The Design and Construction of the Magnetic Instrument Bridge
The main scientific significance of the Galilee was its function for proof-testing equipment and methods for obtaining accurate, repeatable geomagnetic data at sea. The Department of Terrestrial Magnetism of the Carnegie Institution in Washington, DC (DTM/CIW), was established, in part, for this very reason. DTM was also conducting a global land-based survey at the same time that extended well into the 20th century and is documented in their publications, available online. The seaborne portion of this scientific survey was carried out beyond the Galilee's charter period by their purpose-built, nonmagnetic brigantine Carnegie.
In order to distance the magnetic instruments as far away from the inherent magnetic influence of Galilee's hull, DTM specified a sturdy, bridge-like structure be built to support the magnetic instruments (which, for completeness only, will be described in a future post).
As with virtually everything else involved in this project, the instrument bridge evolved through the duration of the DTM charter as priorities and new data requirements changed over the three-year period. The purpose of my project here is to depict the ship as she appeared in 1908, so I will not discuss the earlier versions of the bridge and instrument suits.
Evolution of the instrument bridge plan over the course of the three years of the charter period.
Plan C was the main form utilized for Cruises II and III. The specific instruments were identified by the displayed codes.
(Source: OCEAN MAGNETIC OBSERVATIONS 1905-1916 AND REPORTS ON SPECIAL RESEARCHES BY L. A. BAUER. Director, 1917)
The instrument bridge was constructed on four pairs of wooden trestle legs. The aft three sets of legs were fastened to heavy sill beams spiked to the deck. The forward pair of legs were fastened to the forecastle cabin roof. Only non-ferrous fasteners were used, for obvious reasons.
The bridge railings were about 36 inches high. The bridge itself was normally accessed from the main deck via a long stair/ladder at the aft port corner of the bridge. However, there was also a set of side stairs at the forward end leading down to the starboard forecastle cabin roof. Initially, I think this was originally provided to permit a second way of getting off the bridge in an emergency. However, for Cruise III, it also provided easy access to the cabin roof for access to another instrument that would be installed under the bridge there later.
For DTM Cruises II and III, the forward end of the bridge was about 18 inches from the foremast. For this reason, the lower mainmast stay, which was originally set up to eyebolts on the deck near the foremast, had to be moved up to a heavy bronze ring just above the forward rail of the observation bridge.
A lot of cross-checking between DTM photos and the digital model was required to get everything right. In the end, the only part of the bridge that gave me real headaches was fitting in the aft stairway, next to the mainmast. The stairs were skewed to the port side of the mainmast which caused all sorts of issues trying to connect the upper end of the stairs to the bridge framework. Not fun, and hats off to the shipyard workers that accomplished that task!
So, here are some screen captures showing the overall structure of the instrument bridge. Evidently, when taken to sea for the first time, the bridge must have shown some fore-aft sway, in spite of the as-built angle bracing, so four heavy longitudinal braces were added between the aft pairs of trestles to eliminate that problem.
Portside view of the final version of the magnetic instrument bridge. The preliminary masts were included for reference. (Blender)
From this vantage point, the instrument bridge support trestles look like a mish-mash of struts! (Blender)
Starboard-side view of the bridge showing the forecastle cabin steps. (Blender)
The trestle legs were splayed at the same angle throughout, in spite of the varying height of the bridge above deck.
Probably for aesthetic reasons. (Blender)
Overhead view of the main access steps at the aft end of the bridge, showing how they were skewed to port of the mainmast.
The hatch cover was provided to close the deck opening, presumably in rough weather when taking magnetic readings.
The bridge handrails were dark in the monochrome photos, so I assumed they were mahogany finished bright. (Blender)
Underside of the bridge showing the four angled fore-and-aft anti-sway struts.
At each magnetic instrument position on the bridge deck, there were a pair of deck cross-beams provided to secure them to. (Blender)
Perspective view of the bridge deck looking aft. (Blender)
Next up: Anchors and ground tackle
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CDR_Ret ·
