C ’96 1 
XV. On the Errors in Longitude as determined by Chronometers 
at Sea , arising from the action of the iron in the ships upon 
the Chronometers. By George Fisher, Esq. Communicated 
by John Barrow, Esq. F. R. S. 
Read June 8, 1820. 
The determination of the longitude at sea by timekeepers, 
is so exceedingly easy from the simplicity of the observations 
and calculations employed, and from the general practica- 
bility of the method, as to render chronometers, in the pre- 
sent improved state of navigation, almost indispensable articles 
in the equipment of ships for foreign service ; and I shall feel 
happy if the following observations may, in any way, con- 
tribute to the more accurate determination of the longitude 
by this method. 
The sudden alteration in the rates of chronometers when 
taken on board of ships, has been frequently observed by 
intelligent seamen ; and is generally ascribed to the motion 
of the vessels. Before, however, I attempt to account for 
this alteration, I shall first prove that it actually takes place ; 
and, in order to do this, shall relate the circumstances con- 
nected with the chronometers on board the Dorothea and 
Trent, commanded by Captain Buchan, which occurred du- 
ring the late voyage to the North Pole. 
Soon after the arrival of the ships on the coast of Spitz- 
bergen, the chronometers on board the Dorothea (five in 
number) were found to be rapidly gaining on their former 
