PEN 



PER 



their centre, or by some point within the 

 margin, as in tropseolum. 



PE'NCIL {penicillus, any thing which 

 has its end divided like a painter's brush). 

 In optics, a pencil of rays is a collection 

 of rays which converge to, or diverge 

 from, a point ; in geometry, a pencil of 

 lines is a series of lines which meet in a 

 point. 



PE'NDULUM. Every solid body which 

 is freely suspended, is called a pendulum. 

 A distinction is made between the simple 

 or mathematical, and the compound or 

 physical pendulum. The former is re- 

 garded as an inflexible imponderable 

 line suspended at one end, its other end 

 being considered as a point possessing 

 weight. A pure mathematical pendulum 

 has no actual existence. All such as 

 actually exist are physical pendulums, 

 every material particle of which acts 

 as the heavy point of the simple pen- 

 dulum. A seconds pendulum is one 

 whose vibration occupies one second. 



PENI'NSULA {pene, nearly, insula, 

 an island). A part of a continent which 

 runs out into the sea, and is joined to 

 the main land by only a small portion of 

 its circumference. If the projections of 

 land reach but a little way into the sea, 

 they are called capes, promontories, or 

 headlands. 



PENNATU'LID^ (pennatula, a little 

 feather). A family of cortical polyps, in 

 which the whole animal resembles a 

 feather, the stem supporting lateral 

 branches, upon which the polyps are 

 arranged. 



PENTA'CRINITE (Trei/re, five, Kpivov, 

 a lily). A pedunculated star-fish with 

 five rays, for the most part fossil. The 

 animal is so complicated that the num- 

 ber of separate pieces of stone of which 

 its singular skeleton is made up has been 

 calculated to amount to not fewer than 

 one hundred and fifty thousand. 



PE'NTAGON (Trei/re, five, foivla, an 

 angle). A plane geometrical - figure, 

 bounded by five sides, and consequently 

 having five angles. When the sides and 

 angles are all equal, the pentagon is 

 called regular, and then the square of 

 its side is equal to the sum of the squares 

 of the sides of the hexagon and decagon 

 inscribed in the same circle as the pen- 

 tagon. 



PENTA-GYNIA (yvvrj, a woman). 

 Having five pistils ; an ordinal cha- 

 racter of plants in the system of Lin- 

 naeus. 

 PENTAKE'NIUM. The name given 

 252 



by Richard to that modification of the 

 polakenium, in which the fruit, so desig- 

 nated, has five akenia, or cells. See 

 Cremocarpium. 



PENTA'MERA {n^vre, five, nepo^, a 

 pai%). A section of Coleopterous insects, 

 in '^hich the tarsi of all the feet are five- 

 jointed, the fourth being of ordinary size. 

 Latreille. 



PENTA'NDRIA {wevre, five, Uvij?, a 

 man). Having five stamens; the cha- 

 racteristic feature of the fifth class of 

 plants in the system of Linnaeus. 



PENU'MBRA {pene, almost, umhra, 

 a shadow). A partial shadow ; an 

 intermediate shade between pure sha- 

 dow and light; a shadow which re- 

 ceives only a portion of the rays of 

 a luminous body, when that body has 

 a measurable diameter. It occurs in 

 solar and lunar eclipses, in the form of 

 two diverging spaces, one on each side of 

 the cone of umbra, or pure shadow, 

 caused by the interception of the sun's 

 rays. In a solar eclipse, so long as any 

 part of the sun is visible, the observer 

 is in the penumbra, and not in the 

 umbra or complete shadow. 



PEPERl'NO. An Italian name for a 

 particular kind of volcanic rock, formed, 

 like tuff, by the cementing together of 

 volcanic sand, cinders, scoriae, &c. 



PEPO. A gourd ; a three-celled fleshy 

 indehiscent fruit, with parietal placentae, 

 as the cucumber. Richard uses the term 

 peponida. 



PE'PSIN (TreTTTO), to digest). A pecu- 

 liar animal principle secreted by the 

 stomach and present in the gastric juice. 



PER. A Latin preposition, which, 

 when prefixed to the name of an oxide, 

 indicates the presence of the greatest 

 quantity of oxygen which can exist in 

 a compound, as in per-oxide. 



Bi-per. This double prefix is used, 

 when there is more than one atom of 

 oxygen in the base, as well as an unequal 

 number of atoms of acid and base, as in 

 the 6i-per-sulphate of mercury, where 

 hi indicates the presence of two atoms of 

 acid, and per that the mercury is in the 

 form of a per-oxide. 



PER ACCIDENS. A term formerly 

 employed in philosophical language to 

 denote an effect which does not follow 

 from the nature or essence of the thing, 

 but from some accidental circumstance 

 or quality; in this sense, it is opposed 

 to the term per se : thus, fire burns per 

 se; a red-hot iron burns per accidens. 

 In Logic, the term per accidens is 



