Each wing forms a mobile helix or screw.
Proposition 30 describes the construction of a curve of double curvature called by Pappus the helix on a sphere; it is described by a point moving uniformly along the arc of a great circle, which itself turns about its diameter uniformly, the point describing a quadrant and the great circle a complete revolution in the same time.
If a helix be described round the screw, crossing each turn of the thread at right angles, the distance between two corresponding points on two successive turns of the same thread, measured along this normal helix, may be called the normal pitch; and when the screw has more than one thread the normal pitch from thread to thread may be called the normal divided pitch.
In wheelwork of this kind the contact of each pair of teeth commences at the foremost end of the helical front, and terminates at the aftermost end; and the helix is of such a pitch that the contact of one pair of teeth shall not terminate until that of the next pair has commenced.
Such a complex motion is called screw-like or helical motion; for each point in the body describes a helix or screw round the axis of rotation, fixed or instantaneous as the case may ~ be.
This helix is presented or held near to the antenna, and the length of it shortened until oscillations of the greatest intensity are produced in the helix as indicated by the use of an indicator of fluorescent paper.
The shell of the Pulmonata, though always light and delicate, is in many cases a well-developed spiral " house," into which the creature can withdraw itself; and, although the foot possesses no operculum, yet in Helix the aperture of the shell is closed in the winter by a complete lid, the " hybernaculum," more or less calcareous in nature, which is secreted by the froot.
The species of Helix are all herbivorous, like the Pulmonata generally; snails and slugs are well-known enemies to the gardener.
In the steady motion under no force of such a body in medium, the centre of gravity describes a helix, while the axis escribes a cone round the direction of motion of the centre of ravity, and the couple causing precession is due to the dislacement of the medium.