374 Penfield — Use of the Stereographic Projection ft 



or 



sailing chart with its protractor, page 357, and the maps of 

 the hemispheres, pages 252, 257 and 258, with their accompany- 

 ing protractors, were made during the past summer. It happens 

 that they were drawn upon three kinds of materials as follows ; 

 the best quality of cardboard, cheap cardboard, and high-grade, 

 linen-backed drawing paper. At the time of writing, mid- 

 winter, it is now found that the best quality of cardboard has 

 shown the least shrinkage (J mm in 300 mm ), the second quality of 

 cardboard comes next (l mm in 300) and the linen-backed paper 

 third (li mm in 300). As far as may be told, celluloid seems 



41 



42 



43 



North America. 



80 6a 40 



MR 



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Asia. 

 46 





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South America. 



Africa. 



Australia. 



Maps of the Continents in Stereographic Projection, all drawn to uniform 

 scale. The small circle in each represents the point of tangency. 



about intermediate between the two grades of cardboard. The 

 protractors were made on well seasoned celluloid, nearly a year 

 old, and they fit the maps drawn on cardboard just as well at 

 the time of writing as at the time they were made. The 

 engine divided scales and protractors, previously described by 

 the present writer, have given good satisfaction throughout a 

 whole year, the expansion and contraction of paper being not 

 sufficiently greater than that of celluloid to cause serious 

 trouble. Thus it may confidently be asserted that reliable 

 measurements may be made with scales printed on well seasoned 

 celluloid, provided they are used with maps printed at about 

 the same season of the year and, preferably, on cardboard. 



