FINDING THE WAY AT SEA. 721 



other : he brings them into view together, and then, looking at the 

 sextant to see how much he has had to move the swinging arm which 

 carries the reflecting glasses, he learns how high the sun is. This being 

 done at noon, with proper arrangements to insure that the greatest 

 height then reached by the sun is observed, at once indicates the lati- 

 tude of the observer. Suppose, for example, he finds the sun to be 

 40 above the horizon, and the Nautical Almanac tells him that, at 

 the time the sun is 10 north of the celestial equator, then he knows 

 that the celestial equator is 30 above the southern horizon. The pole 

 of the heavens is, therefore, 60 above the northern horizon, and the 

 voyager is in 60 north latitude. Of course, in all ordinary cases, the 

 number of degrees is not exact, as I have here for simplicity sup- 

 posed, and there are some niceties of observation which would have 

 to be taken into account in real work. But the principle of the method 

 is sufficiently indicated by what has been said, and no useful purpose 

 could be served by considering minutiae. 



Unfortunately, the longitude is not determined so readily. The 

 very circumstance which makes the determination of the latitude so 

 simple introduces the great difficulty which exists in finding the lon- 

 gitude. I have said that all places in the same latitude have the same 

 celestial scenery ; and precisely for this reason it is difficult to dis- 

 tinguish one such place from another, that is, to find on what part of 

 its particular latitude-circle any place may lie. 



If we consider, however, how longitude is measured, and what it 

 really means, we shall readily see where a solution of the difficulty is 

 to be sought. The latitude of a station means how far toward either 

 pole the station is; its longitude means how far round the station is 

 from some fixed longitude. But it is by turning round on her axis 

 that the earth causes the changes which we call day and night ; and 

 therefore these must happen at different times in places at different 

 distances round. For example, it is clear that, if it is noon at one sta- 

 tion, it must be midnight at a station half-way round from the former. 

 And if any one at one station could telegraph to a person at another, 

 " It is exactly noon here," while this latter person knew from his clock 

 or watch that it was exactly midnight where he was, then he would 

 know that he was half-way round exactly. He would, in fact, know 

 his longitude from the other station. And so with smaller differences. 

 The earth turns, we know, from west to east that is, a place lying due 

 west of another is so carried as presently to occupy the place which 

 its easterly neighbor had before occupied, while this last place has 

 gone farther east yet. Let us suppose an hour is the time required to 

 carry a westerly station to the position which had been occupied by a 

 station to the east of it. Then manifestly every celestial phenomenon 

 depending on the earth's turning will occur an hour later at the west- 

 erly station. Sunrise and sunset are phenomena of this kind. If I 

 telegraph to a friend at some station far to the west, but in the same 

 vol. in. 46 



