EXPLANATION OF SCIENTIFIC TERMS. 



15 



bodies have a tendency to motion, with 

 velocities, and in directions which they 

 are constrained to change, in consequence 

 of tbeir reciprocal action on each other, 

 then these motions may be considered as 

 composed of t\vo 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 

 jrero." This is the general principle of 

 virtual velocities, referred to at page 2 of 

 MECHANICS, Treatise ii. 

 PR I SAL 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 

 consequently its other faces are three pa- 

 rallelograms. 



PROPORTION, DIRECT AND IN- 

 VERSE. See Ratio. 

 PROPORTIONALS. See Ratio. 

 PYRAMID See Cone. 

 PYROMETER. See Thermometer. 

 QUADRANT. See Angle. 

 QUARTZ is a hard sparkling stone, ex- 

 tremely abundant in nature, from the 

 common pebble to large mountain veins, 

 and even entire rocks. It is found in 

 crystals with various degrees of transpa- 

 rency, which, when pure, have the name 

 of Rock crystal : of this, the Scotch Cairn- 

 fform (denominated from a mountain 

 where they were once plentiful) is a 

 variety. The purple-coloured rock crys- 

 tals are commonly called amethysts, and 

 the yellow-coloured have the name of 

 topazes. See Corundum. 

 RA D I A TI ON ( Latin radiare) is the shoot- 

 ing forth, in all directions, from a centre. 

 The Latin radius was a shoot or rod, and 

 its plural radii (rays) was used both lite- 

 rally and metaphorically : they were the 

 spokes of a wheel, or the beams shot 

 from the sun. In natural philosophy, 

 whatever seuds emissions in all direc- 

 tions, is said to radiate ; and hence we 

 have not only radiations, or rays of light, 

 but of heat and of sound. Each of these 

 radii is a ray. See Hay. It may here 

 be observed, that the radiant heat of the 



sun passes through glass ; but it is other- 

 wise with terrestrial heat. 



RADIUS. See Angle. 



RAREFACTION is the act of causing a 

 substance to become less dense ; it also 

 denominates the state of this lessened 

 density. The term is more particularly 

 applicable to elastic fluids, which expand 

 so as to fill the vessel in which they are 

 contained after part is extracted. The 

 gas becomes rarefied in consequence o* 

 the partial exhaustion. Liquids are ex- 

 panded by means of heat, and thence be- 

 come thinner or more rarefied. 



RATIO. In comparing two subjects, with 

 regard to some quality which they have 

 in common, and which admits of being 

 measured, that measure is their ratio. 

 It is the rate in which one exceeds the 

 other. Proportion is the portions, or 

 parts of one magnitude that are con- 

 tained in another. When the ratio is 

 commensurable (that is, when it is redu- 

 cible to numbers), it is equivalent to pro- 

 portion ; but the latter term is usually 

 employed in the comparison of ratios, in 

 which case, two equal ratios are said to 

 be proportionals. Thus 3 has to 4 a cer- 

 tain ratio, or proportion ; but the ex- 

 pression 3 is to 4 in the same proportion 

 as 6 to 8, denotes that the ratios of 3 to 4 

 and 6 to 8 are equal ; 3 being the same 

 proportion of 4 as 6 is of 8, that is, three 

 fourths. 



DIRECT AND INVERSE. When 



two quantities, or magnitudes, have a 

 certain ratio to each other, and are, at 

 the same time, subject to increase or di- 

 minution; if, while one increases, the 

 other decreases in the same ratio, or, if 

 while the one diminishes, the other dimi- 

 nishes in the same ratio, the proportions, 

 or comparisons of ratios, remain unal- 

 tered, and those quantities, or magni- 

 tudes, are said to be in a direct ratio or 

 proportion to each other. Thus, if a 

 yard of cloth be worth a pound, ten, or 

 any number of yards will be worth so 

 many pounds, and the proportion of value 

 continues unaltered. 



But, if the magnitudes are such, thit, 

 when one increases, the other necessarily 

 diminishes ; and vice versa, when the 

 one diminishes the other increases, the 

 ratio, or proportion, is said to be inverse. 

 Thus, there is, at any moment, a certain 

 ratio of the length of the day to that of 

 the night ; but this is an inverse ratio ; 

 for, in proportion as the length of either 

 increases, that of the other must diminish. 



RAY is a single radiation from a body 

 which sends out emissions in all direc- 

 tions. See Radiation, and Light. 



RAYS, ABERRATION OF. See Aber- 

 ration. 



CALORIFIC. See Calorific Rays. 



COLOURED. See Colours, and 



Prismatic Spectrum, 



