12 



EXPLANATION OF SCIENTIFIC TERMS. 



POINT, ACTING. See Machine. 



BOILING. See Boiling point. 



FREEZING. See Freezing 



point. 



OF INCIDENCE. See Refrac- 

 tive Power. 



WORKING. See Working point. 



POLARITY (of a magnet). See Magnet, 

 Artificial. 



POLES, of amagnet. See Magnet. Artificial. 



OF A GALVANIC PILE. See 



Galvanic Circle. 



POLYHEDRON. See Rhombus. 



POLYZONAL designates, literally, what 

 is composed of many zones, or belts. 

 The term is applied in this volume to 

 certain lenses, composed of pieces united 

 in rings which are therefore called Poly- 

 zonal lenses. 



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, 



Jire, water, or other inanimate bodies. 



MECHANICAL. See Mecha- 

 nical Power. 



, IN OPTICS expresses the effect 



producible by lenses, or other instru- 

 ments, as magnifying power, healing 

 power, 8fc. 



PRESSURE is the application of force to 

 a resisting body, when that force is in 

 continued contact with the body upon 

 which it is exerted. See Impulse and 

 Percussion. 



ATMOSPHERIC. See At- 



mospheric 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, but in 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, pri- 

 mary. 



PRINCIPLE, D'ALEMBERT'S, in Me- 

 chanics, is this : If several non-elastic 

 bodies have a tendency to motion, with 

 velocities, and in directions which they 

 are constrained to change, in consequence 

 of their reciprocal action on each other, 

 then these motions may be considered as 

 composed of two others ; one which the 

 bodies actually take; and the other such, 



that, had the bodies been acted on by 

 such alone, they would have remained 

 in equilibrium. See Equilibrium and 

 Forces, composition of. 



PRINCIPLE OF VIRTUAL VELOCI- 

 TIES. " When a system of material 

 points, solicited by any force, is in equi- 

 librium, if the system receive a small al- 

 teration in its position, by virtue of 

 which every point describes an infinitely 

 small space, the sum of each force multi- 

 plied by the space described by the point 

 to which it is applied, according to the 

 direction of the force, is always equal to 

 zero." This is the general principle of 

 virtual velocities, referred to at page 2 of 

 MECHANICS, Treatise II. 



PRISM. A prism is a solid contained by 

 plane figures, of which two are parallel, 

 and the rest are parallelograms. 



PRISMATIC SPECTRUM is the various- 

 coloured appearance (Latin spectrum} 

 which a ray of white light exhibits when 

 separated by refraction through a glass 

 prism. The prism of the opticians is 

 triangular ; that is, its two ends are pa- 

 rallel, equal and similar triangles, and 

 conseqxiently its other faces are three pa- 

 rallelograms. 



PROGRESSION, in a general sense, is 

 merely going forward, but as the word is 

 understood, it is presumed that the pro- 

 gress is in a determinate order. It is 

 motion measured by some scale whatever 

 that scale may be. 



, ARITHMETICAL, is a 



series of numbers, or quantities, in which 

 each term differs from that which pre- 

 cedes it by a fixed number, or quantity, 

 called the ratio. Thus 3, 5, 7, 9, 11, &c. 

 is an arithmetical progression having the 

 ratio, two ; that being the number by 

 which each term differs from the adjoin- 

 ing one. 



GEOMETRICAL, is 



series of numbers, or quantities, in which 

 every two consecutive terms differ by a 

 multiplier which is common to the whole 

 series : this multiplier is the ratio. Thus 

 2, 4, 8, 16, 32, &c. is a geometrical pro- 

 gression, of which every term is double 

 an adjoining one, and whose ratio (or 

 rate of increase) is 2. Whether arith- 

 metical, or geometrical, progressions in- 

 crease or decrease, that is, ascend or 

 descend, the principle, or order of pro- 

 gression, is the same. 



PROPORTION, direct and inverse. See 

 Ratio. 



PROPORTIONALS. See Ratio. 



PYRAMID. See Cone. 



PYROMETER. See Thermometer. 



PYRO SCOPE. See Thermometer. 



QUADRANT. See Angle. 



QUARTZ. See Gloss. I. 



RADIATION See Gloss. I. 



RARITY, in bodies, is the opposite of Den- 

 sitfo which see. 



