It is true that the theory of vortex rings in hydrodynamics is of a simpler type; but electric currents cannot be likened to permanent vortex rings, because their circuits can be broken and the element of cyclic steadiness on which the simplicity depends is thereby destroyed.
In its simplest form the exoskeleton of a typical somite is a ring of chitin defined from the rings in front and behind by areas of thinner integument forming moveable joints, and having a pair of appendages articulated to its ventral surface on either side of the middle line.
The annual rings in a root are often less clearly marked than in the stem, and the xylem-elements are frequently larger and thinner.
For the observation and measurement of rings in crystals polariscopes are constructed on the following principle.
You can use petroleum jelly to hold the sealing rings in place during assembly.
