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States passes a little east of Philadelphia. When mean noon reaches this meridian, it is considered as twelve o'clock throughout all our Eastern and Middle States as far west as Ohio. An hour later, it is considered twelve o'clock in the Mississippi Valley. An hour later, it is twelve o'clock for the region of the Rocky Mountains. In yet another hour, it is twelve o'clock on the Pacific coast. Thus we use four different kinds of time, Eastern time, Central time, Mountain time, and Pacific time, differing from each other by entire hours. Using this time, the traveller only has to set his watch forward or back one hour at a time, as he travels between the Pacific and the Atlantic coast, and he will always find it correct for the region in which he is at the time.

It is by this difference of time that the longitudes of places are determined. Imagine that an observer in New York makes a tap with a telegraph-key at the exact moment when a certain star crosses his meridian, and that this moment is recorded at Chicago as well as New York. When the star reaches the meridian of Chicago, the observer taps the time of its crossing over his meridian in the same way. The interval between the two taps shows the difference of longitude between the two cities.

Another method of getting the same result is for each observer to telegraph his local time to the other. The difference of the two times gives the longitude.

In this connection, it must be remembered that the heavenly bodies rise and set by local, not standard, time. Hence the time of rising and setting of the sun, given in