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    Transatlantic cables and measuring dip.
    From: George Huxtable
    Date: 2003 Mar 21, 23:53 +0000

    Transatlantic cables and measuring dip.
    
    Here's a chance to entwine two separate threads that are presently running
    on this list, as you will see later.
    
    Thanks to the help of Jan Kalivoda and others, I've now managed to read
    that story about repairing cables at sea, and a really interesting read it
    has been. As good an account of storm conditions as I've read in any
    sea-story.
    
    The on-shore instrumentation in the hut took me back to my schooldays in
    the 1940s. Presumably the spot of light that was being watched, for any
    movement, must have been a mirror-galvanometer, looking for an imbalance in
    a Wheatstone bridge, which must have been set up to record the resistance
    of remaining wire up to a short-circuit. This was just the instrumentation
    we used in the sixth-form physics lab for measuring resistance. Wheatstone
    was a major figure in the early days of cable.
    
    The two threads referred to above are combined in the person of Commander
    John B. Blish, of the US Navy, who has been mentioned on this list before.
    He published in March 1903, vol XXIX, No 1 of the proceedings of the United
    States Naval Institute, Annapolis, a paper entitled "The Navigator's
    Prism", which is held at-
    
    Department of the Navy, Operational Archives Branch, Naval Historical
    Center, 805 Kidder Breese SE, Washington Navy Yard, DC 20374-5060.
    
    First, the connection with cables.
    
    He tells of being invited by William Siemens to travel on board his
    company's cable-laying steamer Faraday in the summer of 1900, when it was
    laying a cable between Canso and Fayal.
    
    Comments in square brackets are my own.
    
    He says that the navigation was of a very high order and included- "a
    distance-wire, which geve the exact distance run OVER THE GROUND: a
    deep-sea sounding wire was paid out continuously under a carefully
    regulated strain and in passing over a measuring wheel recorded the run
    with an accuracy comparable to that in measuring distances in land
    surveying. [Well, it's one way of doing precise DR, but out of reach of
    most of us...] Furthermore, the "lee-way" could be quite accurately checked
    by observing the trend of this "ground-log" wire. Another feature was the
    daily noon telegraphic time signal from the Canadian Government observatory
    at Quebec..."
    
    [Presumably there was electrical access to the end of the cable that was at
    the centre of the spool.]
    
    [Here comes the reference to anomalous dip-]
    
    On deck no less that nine experienced navigators took sights from early
    dawn until dark and half-hourly measurements on the Sir William Thompson
    compass with its accurate shadow-pin, [think of it, nine navigators! It's a
    wonder they could agree about anything!]
    
    In sextant work these observers habitually used the long astronomical
    telescopes in their sextants; these and all other operations were favored
    by clear weather and a smooth sea, in which the 10,000 ton Faraday with her
    deep bilge-keels and the restraining pull of the cable over the stern rode
    as steady as the proverbial church. All sight were worked out with 7-place
    logarithms and a mean of the various positions was accepted after throwing
    out any which differed a mile from that mean.
    
    But in spite of this, there came a bright day with a seeming perfect
    horizon when every "line of position" was bad and although the nine
    sextants gave the same meridian altitude to within a half minute, yet the
    noon latitude was nearly three miles north of the line on which the ship
    was supposed to be.. With unfaltering judgment, Mr Brittle, the cable
    engineer in charge, held his course and the next day's sights put the ship
    back on the line which had been laid off two days before.
    
    All these scientific appliances and all the care and skill of these trained
    navigators had been bowled over by a change in the atmosphere whereby the
    sea horizon was raised above its normal place, and the actual "dip of the
    horizon" was less than that taken from the "dip table".
    
    Having no means of measuring the actual dip, the sights were all rejected
    and the location of the cable on the chart was plotted by "dead reckoning"
    from the "distance-wire" records, as if the sun and stars had not been seen
    on that day."
    
    His mention of stars seems a bit odd, as measuring a number of stars at
    differing azimuths would be expected to give clearly discordant position
    lines, show up the anomalous dip, and allow it to be averaged out.
    
    He went on to patent the Blish Prism, a sextant attachment for measuring
    dip. This is a rectangular block of glass with its ends faced as 45-degree
    bevels, arranged to act as a backwards-facing periscope. A pair of mirrors
    rigidly mounted in a frame would do the same job, and that is what I have
    added to my plastic sextant. The periscope is mounted above the sextant so
    that its lower angled face protrudes into the light path that would
    normally arrive at the index mirror, so instead the index mirror provides a
    view of the horizon behind the observer's head. The periscope has to be
    long enough that the light-ray from the backward horizon can clear the top
    of the navigator's head, which depends on the radius of the sextant arm,
    the height of his head, and the size of his cap or hairstyle.
    
    In use, the sextant is always set to somewhere near zero degrees on the
    scale, adjusted so the fore and aft horizons coincide, and its reading
    taken. Then the sextant is INVERTED, (rotated through 180 deg about the
    line between the fore horizon and the eye), again the two horizons are
    aligned, and a new reading taken. The difference between these readings is
    then four times the dip.
    
    To those who correctly argue that the periscope would be trying to look
    through the observer's chest in that second measurement, I should explain
    that the observer has to cock and twist his head and body over to one side
    so that he is out of that line of sight, keeping the sextant vertical. A
    bit awkward, but not really a difficult contortion.
    
    Used in the way described above, it's not at all important that the
    included angle between the reflecting faces subtends exactly 90 degrees,
    nor does its alignment to the sextant need to be very precise.
    
    Another use Blish proposes for such a prism (or mirror-combination) is for
    use on its own, (independent of the sextant) in pilotage waters. Hold it
    horizontally, and look into it at one end to view the horizon behind, with
    the incoming light brushing past your ear. (It's a gadget teachers would
    find useful in an unruly class) If you can see a beacon behind, through the
    periscope, and it aligns with another beacon ahead seen just OVER the
    periscope, then you are on a direct line between the beacons. We are all
    accustomed to keeping to a transit given by two markers at the same
    azimuth, but this device allows you to keep to an INTERNAL transit (as I
    would call it) that you can devise between two markers or natural features
    in opposite directions, such as channel-markers ahead and astern. Useful?
    
    A suitable prism is available under the name of "Dove prism" from Edmund
    Scientific. I think you would need the largest size, which is 106 mm
    overall length (between the pointy edges), and this in 2001 in the UK was
    on sale for �130 from uksales{at}edmundoptics.com. This may be a heavy chunk
    of glass to attach to a sextant and you may prefer to choose a
    mirror-periscope, as I have.
    
    In conclusion, Blish describes some instances of anomalous dip measured
    using his dipmeter on a US Navy vessel on the coasts of California and
    Lower California. Over much of the period the dip didn't deviate by much
    more than 1 minute or so from its predicted value. However, on 16 Oct 1901,
    around 33N and 117W, there was a period of several hours when the dip was 2
    to 3 minutes too small, and on Oct 21-22, around 32N 118W, dip was found to
    exceed the predictions by 5, 6, and at one time 7 minutes. There was
    nothing in the weather observations to indicate anything might be amiss.
    
    The Navy named a small oceanographical survey vessel the John B Blish in
    his honour, which had an ignominious end as one of the vessels sacrificed
    in a nuclear bomb test.
    .
    =============
    
    Now I should get round to answering Richrd Pisko, who asked-
    
    Is there a handy web reference or drawing?  Most of the
     hits on Google
    seem to be referring to the magnetic dipmeter.  I wonder if
    it would look something like a WWI or WWII rangefinder
    without the two end prisms / mirrors?
    
    Yes, if you ask in a museum about a dipmeter they will assume you refer to
    magnetic dip.
    
    I know of another type of dipmeter which was made by Carl Zeiss and was
    used by the Carnegie oceanographic expeditions in the 1920s. I havent seen
    one, but I think it's rather as Richard indicates. The observer looks
    through a telescope at a pair of prisms (could be mirrors) with reflecting
    surfaces at 45 deg to his sight line. These provide a split view of the
    horizon to his left and the horizon to his right. One of the prisms or
    mirrors can be tilted sideways slightly by a calibrated fine-adjustment
    until the two views of the horizon are aligned. Having taken a reading, the
    instrument is inverted and the measurement repeated. The difference between
    the readings is 4 times the dip.
    
    Such an instrument needs to cover a small scale of only a few minutes, and
    I think it should be within the ability of any good model-machinist. But
    not me.
    
    George Huxtable
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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