> as lemmas

1867 P. G. Tait tr. Helmholtz in London, Edinb. & Dublin Philos. Mag. 4th Ser. 33 486 By vortex-lines..I denote lines drawn through the fluid so as at every point to coincide with the instantaneous axis of rotation of the corresponding fluid element.

1878 W. K. Clifford Elem. Dynamic iii. 200 A curve such that its tangent at every point is in the direction of the spin at that point is called a vortex-line.