7 2o THE POPULAR SCIENCE MONTHLY. 



readily explained without a globe or diagrams, but may be thus indi- 

 cated : Suppose a circle to run completely round the earth, through 

 Greenwich and both the poles ; now, if this circle be supposed free to 

 turn upon the polar axis, or on the poles as pivots, and the half which 

 crosses Greenwich be carried (the nearest way round) till it crosses 

 some other station, then the arc through which it is carried is called 

 the longitude of the station, and the longitude is easterly or westerly 

 according as this half-circle has to be shifted toward the east or west. 

 A complete half-turn is 180, and, by taking such a half-turn either 

 eastwardly or westwardly, the whole surface of the earth is included. 

 Points which are 180 east of Greenwich are thus also 180 west of 

 Greenwich. 



So much is premised in the way of explanation to make the present 

 paper complete ; but ten minutes' inspection of an ordinary terrestrial 

 globe will show the true meaning of latitude and longitude more 

 clearly (to those who happen to have forgotten what they learned at 

 school on these points) than any verbal description. 



Now, it is sufficiently easy for a sea-captain in fine weather to de- 

 termine his latitude. For places in different latitudes have different 

 celestial scenery, if one may so describe the aspect of the stellar heav- 

 ens by night and the course traversed by the sun by day. The height 

 of the pole-star above the horizon, for instance, at once indicates the 

 latitude very closely, and would indicate the latitude exactly if the 

 pole-star were exactly at the pole instead of being merely close to it. 

 But the height of any known star when due south also gives the lati- 

 tude. For, at every place in a given latitude, a star rises to a given 

 greatest height when due south ; if we travel farther south, the star 

 will be higher when due south ; if we travel farther north, it will be 

 lower ; and thus its observed height shows just how far north of the 

 equator any northerly station is, while, if the traveller is in the South- 

 ern Hemisphere, corresponding observations show how far to the south 

 of the equator he is. 



But commonly the seaman trusts to observation of the sun to give 

 him his latitude. The observation is made at noon, when the sun is 

 highest above the horizon. The actual height is determined by means 

 of the instrument called the sextant. This instrument need not be 

 here described ; but thus much may be mentioned to explain that pro- 

 cess of taking the sun's meridian altitude which, no doubt, every one 

 has witnessed who has taken a long sea-journey. The sextant is so 

 devised that the observer can see two objects at once, one directly and 

 the other after reflection of its light ; and the amount by which he has 

 to move a certain bar carrying the reflecting arrangement, in order to 

 bring the two objects into view in the same direction, shows him the 

 real divergence of lines drawn from his eye to the two objects. To 

 take the sun's altitude, then, with this instrument, the observer takes 

 the sun as one object and the horizon directly below the sun as the 



