latitude, "The sun is rising here," and he finds that he has to wait exactly an hour before the sun rises there, then he knows that he is one hour west of me in longitude, a most inexact yet very convenient and unmistakable way of speaking. As there are twenty-four hours in the day, while a complete circle running through my station and his (and everywhere in the same latitude) is supposed to be divided into 360°, he is 15° (a 24th part of 360) west of me; and, if my station is Greenwich, he is in what we, in England, call 15° west longitude.[1]
But what is true of sunrise and sunset in the same latitudes and different longitudes, is true of noon whatever the latitude may be. And of course it is true of the southing of any known star. Only unfortunately one cannot tell the exact instant when either the sun or a star is due south or at its highest above the horizon. Still, speaking generally, and for the moment limiting our attention to noon, every station toward the west has noon later, while every station toward the east has noon earlier, than Greenwich (or whatever reference station is employed).
I shall presently return to the question how the longitude is to be determined with sufficient exactness for safety in sea-voyages. But I may digress here to note what happens in sea-voyages where the longitude changes. If a voyage is made toward the west, as from England to America, it is manifest that a watch set to Greenwich time will be in advance of the local time as the ship proceeds westward, and will be more and more in advance the farther the ship travels in that direction. For instance, suppose a watch shows Greenwich time; then when it is noon at Greenwich the watch will point to twelve, but it will be an hour before noon at a place 15° west of Greenwich, two hours before noon at a place 30° west, and so on: that is, the watch will point to twelve when it is only eleven o'clock, ten o'clock, and so on, of local time. On arrival at New York, the traveller would find that his watch was nearly five hours fast. Of course the reverse happens in a voyage toward the east. For instance, a watch set to New-York time would be found to be nearly five hours slow, for Greenwich time, when the traveller arrived in England.
In the following passage these effects are humorously illustrated by Mark Twain:
"Young Mr. Blucher, who is from the Far "West, and on his first voyage" (from New York to Europe) "was a good deal worried by the constantly-changing 'ship-time.' He was proud of his new watch at first, and used to drag it out promptly when eight bells struck at noon, but he came to look after a while as if he were losing confi-
- ↑ In this case, he is "at sea" (which, I trust, will not be the case with the reader), and, we may suppose, connected with Greenwich by submarine telegraph in course of being laid. In fact, the position of the Great Eastern throughout her cable-laying journeys, was determined by a method analogous to that sketched above.
