HOW MAPS AKE MADE. 425 



horizon — the line where the sea meets the sky — we measure altitudes 

 from this horizon ; but on land we have no true horizon, and then we 

 use, what is more accurate, an artificial horizon, which is a cui) of 

 mercury in which the heavenly body is reflected. Measure the angle 

 between the real body in the sky and its reflection in the mercury, and 

 half the angle is the true altitude. 



Projection. — Having discovered our position on the sarl'ace of the 

 globe, we come to the representation of it on a flat sheet or map. 



The Latiu dictionary tells us that '-mappa" is a sheet or napkin. 

 iS^ow, the surface of the globe is curved, and in vi map we have only a 

 flat surface to represent it on, and we shall for a short while study how, 

 as it is the basis of all map-drawing. The conventional representation 

 on a flat surface of the curved surface of the earth is called projection, 

 because in its fundamental idea it is a i)i«'ture of the globe projected or 

 thrown forward from the eye on to a flat sheet; but this idea of it is so 

 confusing to the mind unaccustomed to think out such things that, 

 although it is the invariable way of describing it in all text books, I 

 have preferred to show you three forms of projection without assuming 

 any ideal throwing of the rays on to planes. 



The first I show is the modified sttreographic or equi-globular i^ro- 

 jection (Plate xxii), invented by Philip de la Ilire about the e)id of the 

 seventeenth century. A simple way of investigating this projection is 

 to fit an iron ring over the center of the globe, and stretch tightly, from 

 the North Pole to the South, India rubber bands that coincide with the 

 meridians of longitude on the globe, fastening them firmly to the ring 

 at the polos. Similarly stretch India-rubber bands over the parallels of 

 latitude, fastening them to the iron ring and to the meridians where 

 they cross. While the ring is kept on the globe these India-rubber 

 bands show the parallels and the meridians on the sphere. When the 

 ring is lifted off the globe the India-i ubber shrinks to a plane and shows 

 exactly the lines of the stereographic projection. This is the proj<^ctioii 

 used in all atlases for the world in hemispheres, for continents, and for 

 large surfaces. It gives, indeed, a notion of rotundity and a general 

 idea of projection, but the central portions are shrunk in, and the edges 

 are distorted. 



The next projecti(^u to be studied is Mercator's. Mercator was a 

 Fleming who lived in the sixteenth century. He was almost an exact 

 contemporary of John Knox, He was a writer on theology and geog- 

 raphy. His real name was Gerard Kremer, which name, meaning mer- 

 chant, he Latinized, in accordance with the custom of the day, iuto 

 Mercator. 



His invention is very clever. The ccmstruction of it is a little com- 

 plicated, and is generally shirked in text books, but the actual idea is 

 very simple, and I have here designed a piece of apparatus to illustrate 

 exactly what I believe Mercator did wlien he evolved his system. 



