Percussion. The striking of bodies against each other. The force of this, depends upon the momentum of the striking body.
Period. The time required for the revolution of one of the heavenly bodies in its orbit.
Perpendicular. Making an angle of 90 degrees with the horizon. When two lines which meet, make an angle of 90 degrees, they are perpendicular to each other.
Phases. The various appearances of the disc, or face of the moon, and of the planets; that portion of them which we see illuminated by the rays of the sun.
Phenomenon. Any natural appearance is properly so called; the term, however, is usually applied to extraordinary appearances, as eclipses, transits, &c.
Piston. That part of a pump, or other engine which is made to fit into a hollow cylinder, or barrel; and to move up and down in it, in order to raise water, or for any other purpose.
Plane. A perfectly flat surface. The plane of the orbit of a planet, is an imaginary flat surface, extending to every part of the orbit.
Planet. Those bodies which revolve round the sun, in orbits nearly circular. They are divided into primary, and secondary; these latter are also called satellites, or moons; they revolve round the primary planets, and accompany them in their courses round the sun.
Plumb-line. A string, or cord, by which a weight is suspended; it is used for the purpose of finding a line perpendicular to the horizon; the weight being always attracted towards the centre of the earth.