A body whose parts yield to the slightest force when impressed, and by yielding, are easily moved against each other. Fluids are divided into elastic and non-elastic. Elastic fluids are those which may be compressed into an exceeding small compass, but which, on removing the compressing force, resume their former dimensions; and these are distinguished as airs or gases. Non-elastic fluids are those which occupy the same bulk under all pressures, or if compressible, it is only in a very slight degree, as water, oil, etc, and these are denominated liquids, except in the case of metals in fusion. The physical nature, laws, and effects, of wore-elastic fluids at rest, constitute the science of Hydrostatics, and when in motion, of the science of Hydraulics, or Hydrodynamics; those that relate to elastic fluids appertain to Pneumatics.