“It is found that the cord covering the curved surface is twice as long as the one covering the flat surface. So the area of the entire curved surface of a sphere is equal to the area of the surface of 4 circles like the one measured.” -Foster, 1921
Ellsworth D. Foster ed. The American Educator (vol. 1) (Chicago, IL: Ralph Durham Company, 1921)
2400×768, 192.3 KiB
1024×327, 34.3 KiB
640×204, 18.3 KiB
320×102, 6.9 KiB