OF LOGICAL CLASS-FREQUENCIES, ETC, 
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§ 31. For the Ijenefit of tliose who would like to construct actual models of the 
surfaces shown in figs. 10, 11, 12, I again give helow sketches of their developments, 
po’s. 13—15. These sketches are drawn to half the scale of the preceding 
projections ; the notation for the angles is tlie same as in figs. 6-9. 
