Page:Popular Science Monthly Volume 11.djvu/189

Rh represents the danger which the merchant-ships of Liverpool actually were subjected to for many years on account of erroneous running of their chronometers, and because the sea-rates varied from the shore rates. It must also be remembered that from this table all cases of vessels which were shipwrecked (on this and other accounts) are omitted, so that, no matter how impossible it may at first sight seem to be that such enormous errors existed, it is yet a matter of fact that the errors are under and not over stated.

 Table showing Error of Longitude in Geographical Miles on the Equator, deduced from 1,700 Chronometers.

Examining the table in detail, it becomes necessary to recollect that it is a matter of record that these actually were the errors of chronometers carried on a large number of ships sailing out of Liverpool. The average errors derived from no less than 1,700 chronometers are enormous, being as great as thirty-three miles for a voyage of four months.

Among the many vessels carrying these instruments were a large number going on long voyages to India, Australia, and South America, and in many cases these vessels would necessarily be between three and four months or more on the voyage, often without sighting land. It appears from this table that the average error to be expected on such a voyage, and with such chronometers as they had (up to 1863), was no less than thirty-three miles! It is plain that no such errors are to be found in the chronometers used by our own naval vessels, nor were American merchant-vessels during the same period so badly provided for, but it is certain that English vessels were provided on the whole with extremely poor instruments.

It is plain that several causes were here acting. The chronometers furnished to these ships were on the average very poor. This fault