+ p"( X", Y". /") = h. 



/(X , ',Y' , ,Z") + p'(X',Y',Z') - °- 



',Z') + p'(\",Y",Z") 



(X", Y", Z") = | 



0) Cf. 54, ch. III, § 75, pp. 107-108 et ch. XI, § 181, p. 249; 76, ch. 11, 

 §§ 17-18, pp. 13-U et ch. VII. § 105, p. 91 ; 66, S 15, p. 83 : 53 : 57 : Ht : etc. 

 ( 2 ) Cf. 56, ch. I, p. 280; 76, ch. II, s 19, p. U. 



