**Zz**, Septal strands.- We may also write ur 1 = I +zu 1+ &c., since z is very small compared with u, and expressing u in terms of w by (25), (we find l 21- mv i fi(z) i I +z(c R w + ' R 2 w) do) = 27rmoti(z) I -f-
**ZZ**(Ki + R2/ This then expresses the work done by the attractive forces when a particle m is brought from an infinite distance to the point P at a distance z from a stratum whose surface-density is a, and whose principal radii of curvature are R 1 and R2. - But, assuming the distributive principle, the product of two lines appeared to give the expression xx' - yy' -
**zz**' +i(yx' +xy')+j(xz' i j (yz' +zy'). - He had now the following expression for the product of any two directed lines: xx' - yy -
**zz**' +i(yx'+ xy')+ j(xz' '+zx') +ij(yz' - zy').

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