OTAGO HARBOUR MOUTH

Evening Star, Issue 21077, 14 April 1932, Page 5

OTAGO HARBOUR MOUTH

NEW LEADING BEACONS Nearly all the works of man have to pass the test of actual use. The steamer must make a trial trip to show that she can travel at the speed contracted for. The locomotive has to go on to the service track and draw the load for which it was designed. The pump is not paid for until it works to its theoretical power. But there are exceptions, even in engineering. One of these is the construction of beacons and guiding lights. Their planning on theory must come out right in practice, because the preliminary study as to bearings and tides and visibility takes into account all the elements of variableness. The members of the Otago Harbour Board’s engineering staff who recently set up new leading lights at the Heads knew exactly beforehand that this work must be all right. Consequently the public has not watched the results. None of the board’s self-appointed critics have made a_ trip to the Spit or Harrington Point in the hope of ferreting, out a defect upon which a “ I told you so ” could be based. Nevertheless, though shipmasters and all concerned have rested content on the engineering skill and knowledge brought to bear, plus the faithfulness of expert workmen and labourers, there must be many points about the operation respecting which all the facts are not generally known, and it is the purpose of this article to supply information as to some of those matters. Incidentally, it may . be recorded that it was a real pleasure trip, and one available to all and sundry at slight cost. The ordinary motor run on the Peninsula is one way by the upper and the other way by the lower road, with Portobello as the terminus. By than tour one enjoys a delightful prospect on either hand. It is well worth while. But to get a true idea of the expansiveness and the romance of the Otago Harbour it is necessary to continue the jaunt beyond the settled and trimmed stretch of road and gy on to a view of the country in and beyond Lower Portobello, where the dwellings are only here and there, where there is rich land awaiting intensive cultivation, where in the perhaps not distant future many of our town-dwellers who desire restful retreats will be building houses that fit in with the scenery. Nobody need be frightened about the road. It is tar-sealed from the Anderson’s Bay bridge to Macaiidrew Bay, thence to Portobello dressed with screenings, thence in fair order for a country road as far as the parking area near -w Kaik, and from that point on to the lighthouse the going is quite safe in daylight for a sound car with a sober driver.

The one notable change in the countryside from the Kaik onwards in recent years is that the steep slope between Harrington Point and Wickliffe Bay is now green interspersed with white. ' Twenty years ago the slope seemed to be pure wind-blown sand, and was so on the surface, the whiteness making a* recognisable landmark. Nowadays the accumulation of sand has ceased, or appreciably diminished, and marram and lupin pretty well obliterate the whole face. It is a marvellous transformation that may be the prelude to closer settlement. THE TWO BEACONS.

All this', however, by the way. The subject in hand is the repositioning of the leading beacons so as to accurately direct navigators clear of the sandbanks when approaching and' entering the Heads. There are two such beacons, each fitted with Aga flashing lights. The outer one, defining the centre of the channel, is abreast of Howlet Point, where the channel is approximately 600 ft wide. This beacon is a four-pile dolphin carrying a red triangular structure 16ft high and 10ft wide at the base. Timber piles 45ft long were used. It shows a flashing white light for O.Ssec, and is then dark for o.7sec, making sixty flashes per minute. The height of this light above high water mark is 20ft, and it is visible to an approaching vessel at a distance of five miles. ' The inner beacon, on the rock at the foot of the cliff near Harrington Point, is a white triangular structure with a rod stripe down the centre. It is 25ft high and 16ft wide at the base. The white flashing light shows for 2.ssec, and the’ darkness is for 2.ssec, the flashes being twelve per minute. The height of this light is 64ft above high water mark, and is visible for ten miles, the arc of visibility 36dcg on each side of the leading line. The two beacons arc about threequarters of a mile apart. A navigator picks up the channel beacons before breasting the inner leading beacon. The two old beacons on the land are not dismantled, but efface'Jr By being painted green of a shade that exactly matches the vegetation, so that they are not seen unless by a person close to who has sharp eyesight. They are in the meantime dummies, but are leftstanding on the off chance that some shifting of the channel in the future may cause them to be brought into usd again. THE AGA LIGHT. Some of our readers will no doubt bo interested in a few particulars about this Aga light, seeing that our harbour is served by it, using the Galen system. By such means any desired light power and character may be obtained. An Aga signal will act automatically and uninterruptedly for any predetermined length of time without any attendance whatever, and, a consequence, with a minimum of working expenses. With regard to intensity as well as intrinsic brilliancy, the Aga light is far superior to all systems hitherto used in lighthouse service. The illuminant of the Aga light is dissolved acetylene—that is, acetylene completely purified and dried and safely stored in easily transportable steel cylinders, called gas accumulators. The special character of light is obtained through the Aga flasher (Daleu’s patent). The determining factors are:—(l) The desired range of light; (2) the desired light character; and (3) the desired service capacity. To give the desired range the light must have ,a certain light power, and a certain height of focal plane. The burner and the lens must be selected accordingly. The size of the burner combined with the light character chosen determines the daily gas consumption. The. capacity of the gas storage is determined by the length of time during which the light has to serve without recharging. The gas storage is composed of one or more gas accumulators holding sufficient gas for the required time. Acetylene alone under pressure is explosive, but dissolved m acetone and stored in receptacles (gas accumulators) filled with a porous substance is absolutely nonexplosive.

The Aga light apparatus consists of governor, flasher, and burner stand, with burner and pilot burner. The Aga governor (Dalon’s patent) reduces the high variable pressure .of tbc gas coming from tbc accumulator to a low constant one suitable for the ilashcr. From the governor the gas passes through two different pipes, one leading to the flasher, the other to a pilot burner placed on the main burner. The

Aga, flasher produces distinct and characteristic light and dark periods by automatically opening and closing the gas passage to the burner. When a predetermined quantity of gas has passed into the flasher, causing a leather membrane to use a certain amount, the inlet valve in the flasher closes and the outlet valve to the burner opens, allowing the gas in the flasher to pass to the burner, where it is ignited by a constantly burning pilot flame. This gas being consumed, the outlet valve closes and the inlet valve opens, allowing gas to enter the flasher again. Any combination of light and dark periods can be obtained by an Aga flasher. THE LIGHTHOUSE. The Taiaroa Head lighthouse is a solid stone structure built in 1868, and is situated about 196 ft above sea level. It contains an Aga red light showing about eighteen miles to sea. This light is controlled by an ingenious contrivance called an Aga sun valve. This Aga sim value performs the functions of a keeper, extinguishing the lights at sunrise and lighting them again at the approach of darkness independently of the hour. The Aga sun valve is actuated entirely by light, and is not influenced by changes in the temperature. Its construction is based on the well-known physical law that absorbed light is transformed into heat. It consists of a system of metal rods protected by a strong glass cylinder. The central rod is coated with lampblack, which gives it the property of absorbing light, while the three rods surrounding it are polished, and thus reflect the light. All four rods expand in the same degree under the influence of heat, but only the central rod expands under the influence of light. The additional expansion caused thereby is used to operate a valve which controls the passage of gas to the -main burner. At the approach of daylight the black rod expands and closes the valve. At the approach of darkness it contracts, and the valve opens. The continuously-burning pilot flame attached to the main burner and fed directly from this governor ignites tbe gas. The average saving in gas by using a sun valve is about .30 per cent., but the saving to a certain extent depends on the climate.