Page:Popular Science Monthly Volume 11.djvu/187

Rh times in advance of it, and sometimes later; so that the moment when the sun was highest at a certain place does not mark a determinate instant unless the day of the year is also given.

It is necessary to remember this, and to insist somewhat upon it, as the idea that the local noon as determined by clocks and watches is a sort of naturally determined epoch is widely spread, while the fact is that it is an artificial epoch, which can only be fixed by a somewhat difficult astronomical observation and a subsequent computation. The farm-laborer who eats his dinner in the field at the time that shadows cast by the sun point north and south is the victim of his own ignorance, as he sometimes anticipates the noon of watches and clocks by more than a quarter of an hour, and is sometimes equally in retard. The improvement of the balance-watch upon the clepsydra or the hour-glass and other early time-keepers caused the change to be made from apparent to mean time, and the increasing requirements of a complex civilization demand more and more attention to the keeping of accurate standard time. One of the most important functions of observatories is the determination of such a standard of time, and if this were their sole function the expense of maintaining them would be fully repaid.

If the standard time is important to the man of business in making his appointments and regulating his affairs, to the traveler in providing railways with a correct time by which to govern the movements of trains, and in general to every citizen in his daily occupations on land, it is vital to the successful and safe navigation of the ocean. Every ship that sails for a foreign port must before her departure know the correction of her chronometers to Greenwich time (that is, the number of seconds they are fast or slow on that time), and besides this their rate (or the number of seconds they daily gain or lose). Provided with good chronometers and with these data well determined, a ship sails from her port with the power of determining on any day her position on the earth's surface.

A simple observation of the altitude of the sun at noon gives, by a short computation, her latitude, and a determination of the angular distance of the sun east or west of her meridian gives the local time. The difference of the local time of the ship and the Greenwich time, as shown by her chronometers, gives her longitude. Latitude and longitude being known, her place on the chart can be put down with but little uncertainty. This is daily done, if possible, on every one of the ships sailing out of New York City, and on the skill of her officers, the goodness of her chronometers, and the accuracy of their rates, depends the safety of her passengers and cargo. To all men of business, then, in their appointments and affairs on shore and in their commercial ventures by sea, the fact that a standard time is easily attainable and perfectly correct is of no slight importance. To travelers, whether by sea or land, it is truly a matter of life and