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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Cable repair story
From: George Huxtable
Date: 2003 Mar 23, 19:15 +0000
From: George Huxtable
Date: 2003 Mar 23, 19:15 +0000
Cable repair story. Let me hypothesise about what those celestial observations were for, in that interesting story about repairing cables, http://atlantic-cable.com/Article/Combe/ One of the first and most valuable uses of a telegraph cable was to establish the precise longitude of places along its route, because the cable supplied rather accurate time signals for GMT. To do this job, an accurate celestial observation for local-time was needed, best from a star or, better, stars. That having been done once, the longitude was then deduced and noted, and that measurement didn't need to be repeated ever again, unless the technology improved. Can we presume, then, that the longitude of the onshore cable hut had been precisely determined, in this way? Subsequently, a cable break might occur, somewhere in mid-ocean. To drag a grapnel for it, the repair vessel had first to find it, so had to return to the line along which it had been laid. The closer the repair vessel could get to that line, the easier the job would be. We have learned from Blish that as it was being laid, a cable could provide time-signals on board, so that longitudes as well as latitudes could then be accurately noted (within the limitations allowed by anomalous dip, etc). The cable ship didn't need to rely on its on-board chronometers, nor on lunar distances (which would have been hopelessly inaccurate for its purpose). For the repair vessel to return to the presumed position of the fault, to lift the cable for repair, it then required the best possible estimate of its own position. That included both latitude and longitude. It no longer had access to the cable for time signals. Instead. it was necessary to rely on the ship's on-board chronometers, and any error in those chronometers needed to be found. Also the chronometers' rate of gain or loss needed checking. In any case, the repair vessel would need to call at the onshore cable hut so that the cable engineer could determine how far along the cable the fault had occurred. Presumably, the vessel would anchor as close as possible to the hut, then send a boat party ashore. If it was down-line from the fault, the hut would no longer have acess to time signals from the cable. Because the longitude of the hut had been predetermined accurately, a precise measurement at the hut of local time, using the Sun (or better, a star) from the hut in a time-sight, would provide a precise measure of Greenwich Time. This time could then be referred to that of the ship's chronometers by carrying a hack-watch back by boat to the ship, or perhaps by firing a gun. The anchored position of the ship would also need to be known with respect to the hut, and so a bit of triangulation would be called for, using a measured baseline between points from which an observer could see both the ship and the hut. Perhaps it was this latter part of the operation that called for the cleared area of level ground? Repeating the measurement of chronometer error over a few days would establish its "rate" of gaining or losing, for correcting future clock observations. I'm not saying "this was how, and why, it was done". It's no more than speculation, by someone who knows little about cable technology 100 years ago, asking whether that explanation is plausible. George Huxtable ================================================================ contact George Huxtable by email at george@huxtable.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. ================================================================