PER 



PET 



and diminution of lustre, involving in 

 one or two cases a complete extinction 

 and revival. One of the most remark- 

 able of these is the star Omicron, in the 

 southern constellation Cetus, first noticed 

 by Fabricius in 1596. 



PERIODICAL WINDS. Winds which 

 blow half the year in one direction, and 

 half the year in the opposite direction. 

 October and April are the two months, in 

 which the change in the direction of 

 these winds usually takes place. See 

 Monsoon. 



PERKINS'S STEAM-ENGINE. A 

 modification of the steam-engine, in 

 which there is no proper boiler, but the 

 water is heated to a high temperature in 

 a cylindrical vessel, called a generator, 

 thence discharged through a valve, and 

 immediately converted into steam, hav- 

 ing a pressure equal to that of about 35 

 atmospheres, or of about 500 lbs. on the 

 square inch ; the steam at this high 

 pressure is then admitted into a cylinder 

 only 18 inches long and 2 inches wide, 

 the piston of which would perform 200 

 strokes per minute, and exert a force 

 equal to 10 horse power. 



PERLATE ACID. The name given 

 by Bergman to the acidulous phosphate 

 of soda ; the phosphate of soda had been 

 previously called sal mirahile perlatum. 

 It was named by Guyton-Morveau, 

 ouretic acid. 



PERMANENT INK. A solution of 

 nitrate of silver, thickened with sap 

 green or cochineal, 



PERMANENT WHITE. Sulphate of 

 baryta. At a high temperature it fuses 

 into an opaque white enamel, which is 

 used in the manufacture of fine earthen- 

 ware, and as a pigment. 



PERMEABILITY {per, through, meo, 

 to pass). That property of certain bodies 

 by which they admit the passage of other 

 bodies through their substance. The 

 cellular tissue of plants is permeable by 

 fluids, though at the same time imper- 

 forate. 



PERMIAN SYSTEM. Zechstein. 

 Magnesian Limestone Formation. Under 

 these names is designated a series of 

 ■well-knovvn deposits which succeed the 

 carboniferous rocks, and sometimes pass 

 into them by insensible gradations. 



PERMUTATIONS. By this term is 

 meant the number of changes which any 

 quantities, a, b, c, d, &c., may undergo 

 with respect to their order, when taken 

 two and two together, three and three, 

 &c. &c. The number of changes which 

 255 



may be rung upon twelve bells is 

 479,001,600. See Combinations. 



PE'RONATE {pero, a sort of high 

 shoe). A term applied in botany to the 

 stipes of fungaceous plants, which is 

 thickly laid over with a woolly substance, 

 ending in a sort of meal. 



PEROXIDE. A term applied in che- 

 mical nomenclature to denote the highest 

 degree of oxidation, of which a compound 

 is capable. See Per. 



PERPENDICULAR. A term applied 

 in geometry to that straight line which, 

 falling upon another straight line, makes 

 the adjacent angles equal to each other. 

 Each of these angles will be a right 

 angle. 



PERSEUS. A northern constellation 

 consisting of fifty-nine stars, the prin- 

 cipal of which is Algenib. 



PERSISTENT. A term applied to 

 those parts of plants which do not fall at 

 the usual period, as the corolla of cam- 

 panula, the calyx of paeonia. See Cadu- 

 cous. 



PERSONATE {persona, a mask). A 

 term applied to that form of the gamo- 

 petalous corolla, in which the limb is 

 unequally divided : the upper division, 

 or lip, being arched ; the lower prominent, 

 and pressed against it, so that the whole 

 resembles the mouth of a gaping animal, 

 as in antirrhinum. See Ringent. 



PERSO'NIFICATION. A metaphor- 

 ical mode of speech by which inanimate 

 objects or abstract ideas are repre- 

 sented as possessed of life and action. 

 Every scholar has enjoyed Virgil's famous 

 personification of the grafted tree : — 

 *' Miraturque novas lirondes, et noQ sua poma." 



PERSPE'CTIVE. The application of 

 geometrical rules to the delineation of 

 objects on a plane surface, as connected 

 with the arts of design. 



PERTURBATIONS {perturbo, to dis- 

 turb). A name applied to those inequa- 

 lities in the lunar and planetary motions, 

 which arise from the universality of at- 

 traction. Thus, not only does the sun 

 attract the earth, and the earth the moon, 

 but these attractions are all reciprocal ; 

 and not only is this the case, but each 

 individual planet attracts, and is attracted 

 by, all the rest. 



PETAL {neraXov, a leaf). In Botany, 

 a flower-leaf; one of the divisions of a 

 corolla. In some plants, as in nymphaea, 

 the filament is expanded, and resembles 

 a petal, and is then called petaloid. 



PE'TALITE. A felspathic substance, 

 in which lithia was first discovered by 



