Swinging ship for the precise determination of deviations in magnetically‐disturbed harbors

In the ordinary method of swinging ship, the vessel is held on the selected headings successively as for example, E, SE, S, SW, W, NW, N, and NE and on the swing with the other helm the order is reversed, NE, N, NW, W, SW, S, SE, and E. When the region is magnetically disturbed and the highest precision is sought there are two points to be considered. a. In making a swing under ideal working conditions, that is perfectly calm weather, absolutely smooth water, with no current, and spending the same amount of time at uniform speed on each heading the vessel's course throughout the swing will describe a regular polygon as in Figure 1 in which the vessel on port swing, heads E from D to E, SE from E to F, etc., and on the starboard helm NW from F to E and W from E to D, etc. If the swings with both helms are made under these ideal conditions at the same speed and spending the same amount of time on each heading, the vessel will pass over the same bottom on opposite headings of the two swings, and the results for any two opposite headings of the two swings would be strictly comparable as regards irregular distribution of the local disturbance. This statement would not, however, hold for any other headings.