KNOWLEDGE 



iuiteniise c 

 potassic 

 a ring of is 



1* of dissected parts, such as 

 drop of saturated solution of 

 ' Ii a cover kept in its place bj^ 



U^ 



FINDING THE WAY AT SEA. 



Br Richard A. Proctok. 



(Contintted from p. 150.) 



JNFORTUNATELY, the longitude is not determijied 

 so readily. The very circumstance which makes 

 the determination of the latitude so simple introduces 

 the great difficulty which exists in ending the longitude. 

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

 same celestial scenery ; and precisely for this reason it 

 IS difficult to distinguish one such place from another, 

 that is, to find on what part of its particular latitude- 

 circle any place may lie. 



j§ If we consider, however, how longitude is measured, 

 :md 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 towards either pole the station 

 is ; its longitude means how far round, from some fixed 

 longitude, the station is. Bat it is by turning round 

 on her axis that the earth causes the changes which we 

 eall day and night ; and therefore these mtSt happen at 

 different times in places at different distances round. 

 For example, it is clear that if it is noon at one station 

 it must be midnight at a station half-way round fi-om 

 the former. And if any one at one station could tele- 

 graph toa 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 haU-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 

 iinother is so carried as presently to occupy the place 

 which its easterly neighbour 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 pheno- 

 menon depending on the earth's turning will occui- an 

 hour later at the westerly 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 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, w-hile a complete circle running through my station 

 and his (and everywhere in the same latitude) is sup- 

 posed to be divided into 360 degs., he is 15 degs. (a 24th 

 part of .H60) west of me ; and if my station is Greenwich, 

 he is in what we, in England, call 15 degs. west longi- 

 tude.* o "=> 



But what is true of suni-ise and sunset in the same 

 latitudes and in different longitudes, is true of noon 

 whatever the latitude may be. And, of course, it is true 



* In this case, he is " at sea" (which, I trust, will not be the 

 case with the reader), and, we may enppose, connected with 

 Greenwich by a gnbmarine tslegraph in conrse of being laid. In 

 fact, the position of the (ireat Eastern, thronghont her cable-laying 

 journeys, was determined by a method analogous to that sketched 



ulecha 



noon later, while every static; 

 earlier, than Greenwich (or ^^ - 

 employed). 



I shall presently return tn 

 tnde is to be determined \\ 

 safety in sea voyages. But 1 

 what happens in sea voyages win ir iliu iiMi- 

 largely. If a voyage is made towards the 

 England to America, it is manifest that a watch set to 

 Greenwich time will be in advance tf tl,. Licil time as 

 the ship proceeds westwards, : i T " 1 ;iiid more 



in advance the farther the sli', direction. 



For instance, suppose a watc; -h time; 



then when it is noon at Greer. \ will point 



to twelve, but it will be an hour 1 .cf. re ii-cn 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 towards 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 ill". Blucher, who is from the Far West, and 

 on his first voyage " (from New York to Europe) "was a 

 good deal woraied 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 confidence in it. Seven days out from New York 

 he came on deck, and said with great decision, ' This 

 thing's a swindle!' 'What's a swindle?' 'Why, 

 this watch. I bought her out in Illinois — gave 150 dols. 

 for her, and I thought she was good. And, by George, 

 she is good on shore, but somehow she don't keep up her 

 lick here on the water — gets sea-sick, maybe. She skips : 

 she runs along regular enough till half-past eleven, and 

 then all of a .sudden she lets down. I "ve set that old 

 regulator up faster and faster, till I 've shoved it clean 

 round, but it don't do any good ; she just distances everj- 

 watch in the ship,* and clatters alonp i" " -y'l -^'s 

 astonishing till it's noon, but them "eiLli^ ' ' y- 



gets in about ten minutes ahead of her . ' it 



know what to do with her now. Slu '- c i', . : ij -).,. 

 can, — she's going her best gait, but it wm't siive her. 

 Now, don't Tou know there ain't a watch in the ship 

 that 's makino- better time than she is ; but what does it 

 signify ? When you hear them '^ eight bells," you '11 find 

 her just ten minutes .short of her score — siu-e.' The ship 

 was gaining a full hour every three days, and this fellow 

 trying to make his watch go fast enough to keep u 

 ~ ' " '1 c.;,! lie had pushed the 



to her. But, 

 up as far as I't ■ 

 gait,' and so i 

 and see the >i, 

 captain, and lie 



'gulator 

 n its best 

 to fold his hands 

 sent him to the 

 mystery of 'ship 



* Becanse set to go " fast." Of conrse, the other watches on 

 board would be left to go at their usual rate, and simply put 

 forward at noon each day by so many minutes as corresponded to 

 the run eastwards since the preceding noon. 



