The self-gravitation of comets is shown to be negligibly small compared with the differential force due to the Sun, and hence the separate particles of comets move practically in independent orbits, except possibly for comets that recede to several hundred astronomical units. A comet cannot be regarded as possessing any permanent shape. The changes in dimensions perpendicular to the general orbital plane are shown to be large. Near perihelion every particle of the comet must cross through this plane. Collisions between a proportion of the particles will occur and reduce them to finer dust, thereby producing particles sufficiently small for the effect of radiation pressure to create a tail. Changes of dimensions in the orbital plane during a revolution are far smaller. Any slight differences of period must gradually result in particles of the comet being distributed right round the orbit. The possible contribution of the process to the acceleration of Encke's comet is briefly discussed.