Art. XL.—The Wellington Tide-gauge.
[Read before the Wellington Philosophical Society, 1st July, 1908.]
The Wellington tide-gauge differs materially from all other tide-gauges, and, as its design avoids most of the sources of error in the usual patterns, an account of its great advantages is now submitted; but in order to appreciate these advantages it will be necessary to refer briefly to the essential features of the usual forms.
In vol. xvi* of the G.T. Survey of India it is stated that “the object aimed at in any complete system of tidal observations is to obtain the height of the tide above some fixed mark or datum for every instant of time during a more or less extended period.… This object is attained graphically by causing the rise and fall of the Water to communicate its motion, by mechanical means, to a pencil which traces a line on paper wound round a drum turned by clockwork once in twenty-four hours.… An instrument such as above briefly described is called a self-registering tidegauge, and of these various forma have from time to time been constructed. The best form is, according to the opinion of Sir William Thomson, one in which the drum is inclined to the vertical, as this enables the friction between the pencil and the paper to be nicely regulated. The pattern almost ex-
[Footnote] * “Details of Tidal Observations,” p. 9.
clusively used in India is that known as Newman's pattern, in which the drum is horizontal, the only exception being a small gauge at Prince's Docek, Bombay, where the drum is vertical.”
Full details of the Indian gauges are given in vol. xvi, while a description of the Newman gauge is also given in Baird's Manual.* The horizontal drum of the Newman gauge is 5 ft. 3 in. long and exactly 24 in. in circumference. The drum revolves by clockwork once a day, and has attached to it the record-paper. A pencil moves in a slide along the top of the drum and to the pencil-carriage a wire is attached which is actuated by the float through cog-wheels. In this arrangement there are many defects: “First, the drum for the diagrams was not quite circular in section, the ellipticity in some cases being very marked. Secondly, the drum was not stable in every position of its revolution—i.e., when disconnected from the clock and turned by hand it assumed a certain position of its own accord.”
To overcome the first defect it was necessary to make the clerk in charge mark the exact position of the pencil on the diagram at four different times of the day, then to redivide the diagram from these marks. Thus the whole advantage of a ruled diagram was lost, as also many of the advantages of a sell-registering gauge, as constant attention of the clerk in charge is required. The second defect was ingeniously overcome by a special backlash weight. There were many other defects, particularly in the adjustment of the drum in relation to the clock, and to overcome all these defects so many additions were necessary as to render the gauge a complicated piece of mechanism requiring skilled attention.
In the Indian gauges constant attention is required to prevent the error of the clock becoming serious. The duties of the clerk in charge of an Indian tidal observatory involve constant attendance at 7 and 10 a.m. and 4 and 6 p.m. daily, and where labour is cheap (£2 to £2 10s. per mensem) this attention can be easily arranged for; but in other parts of the world gauges requiring less attention are necessary.
The Wellington tide-gauge was designed by Mr. William Ferguson, M.A., M.Inst.C.E., formerly Engineer and Secretary to the Wellington Harbour
[Footnote] * “A Manual for Tidal Observations,” by Major A. W. Baird. London: Taylor and Francis, 1886.
Board. It was erected in 1887, and is at present situated in N shed, on Jervois Quay, Wellington, as shown in fig. 1.
The general design of the gauge is shown in fig. 2. The cylinder or drum is 4 ft. long and 0.4 ft. in diameter.* It is horizontal, and carries a wheel 2.4 ft. in diameter on the end of its axle. The float is attached by a band to the circumference of the wheel. A smaller wheel carries a counterweight to keep the float-band always taut. The pencil-carriage moves horizontally at a uniform rate, and is in train with the driving-chain of the eight-day lever clock. Its motion is approximately 6 in. a day, so that one week's record is obtained on the drum.
It will be seen that the design of this gauge avoids all the defects of the Newman and other forms of gauge. Thus, owing to the smaller diameter of the drum, it can be readily turned truly circular, and, as it is the amount of motion under the pencil that has to be measured, small ellipticity of
section does not vitiate the record, and, although the drum is naturally turned as evenly as possible, any defect in balancing is immaterial, as the counterweight can be adjusted to overcome it. It is, however, in the adjustment of the time that the greatest improvement becomes apparent. The clock is wound up once a week, and when wound up the pencil-carriage is drawn to the left end of the drum. A new record-paper is attached to the drum, which is then revolved underneath the pencil, and on the line so drawn the correct local mean time is written, independent of what time is shown on the clock. At the end of the week the drum is again turned round,
[Footnote] * The range in Wellington is about 4 ft. or 5 ft., so that when used elsewhere the ratio of the diameters of the drum and wheel must be altered to keep the record within practical limits.
so that the pencil draws a second line: the correct local mean time is written on this line, and again no record is made of the time shown on the clock. In fact, the sole purpose of the clock is to give uniform motion to the pencil, and so long as the clocks rate does not materially vary the record will be correct. And at any time during the week a check line can be similarly drawn, and the correct local mean time noted on it. A check on the rate of the clock is obtained by measuring the distance between the end lines on the record.
Fig. 3 shows a portion of the record of this gauge, beginning 1908, July 11. The first line was drawn at 9.55 a.m., and a check line is shown at 11.45 a.m. The last line (not shown) was drawn on July 18 at 9.20 a.m., while other check lines (not shown) were drawn on July 14 at 12.40 and 13.00. The lines showing midnight are drawn in by scale from the first and last lines on the diagram.
Particulars of the Wellington gauge were recently sent to Sir, G. H. Darwin, who replied, “I never heard of a tide-gauge such as that you describe, but there seems no reason why it should not work very well.”
Harmonic Tidal Constants.
Through the courtesy of Mr. Thomas Wright, of “Lyndhurst,” Victoria Drive, Eastbourne, Sussex, the harmonic tidal constants for Wellington and Auckland are here recorded. Mr. Wright states, “They have been obtained by the aid of Government grants from the Royal Society, and were deduced by methods devised by Sir G. H. Darwin, and will be presented for publication in the Proceedings of that Society. The constants for Wellington were obtained from twelve months' observations, the epoch to which
the figures refer being 1901, January, 1 d. 0 h. The constants for Auckland were obtained from observations covering a period of the same length, the epoch being 1900, May 1.”
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Auckland. Epoch 1900, May 1||Wellington. Epoch 1901, January 1.|
|Ao = 5871 ft||Ao = 37.74 ft|