EXPLANATION OF SCIENTIFIC TERMS. 



11 



from the Greek, and signifies a generator 

 of sours ; but acids have been sinco dis- 

 covered, which contain no oxygen. It 

 unites with metals and other substances 

 during their combustion, and forms 

 Oxides, what were formerly called Calces. 

 Hence we say that those; substances are 

 Ojrydabfe, and speak of the Oxides of iron, 

 of h-ad, tScc. See De-Ofydalion. 



PARABOLA. See Cone and Conic Sec- 

 tions. 



PA R A BOLOI D. See Conoid. 



PARALLEL LINES. When two straight 

 lines, in the same plane, are so directed 

 that, however much they might be length- 

 ened, they would never approach nearer 

 to, nor recede from one another, they 

 are said to be parallel. 



PARALLELOGRAM is a right-lined and 

 four-sided figure, whose opposite sides are 

 parallel, and consequently equal. When 

 its angles are right-angles, the figure is 

 usually called a Rectangle or Oblong. If 

 all its sides, as well as angles, are equal, it 

 is a Square. See Rhombus. 



PARALLELOPIPED. A parallelepiped 

 is a solid having six sides, each of which 

 is a parallelogram, and consequently al- 

 ways equal to and parallel to its opposite. 

 It is a prism whose base is a parallelo- 

 gram. See Prism. 



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. 



PENCIL OF LIGHT. See Light, Ray 



of. 



PENUMBRA (Latin pene, almost, and 

 umbra, a shadow) is a partial shadow ; 

 that is, a shadow which only receives a 

 portion of the rays of a luminous body, 

 when that body has a measurable diame- 

 ter. This is well explained in Newton's 

 Optics, at page 23. 



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 

 forca 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 

 point is termed the Centre of percussion. 

 When the percutient body revolves round 

 a fixed, the centre of percussion is the 

 same with the Centre of oscillation, and 

 if all the parts of the percutient body be 

 carried forward with the same celerity 



(which is uot the case of tjie pendulum), 



the centre of percussion is the same with 

 the centre of gravity. 



PERCUSSION, FORCE OF.- See Mo* 

 mentum. 



PERIGEE. See Apogee. 



PERIHELION. See Aphelion. 



PERIMETER. The length of the whole 

 bounding line of any plane figure, of 

 whatever parts or shapes that line may 

 consist, is termed the perimeter of the 

 figure. The length of the bounding line 

 of a circle (and, perhaps, of any curve 

 which returns upon itself) is its circum- 

 ference. 



PERPENDICULAR. See Angle. 



PETUNTSE. See Feldspar. 



PHENOMENON (plural Phenomena} is a 

 Greek word, signifying an Appearance, 

 and is limited, in our language, to denote 

 those appearances in nature, whether dis- 

 covered by direct observation or experi- 

 ment, for which there is no obvious 

 cause. An Hypothesis is an assumed 

 cause, by which we endeavour to account 

 for a particular class of phenomena ; and 

 that hypothesis is best which solves the 

 greatest number. 



PHLOGISTON (from the Greek phlego, to 

 burn) is a name given by the older che- 

 mists to an imaginary substance, which 

 was the principle of inflammability. Ac- 

 cording to them, every combustible body 

 was formed of an incombustible base, 

 united to this phlogiston, which escaped 

 into the atmosphere during the combus- 

 tion. This process is now attributed to 

 the union of certain known substances 

 (chiefly oxygen), which are therefore 

 called Supporters of Combustion. 



PHOTOMETER (Greek phos, light, and 

 metron, a measure) is an instrument for 

 measuring the different intensities of 

 light. These instruments are variously 

 constructed ; but those of Rumford and 

 Leslie are the most generally known. 



PHYSICS (Greek physis, nature) is that 

 science which is employed in observing' 

 the phenomena, and investigating the 

 constitution, powers, and effects of the 

 several bodies in nature. Aristotle, that 

 celebrated Greek, whose writings formed 

 the text-books of the schools for so many 

 centuries, after having treated of Phy- 

 sics, or Nature, added certain disquisi- 

 tions, concerning- Being in general, the 

 Soul of Man, and the Deity. These were 

 termed his Metaphysics (meta, beyond), 

 because they Avere distinct, or beyond 

 what he understood by Physics, or Na- 

 ture. 



PILE, GALVANIC, or VOLTAIC. See 

 Galvanic Circ/e. 



PISTON. See Gloss. I. 



PLANE, TANGENTIAL See Tangent. 



, HORIZONTAL. See Horizon 



, VERTICAL. See Horizon. 



PLUMBAGO. See Carbon. 



PLUNGER. See Piston. 



