72 PRACTICAL EXERCISES IX GEOGRAPHY 



crease of interior temperatures may ail l^e shown on this earth section. 

 Comparisons of local and general distances and heights ma}' be made 

 by drawing them to scale. 



Several methods of map projection may be illustrated. First the 

 necessity for projection should be shown by the impossibility of 

 smoothly laying a paper, cut to match a continental outline, upon 

 the surface of a globe. The mercator (or stove pi))e and cannon ball), 

 the conical, and the gnomonic projections may be easily constructed; 

 their difficulties may l)e magnified if clothed in mathematical lan- 

 guage or minified if talked about familiarl}-. After a network of 

 meridians and latitude circles is drawn out a continental outline may 

 be i)latted from a table giving the latitudes and longitudes of a num- 

 l)er of points on the coast line. Greenland and Soutii America on 

 Mercator projection, Greenland on Mercator and conical projection, 

 the margin of the unexplored areas in tlie Arctic and Antarctic re- 

 gions on gnomonic projections all afford good i)ractice for platting. 

 Comparison of distances on globes and on maps serves to detect the 

 distortion characteristic of each kind of projection. A great-circle 

 sailing coarse V»etween San Francisco and Yokoliama, as determined 

 on a globe, Jna}' l>e transferred to any projection l)y the latitude and 

 longitude of a number of points on its path. The same may be tried 

 on a polar gnomonic |)rojection of the great southern ocean for a V03'- 

 age from Cape Korn to Tasmania, 'i'he resuhs in tlie two cases are 

 interesting and instructive. From my own exjierience with school 

 teachers in i»rol)lems of this kind, it is necessary to conclude that 

 geometry must, as a rule, have been very l)adly taught to them. 



Terrestrial magnetism affords some interesting exercises, if time can 

 Ije allowed to them. The local variation of the magnetic needle has 

 already been determined. Charts published by the Coast Survey and 

 elsewhere give, by means of lines of equal variation, the values of 

 local variation atan}- desired point. Local values thus obtained may 

 be copied off on the blackboard, and the pupils may then write in 

 the values on a Mercator map of the world (of their own construction, 

 if desired), or on an outline map of the United States. The values 

 thus charted aff"ord practice in drawing lines of equal variation. The 

 accuracy of the work can be tested by comparing the results with the 

 original chart. A variation on this exercise may be made by drawing 

 arrows at various stations to represent the local direction of magnetic 

 north. Extend the arrows, curving them, if necessary, so that they 

 shall not cross each other ; they will then represent magnetic me- 



