, which were also always made by screw- ing up against the spring, some slow residual motion took place, causing much inconvenience and difficulty in obtaining accuracy; this may also possibly be due to a similar cause.
The extreme damp, by condensation on the object-glass and eyepiece-lens, rendered frequent cleaning of these necessary, and this, often necessitating re- adjustments for runs, was a source of trouble.
Horizontal Angles-An odd number of zero points being desirable with a two-microscope instrument, it was decided in the base-extension figure to take five zero points (the settings being at intervals of 72°), with a swing on each face, whilst the R.O. was intersected at the beginning and end only of each setting.
The mean difference between F.R, and F.L. measurements of angles on the same zero, chiefly due to effects mentioned above, amounted to nearly 3 seconds, whilst the maximum difference between means on different zeros amounted to 12 seconds.
The resulting triangular errors are shown on attached hase-extension diagram, the maximum error being 2.5 seconds, and the mean (of three triangles) being 1.66 seconds.
The probable error of a single angle works out at 0.61 second.
Vertical Angles.-Observations were taken between 2 p.m. and 3 p.m. They included two sets, with observations on each face. The maximum difference between angles observed in two sets was 12 seconds, and the mean difference about 4.1 seconds. Refraction. The co-efficient of refraction for the locality, based on the four sets of reciprocal observations, is 21. On the Navua flats enormous differences in vertical refraction on different days were noticed. On some cloudy days the whole of the terminal signals were inter-visible for most of the day. On other days the vane only was visible in the evening.
The atmosphere, probably owing to its dampness, shows a large amount of lateral refraction, rendering great care in intersections necesary.
Shelter. The quadripod beacon is easily converted into an observation hut with a few canvas sheets, and with a well-thatched roof the instrument can be left on its stand. This was invaluable in the windy and rainy weather met with in the hills.
Cases. The leather cases suffered greatly from the weather, the leather becom- ing sodden and mildewed. The glue from the padding ran out through the stitching.
Oil-cloth coverings to all leather cases are necessary.
Huts. The building of a small Fiji house at a hill point, the main framework of which can be put up at the time of beaconing, whilst the completion by natives from the nearest village can be arranged for, was found invaluable. Though an additional expense and labour at the time they greatly facilitate the theodolite work, give better protection to personnel and equipment, and by enabling tents to be dispensed with, save expense in carriers.
Astronomical Observations.-Determination of an initial azimuth and latitude has been deferred until a greater choice of suitable points can be obtained, and the weather at night is more favourable.
(D) MEAN SEA LEVEL.
A straight, hollow bamboo about 13 feet long was fixed in a vertical position against, and near to the end of, the pier at Naitonitoni, its foot resting on the bottom about 4 feet below low water. A number of holes were bored at its lower end and measurement could be made by a straight graduated wooden rod with cross- piece at the top to rest on the top of the bamboo. This rod was inserted at frequent intervals during high and low water (daily during the period February 10th to March 10th), and the highest and lowest water-marks noted and measured, whilst
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