PER 



PER 



known by the name of "Rock fish,'* and 

 in some districts by that of" Basse." It 

 is brought to our markets in great num- 

 bers during the winter, and is in high es- 

 timation us food. Doctor Mitchell is per- 

 haps the first describer of the " Rock ;" 

 under the specific name which we have 

 here adopted ; he has subsequently 

 thought proper to reject this appellation, 

 and has substituted his own name, Mit- 

 chili. But as he gives us no reason what- 

 ever for an alteration of so much impor- 

 tance, we cannot concur in adopting it. 

 We do not know of any fish belonging to 

 the genus as it now stands, to which the 

 same name has been applied : the V, stria- 

 tus of Liiin. is not a true Perca according 

 to the late arrangements, and therefore 

 cannot be taken into consideration in this 

 case by any one who adopts these ar- 

 rangements. 



PERCEPTION, in logic, the first and 

 most simple act of the mind, whereby it 

 perceives or is conscious of its ideas. 



In bare perception, the mind is for the 

 most part only passive ; yet impressions 

 made on the senses cause no perception, 

 unless they are taken notice of by the 

 mind, as we see in those who are intently 

 busied in the contemplation of certain ob- 

 jects. It ought also to be observed, that 

 the ideas we receive by perception are 

 often altered by the judgment, without 

 our taking notice of it ; so that we take 

 that for the perception of our senses, 

 which is but an idea formed by the judg- 

 ment : thus a man who reads or hears, 

 with attention, takes little notice of the 

 characters or sounds, but of the ideas ex- 

 cited in him by them. 



The faculty of perception seems to be 

 that which constitutes the distinction be- 

 tween the animal kingdom and the infe- 

 rior parts of nature. Perception is also 

 tiie first step towards knowledge, and the 

 inlet of all the materials of it ; so that the 

 fewer senses a man lias, and the duller 

 tiie impressions that are made by them 

 are, the more remote he is from that 

 knowledge which is to be found in other 

 men, 



PERCH, or PEARCH. See PERCA. 



a measure of length equal to 

 live yards and a half. See MEASURE, 



PERCUSSION, in mechanics, the im- 

 pression a body makes in falling or strik- 

 ing upon another, or the shock of two 

 bodies in motion. See MOTION. 



Percussion is either direct or oblique ; 

 direct, when the impulse is given in a Line 

 perpendicular io the point of contact ; and 

 oblique, when it is given in a line oblique 



to the point of contact. The ratio which 

 an oblique stroke bears to a perpendicu- 

 lar one, is as the sine of the angle of inci- 

 dence to the radius. Thus, let a b (Plate 

 XII. Miscel. fig. 13) be the side of any 

 body on which an oblique force falls, with 

 the direction d a , draw d c at right an- 

 gles to d b, a perpendicular let fall from 

 d to the body to be moved, and make a d 

 the radius of a circle : it is plain that the 

 oblique force da, by the laws of composi- 

 tion and resolution of motions, will be re- 

 solved into the two forces d c and b d ,- 

 of which d c, being parallel to a b, hath 

 no energy or force to move that body; 

 and, consequently, d b expresses all the 

 power of the stroke or impulse on the 

 body to be moved : but d b is the right 

 sine of the angle of incidence d a b; 

 wherefore the oblique force d a, to one 

 falling perpendicularly, is as the sine of 

 the angle of incidence to the radius. 



PERDICIUM, in botany, a genus of 

 the Syngenesia Polygamia SuperHua class 

 and order. Natural order of Composite 

 Discoidea:. Corymbiferje, Jussieu. Essen- 

 tial character : corollets bilabiate ; down 

 simple ; receptacle naked. There are six 

 species. 



PERENNIAL, in botany, is applied to 

 those plants whose roots will abide many 

 years, whether they retain their leaves in 

 winter or not : those which retain their 

 leaves are called ever-greens ; but such 

 as cast their leaves are called deciduous. 

 Some of these have annual stalks, which 

 die to the root every autumn, and shoot 

 up again in the spring. 



PERFECT, in arithmetic. Perfect num- 

 ber is, that all whose aliquot parts added 

 together, make the same number with the 

 number whereof they are such parts. Thus 

 six is a perfect number, being equal 1 -j- 

 2+3: so also is 28=l-f 2+4+7 -f 14. 

 Mathematicians have been at considerable 

 pains to investigate the perfect numbers, 

 but with no great success, the following 

 are given as the first six perfect numbers : 



6 



28 



496 



8123 



33550336 



8539869056 



PERGULAU1A., in botany, a genus of 

 the Pentandria Digyuia class and order. 

 Natural order of Contort*; Apocinese, 

 Jussieu. Essential character : contorted ; 

 nectary surrounding the gentials with five 

 sagittate d cusps; corolla salver-shaped. 

 There are five species. 



PiiRlANTHiUM, in botany, the/ower- 



