724 THE POPULAR SCIENCE MONTHLY. 



compiler would be tolerably sure to do, unless he had a mathematical 

 friend at his elbow) that, by voyaging eastward at such and such a 

 rate, a traveller can always have the moon " full" at night, in what an 

 unpleasant predicament would the mistake have placed him! Such 

 things happen, unfortunately ; nay, I have even seen works, in which 

 precisely such mistakes have been made, in use positively as text-books 

 for examinations. On this account, our fiction writers must be careful 

 in introducing science details, lest peradventure science-teachers (save 

 the mark !) be led astray. 



It need scarcely be said that no amount of eastwardly voyaging 

 would cause the moon to remain always " full " as seen by the voyager. 

 The moon's phase is the same from whatever part of the earth she may 

 be seen, and she will become " new," that is, pass between the earth 

 and the sun, no matter what voyages may be undertaken by the in- 

 habitants of earth. Mr. Clemens has confounded the monthly motion 

 of the moon with her daily motion. A traveller who could only go 

 fast enough eastward might keep the moon always due south. To do 

 this he would have to travel completely round the earth in a day and 

 (roughly) about 50^ minutes. If he continued this for a whole month, 

 the moon would never leave the southern heavens ; but she would not 

 continue " full." In fact, we see that the hour of the day (local time) 

 would be continually changing since the traveller would not go round 

 once in twenty-four hours (which would be following the sun, and 

 would cause the hour of the day to remain always the same), but in 

 twenty-four hours and the best part of another hour; so that the day 

 would seem to pass on, though very slowly, lasting a lunar month in- 

 stead of a common day. 



Every one who makes a long sea-voyage must have noted the im- 

 portance attached to moon observations ; and many are misled into 

 the supposition that these observations are directly intended for the 

 determination of the longitude (or, which is the same thing in effect, 

 for determining true ship-time). This, however, is a mistake. The 

 latitude can be determined at noon, as we have seen. A rough ap- 

 proximation to the local time can be obtained also, and is commonly 

 obtained, by noting when the sun begins to dip after reaching the 

 highest part of his course above the horizon. But this is necessarily 

 only a rough approximation, and quite unsuited for determining the 

 ship's longitude. For the sun's elevation changes very slowly at 

 noon, and no dip can be certainly recognized, even from terra firma, far 

 less from a ship, within a few minutes of true noon. A determination 

 of time effected in this way serves very well for the ship's " watches," 

 and accordingly when the sun, so observed, begins to dip, they strike 

 " eight bells " and " make it noon." But it would be a serious matter 

 for the crew if that was made the noon for working the ship's place; 

 for an error of many miles would be inevitable. 



The following passage from " Foul Play " illustrates the way in 



