1914] FLUID MOTIONS 243 moving layer, the liquid might then be able to close in behind, and success would be witnessed by a greatly diminished resistance. When a tolerably fair-shaped body moves through fluid, the relative velocity is greatest at the maximum section of the solid which is the minimum section for the fluid, and consequently the pressure is there least. Thus the water-level is depressed at and near the midship section of an advancing steamer, as is very evident in travelling along a canal. On the same principle may be explained the stability of a ball sustained on a vertical jet as in a well-known toy (shown). If the ball deviate to one side, the jet in bending round the surface develops a suction pulling the ball back. As Mr Lanchester has remarked, the effect is aided by the rotation of the ball. That a convex surface is attracted by a jet playing obliquely upon it was demonstrated by T. Young more than 100 years ago by means of a model, of which a copy is before you (fig. 9). D Fig. 9. A plate, bent into the form ABC, turning on centre B, is impelled by a stream of air D in the direction shown. It has been impossible in dealing with experiments to keep quite clear of friction, but I wish now for a moment to revert to the ideal fluid of hydrodynamics, in which pressure and inertia alone come into account. The possible motions of such a fluid fall into two great classes—those which do and those which do not involve rotation. What exactly is meant by rotation is best explained after the manner $f Stokes. If we imagine any spherical 16—2re as that which is encountered when the ship is replaced by a thin plane moving edgeways.