Page:Popular Science Monthly Volume 11.djvu/188

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176
THE POPULAR SCIENCE MONTHLY.

death. The watches of railway employés are usually set by one clock, but a difference of one or two minutes on a crowded road may bring about the most fatal results, as the reports of the various railway commissions will show. If a ship leaves New York supposing her chronometer which is regulated to Greenwich time to be losing two seconds a day, while it is really losing six, every day she is really about a mile farther west than her reckoning shows her to be, and in a voyage of a month she will suppose herself to be too far west by thirty miles. Such a result may be attended with the most disastrous consequences, and that it does not oftener so result is due to the skill and watchfulness of sea-captains, a class of men whose vigilance and faithfulness are too little appreciated.

That such accidents do occur is brought constantly before us, in the reports of marine disasters as given in the newspapers and elsewhere, and every year a large volume is published by the English Government—the "Report of Wrecks and Casualties," etc.—in which the details are given. A simple inspection of the wreck-chart appended to this bulky annual volume, where every vessel wrecked during the year has the place of her loss indicated by a dot on the map, shows how frequent such losses are. I know of no simpler way of presenting the risks run, when the actual wreck is not incurred, than by giving the following table from the report for 1863 of Mr. Hartnup, Director of the Observatory of Liverpool, an observatory founded especially for the care of the chronometers of merchant-ships.

The work of this observatory has been continued for many years, and a large mass of statistics concerning the running of the chronometers of ships sailing out of Liverpool has been accumulated and partially discussed.

In the earlier history of the observatory, its attention was confined to the rating of chronometers, and, when any chronometer was sent to a ship with a given correction and rate, a record was kept of the fact.

On the return of the chronometer to Liverpool every effort was made to find the correction and rate which were given at the foreign port to which the ship was bound, and in this way a vast amount of statistical information concerning the running of the chronometers of merchant-ships out of Liverpool was accumulated.

In the following table, which summarizes these statistics, the first horizontal column contains the length of the voyage in months; the second, the average error of longitude in geographical miles on the equator, deduced from the means of 1,700 chronometers; and the remaining columns show the average error of the best ten instruments in one hundred, of the second best ten, etc. I have only taken so much of the table as would include a voyage of four months, since a vessel could hardly be without means of correcting her chronometer for a much longer time than this. We may fairly say that this table