Physalia, or the Portuguese Man-of-War, is an Hydrozoan and the type of the Physalidne, a sub-order of the Siphonophora (q.v.). The body consists of a large float or pneumatophore, which is bluntly spindle-shaped in form and little less than a foot in length ; from this hangs down a scries of tentacles and appendages, but these do not include
Dynamics of a system includes, finally, dynamics of an elastic or of a viscous body. The secondary sciences are divided by Clerk Maxwell into four main divisions: - (1) Theory of gravitation; (2) theory of the action of pressure and heat in changing the dimension and state of bodies; (3) the theory of radiance; (4) electricity and magnetism. The theory of gravitation is applied to the study of the motions of falling bodies and their weight, and is the basis of physical astronomy, affording a complete explanation of the movements of the heavenly bodies. The theory of the action of pressure and heat deals with - (1) The three physical states : (a.) Solids, their elasticity, viscosity, plasticity, tenacity. (b) Liquids, their elasticity, viscosity, surface tension, cohesion. (r) Gases, their elasticity, viscosity, adhesion. (2) Effects of a rise in temperature on a body; alteration in size and shape or change of state. (3) Thermometry or measurement of temperature. (4) Calorimetry or measurement of the amount of heat in a body, its "thermal capacity and latent heat. (5) Thermodynamics, heat as form of energy, transformation of work into heat and heat into work, Carnot's cycle. (6) Dissipation of energy by friction, conduction, or the actual removal of high-temperature heat by the scattering of hot particles of matter. (7) The propagation of sound, vibrations of strings, rods, plates, and membranes.
The theory of radiance includes the whole science of light and the investigation of radiant heat. Under this head are to be considered: - (1) Geometrical optics, reflection and refraction; mirrors, lenses, optical instruments, such as telescopes, microeither nectocalyces or hydrophyllia. It lives on the surfaces of tropical seas, but stray specimens occasionally float into British waters. When touched they sting like a nettle.