722 THE POPULAR SCIENCE MONTHLY. 



latitude, " The sun is rising here," and he finds that he has to wait ex- 

 actly 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 Green- 

 wich, 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 un- 

 fortunately 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 sta- 

 tion 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. K 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 west- 

 ward, and will be more and more in advance the farther the ship trav- 

 els 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- 



1 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 jour- 

 neys, was determined by a method analogous to that sketched above. 



