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E2 - - - THE SUMNER LINE - - 10/03/2004

The method of navigation involving the use of the Sumner line, or line of position, takes its name from Capt. Thomas H. Sumner, an American ship-master, who discovered it and published it to the world. As proof of its value, the incident which led to its discovery may be related:

Having sailed from Charleston, S. C., November 25th, 1837, bound for Greenock, a series of heavy gales from the westward promised a quick passage; after passing the Azors the wind prevailed from the southward, with thick weather; after passing longitude 21 W. no observation was had until near the land, but soundings were had not far, as was supposed from the bank. The weather was now more boisterous and very thick, and the wind still southerly; arriving about midnight, December 17th within 40 miles, by reckoning, of Tuskar light, the wind hauled SE. true, making the Irish coast a lee shore; the ship was then kept close to the wind and several tacks made to preserve her position as nearly as possible until daylight, when, nothing being in sight, she was kept on ENE. under short sail with heavy gales. At about 10 a. m. an altitude of the sun was observed and the chronometer time noted; but, having run so far without observation, it was plain the latitude by dead reckoning was liable to error and could not be entirely relied upon.

The longitude by chronometer was determined, using this uncertain latitude, and it was found to be 15' E. of the position by reckoning; a second latitude was then assumed 10' north of that by dead reckoning, and toward the danger, giving a position 27 miles ENE. of the former position; a third latitude was assumed 10' farther north, and still toward the danger, giving a third position ENE. of the second 27 miles. Upon plotting these three positions on the chart, they were seen to be in a straight line, and this line passed through Smalls light.

It then at once appeared that the observed altitude must have happened at all of the three points and at Smalls light and at the ship at the same instant.

Then followed the conclusion that, although the absolute position of the ship was uncertain, she must be somewhere on that line. The ship was kept on the course ENE. and in less than an hour Smalls light was made, bearing ENE. 1/2E. and close abord.


Smalls light is located off the coast of Wales at;

      51° 43' N  05° 40' W

and Tuskar light is located off the coast of Ireland at;

      52° 10' N  06° 12' W

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