Austin (1840; new **eds**., 1841 and 1847), by W.

Denoting the cross-section a of a filament by dS and its mass by dm, the quantity wdS/dm is called the vorticity; this is the same at all points of a filament, and it does not change during the motion; and the vorticity is given by w cos **edS**/dm, if dS is the oblique section of which the normal makes an angle e with the filament, while the aggregate vorticity of a mass M inside a surface S is M - l fw cos **edS**.

Hence if dS and dS' are the areas of the ends, and +E and - E' the oppositely directed electric forces at the ends of the tube, the surface integral of normal force on the flux over the tube is **EdS** - E'dS' (20), and this by the theorem already given is equal to zero, since the tube includes no electricity.

A tube so chosen that **EdS** for one section has a value unity, is called a unit tube, since the product of force and section is then everywhere unity for the same tube.

Every tube of electric force must therefore begin and end on electrified surfaces of opposite sign, and the quantities of positive and negative electricity on its two ends are equal, since the force E just outside an electrified surface is normal to it and equal to a/41r, where a is the surface density; and since we have just proved that for the ends of a tube of force **EdS** = E 1 dS', it follows that adS = a'dS', or Q = Q', where Q and Q' are the quantities of electricity on the ends of the tube of force.

Gilchrist (**eds**.), Science in South Africa (Cape Town, 1905) Reports of the S.