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another second a thousand feet yet farther west, and so on through the twenty-four hours, until noon will once more get back where we are. The obvious result of this is that it is never the same time of day at the same moment at two places east or west of each other. As we travel west, we shall continually find our watches to be too fast for the places which we reach, while in travelling east, they will be too slow. This varying time is called local or astronomical time. The latter term is used because it is the time determined by astronomical observations at any place.

Formerly the use of local time caused great inconvenience to travellers. Every railway had its own meridian which it ran its trains by; and the traveller was frequently liable to miss his train by not knowing the relation between his watch or a clock and the railway time. So in 1883, our present system of standard time was introduced. Under this system, standard meridians are adopted fifteen degrees apart, this being the space over which the sun passes in one hour. The time at which noon passes a standard meridian is then used throughout a zone extending seven or eight degrees on each side. This is called standard time. The longitudes which mark the zones are reckoned from Greenwich. It happens that Philadelphia is about seventy-five degrees in longitude, or five hours in time from Greenwich. More exactly, it is about one minute of time more than this. Thus the standard meridian which we use for the Middle