424 HOW MAPS ARE MADE. 



Hcrscher.s Astronomy, tlie best by far of all ])()i)nlar books on the sub- 

 ject. While hititnde is absolute, longitude, being- diiferenee in time, 

 is relative, for there is no such thing as absolute time; and noon at 

 any place is merely the moment when the snn culminates on the me- 

 ridian. If the observer has a ch)ck whose going he can depend on, 

 and he sets it to, and keeps it always at, Greenwich time, he knows 

 from that clock or chronometer Avhat time it is at Greenwich when any 

 star comes to the meridian. If then he can observe any astronomical 

 phenomenon, such as the meridi;ni passage of a star, he has only to 

 observe the difference of the times recorded on the clock set to Green- 

 wich time and on his local clock, and the difference is the longitude in 

 time. This is the principle on which longitudes are taken at sea, 

 where chronometers can be kept undisturbed, but for explorers on land 

 it is more difficult. 



The moon, however, is a natural clock, very complicated, but still 

 readable to the initiated.* It is continually moving through the stars, 

 and its angular distance from prominent stars is carefully computed 

 for Greenwich, and recorded in the Nautical Almanac for every hour in 

 the year. The observer then iinding the nmoirs position by observa- 

 tion, and recording its local time, can find in the Nautical Almanac 

 when it had the same position at Greenwich, and the difference of the 

 times is the measure of the longitude. 



In old days, when ships met, the first (juestion was, Who are you? 

 the next, What's your longitude ? The invention of the chronometer 

 by Harrison one hundred and twenty years ago has however for sail- 

 ors at least, vastly simplified the finding of longitude at sea: and I find 

 from inquiry among sailors, that "lunars" are practically a lost art. 

 In illustration, I may state that I find from the Nantical Almanac tliat 

 Arcturus comes to the meridian of Greenwich at 11:37 to-night. If I 

 have a clock or chronometer marking true Greenwich time, I shall find 

 that this star will come to our meridian at twelve minutes and forty 

 seconds later; the difference of longitude in time is therefore twelve 

 nunutes and forty seconds, which, converted into angular measure- 

 ment, is .") degrees lOA minutes, the longitude (west) of Edinburgh. 



Note here that at stations differing only in latitude the same star 

 comes to the meridian at the same time, but at different altitudes. At 

 stations differing only in longitude it comes to the meridian at the 

 same altitude, but at different times. The instrument generally used 

 for taking altitudes is the sextant.t At sea, where we have a visible 



* The student should j-ead the beautiful explanation of loujiitude in HerschvVs 

 Astronomy, section 220 et seq. 



t lu reading this paper the sextant, artificial horizon, theodolite, level, i>lano 

 table, and other instruments were all shown and their uses described. These descrip- 

 tions are omitted here, and the student is referred for detailed and illustrated 

 descriptions of these and other instruments to Prof. ]?anl<ine's Manual of Civil 

 Enr/ivcolnr/ (Griffin, Bohn A- Co.), or Mr. Usill's Practical Siirveyiny (Cutshy, Lock- 

 Wood cV 8on, 1«89). 



