14 



EXPLANATION OF SCIENTIFIC TERMS. 



OCTAHEDRON. See Rhombus. 



OCULAR SPECTRA. See Accidental 

 Colours. 



OPACITY (Latin opacus, dark), a state 

 impervious to light. 



OR DIN ATE, of an Ellipse, Parabola, 

 and Hyperbola. See Conic Sections. 



OSCILLATION (Latin, oscillatio, swing- 

 . ing) is particularly applied to designate 

 the motion of a pendulum. 



CENTRE OF. A pen- 

 dulum, when oscillating, has one point in 

 which its whole moving force is concen- 

 trated ; and at which, if it meet with re- 

 sistance, it will instantly stop, without 



- vibration or strain of its other parts : 

 that point is called the centre of oscil- 

 lation. 



A wheel in motion may he considered 

 as an indefinite number of pendulums, 

 each of which has its own centre of oscil- 

 lation. If the wheel be nearly balanced, 

 those several points of oscillation will 

 accommodate themselves, so as to form, 



- in their continued motion, a set of points 

 ecnially distant from the rim. These are 

 Centres of gyration, and in their junction 

 will form a Circle of gyration. 



OVERSHOT WHEEL. See Water- 

 wheel. 



PARABOLA. See Cone and Conic Sec- 

 tions. 



PARABOLOID. See Conoid. 



PARALLEL LINES. When two straight 

 lines in the same plane are so directed, 



, that, however much they might be 

 lengthened, they would never approach 

 nearer to, nor recede from, one another, 

 they are said to be parallel. 



PARHELIA, PARHELIUM, or PAR- 

 HELION (Greek para, near, and helios, 

 the sun), is a mock sun ; an appearance 

 similar to the sun, which occasionally ac- 

 companies halos (See Halo). There have 

 been sometimes seen six or seven of 

 these mock suns at the same time, which, 

 in that case, are denominated by the 

 plural, Parhelia. 



PLNCIL OF LIGHT. See Light, Ray of. 



PERCUSSION, CENTRE OF. Percus- 

 sion is a forcible stroke given by a moving 



body. In taking any particular body, 

 such as a rod of equal thickness, held at 

 one end, and swung forcibly by the hand, 

 so as to strike upon a resisting object, 

 the force of the stroke will be greater or 

 less, according to the part of the rod that 

 shall hit the object. There is one point 

 of the rod in which the whole force of the 

 stroke is concentrated, and the resistance 

 to which would neutralize the blow. That 

 1 "' ; " t is termed the centre of percussion, 

 \ always coincides with that of oscil- 



FORCE OF. See Momentum. 



IETER. The length of the whole 

 ling line of any plane figure, of what- 

 parts or shapes that line may con- 



sist, is termed the perimeter of the figure. 

 The length of the bounding line of a 

 circle (or perhaps of any curve which re- 

 turns upon itself) is its Circumference. 



PERPENDICULAR. See Angle. 



PETUNTSE. See Feldspar. 



PISTON, a short plug, or block, exactly 

 fitted to the bore of a tube, so as to slide 

 outwards and inwards by means of a rod. 

 The piston (with its piston-rod} is a ne- 

 cessary part of the apparatus of a pump. 

 It serves the purpose of exhausting the 

 air from the tube, and is hence commonly 

 called the Sucker. See Suction. A pis- 

 ton is generally accompanied with a valve, 

 otherwise it is a Plunger. 



PLANE, TANGENTIAL. See Tangent. 



PLUNGER. See Piston. 



POINT, ACTING. See Machine. 



BOILING. See Boiling point. 



FREEZING. See Freezing 



point. 



OF INCIDENCE. See Refrac- 

 tive Power. 



WORKING. See Working 



point. 



PORES (of matter). See Volume. 



POWER is that principle which is capable 

 of effecting a change in the state or con- 

 dition of a body. When power is exerted, 

 as in mechanics, it is force, applied for 

 the purpose of producing or preventing 

 motion. In the former case it is termed 

 a moving power, or force, and in the 

 latter a sustaining power ^ or force. Power 

 is latent force. 



ANIMAL, or ANIMATE, is 



the power of a man, or other animal. 



INANIMATE, is that of air, 



fire, tvater, or other inanimate bodies. 



MECHANICAL See Mecha- 

 nical Power. 



IN OPTICS expresses the effect 



producible by lenses, or other instru- 

 ments, as magnifying power, heating 

 power, &c. 



PRESSURE is the application of force to a 

 resisting body, when that force is in con- 

 tinued contact with the body upon which 

 it is exerted. See Impulse and Percus- 

 sion. 



ATMOSPHERIC. See Atmos- 

 pheric Pressure. 



CENTRE OF. When a fluid 



presses upon a surface, there is a point 

 in that surface, at which, if a force be 

 applied in the same line with the pres- 

 sure of the fluid, and equal to the whole 

 of that pressure, butin a contrary direc- 

 tion, this counter-force will exactly 

 balance the whole pressure of the fluid, 

 and that point is called the centre of 

 pressure. 



PRIME MOVER. See Machine. 

 PRIMARY COLOURS. See Colours, 



primary. 

 PRINCIPLE, D'ALEMBERT'S, in Me- 



chanics, is this : If several non-elastic 



