399 



PER, 



PERCEPTION. 



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Peppermint camphor (C^H^O,,), as the stearopten is called, forms 

 colourless prismatic crystals, is fusible at 87 Fahr., and may be volati- 

 lised without undergoing decomposition. Its boiling point is 40<3'4 ; 

 vapour density, 5'S2. It is only slightly soluble in water, but readily 

 so in wood spirit, alcohol, ether, and bisulphide of carbon, and tolerably 

 go in oil of turpentine. Phosphoric acid converts it into MENTHENE. 



Oil of peppermint is much adulterated with turpentine. The latter 

 can generally be detected by the nose; while the pure oil is distinguished 

 from the other mint oils by the deep red-brown colour which is imparted 

 to it by chromate of potash. 



PER. A prefix used in chemistry. [CHEMICAL NOMENCLATURE.] 



PERAMBULATION. A perambulation is a walking through or 

 over ground for the purpose of settling boundaries. A perambulation 

 of a parish for this purpose is made by the minister, churchwardens, 

 and parishioners in Ascension-week, at periods varying from one to 

 three years. Usage will justify the parishioners in following the 

 boundary over any man's laud. Manors and lordships also are or may 

 be perambulated : and there is a writ De Perambulatione facienda, 

 which ought to be sued with the assent of both parties when they are 

 in doubt about the bounds of their lordships or manors. The writ is 

 addressed to the sheriff, who is to execute it and make his return to 

 the justices at Westminster on a certain day; or to the justices of 

 assize, under his seal, and the seals of those who make the perambu- 

 lation with him. The king may direct the writ to other persons to 

 make the perambulation, aa well as to the sheriff. This perambulation 

 made by assent binds the parties and their heirs. But unless both 

 parties who assent to the perambulation are tenants in fee-simple, it 

 seems that the perambulation shall not bind him who is in reversion. 



Questions of boundaries are now generally determined, however, by 

 actions of trespass or ejectment. 



It was an old Roman practice at the time of the Terminalia, in the 

 month of February, to perambulate the boundaries of a district or 

 community. The original boundaries of the territory of Rome, which 

 extended six miles from the city, were perambulated at the Terminalia ; 

 the boundaries between private properties were also perambulated at 

 the Terminalia, and the usual religious offerings were made. This 

 ancient and simple mode of preserving boundaries probably fell into 

 disuse as the land-surveyors became more skilful, and the records of 

 boundaries were better kept. It is stated that the practice fell into 

 disuse with the establishment of Christianity. But this is not pro- 

 bable. The religious ceremonies might be changed or dropped : but 

 the fact of perambulations being still kept up in this country in the 

 OMB of parishes, leads to the conjecture that the practice extended 

 from Rome to other countries which the Romans occupied, and 

 was retained in some form among the Christianised people in the 

 provinces. 



PERAMBULATOR, an instrument in general use for measuring 

 distances on roads, for settling disputes concerning the charges of the 

 drivers of hack-carriages, and for other purposes. It consists princi- 

 pally of a wheel upon which it runs, and an index which shows the 

 number of turns of such wheel reduced into miles, furlongs, poles, and 

 yards. 



The carriage or stock is made of wood, and is about 3 feet long. At 

 one end is a handle for the person who uses it, and the other is fur- 

 nished with sockets in which the axle of the wheel turns ; this end of 

 the stock hns the centre part removed, by which are left two arms 

 between which the wheel works. Upon the stock and just in front of 

 the handle is the dial-plate, with its two hands by which the distance 

 is registered. The wheel is 8 feet 8 inches, or 4 pole, in circumference. 

 Upon one end of the axis of this wheel is a small pinion, which works 

 into a similar pinion at the end of a rod which passes up the stock or 

 carriage to the works beneath the dial-plate. Motion is communicated 

 by means of this rod to a worm or micrometer-screw, which turns 

 once round for each revolution of the carriage-wheel of the perambu- 

 lator. This worm works into a wheel of 80 teeth, which is moved 

 forward one tooth for every 4 pole, and carries a hand or index, which 

 makes one revolution for 40 poles or one furlong. On the axis of this 

 wheel is a pinion of 8 teeth, which works into a wheel of 40 teeth, and 

 on the axis of this second wheel is a pinion of 10 teeth, which moves 

 a wheel of 160 teeth. This last wheel carries another hand, which 

 makes one revolution for 80 of the former. These bunds are arranged 

 in the same manner aa the hour and minute hand of a watch, so that 

 the three circles on the dial-plate are all concentric. The first of these 

 circles is divided into 220, and the second into 40, the number of yards 

 and poles contained in a furlong ; the figures on these circles are read 

 off by the first-mentioned index, that which is attached to the wheel 

 of 80 teeth. The third circle is divided into 80, the number of fur- 

 longs in 10 miles, and to this circle belongs the index attached to the 

 wheel of 160 teeth. The distance is ascertained by reading off the 

 figures in the reverse order in which the circles are given above : 

 divide the number on the first circle by 8, and you will have the dis- 

 tance required in miles, furlongs, poles, and yards. 



The instrument is furnished with a stop or strap, so that after the 

 distance is measured, the perambulator may be conveyed without the 

 in<lx being altered. 



Unlike the pedometer, it requires no regulating, and the only risk of 

 its giving ths distance incorrectly, if well constructed, is in passing 

 ovr nigged and uneven roads, which will of course cause the Index 



to show more than the true distance. In general, however, for short 

 distances, this error is very trifling. 



When about to commence a measurement, the wheel should be 

 turned round until the first-mentioned index points to 220 on the 

 circle of yards. Some are provided with a click and racket, by which 

 this may be done with much less trouble than by the wheel. 



There are other instruments for the same or similar purposes, bear- 

 ing different names, waymaer and odometer ; but the construction of all 

 of them is very similar. 



Way wiser is the name generally given to that form of the instrument 

 which is applied to a carriage, in which, by a slight adaptation to one 

 of the wheels of the carriage, the instrument is made to register 

 the number of turns of such wheel, in the same manner as the per- 

 ambulator. 



PERCEPTION is that power or act of the mind by which it holds 

 communication with the external world. It is distinguished from 

 conception by the circumstance that its objects are in every instance 

 supposed to have an actual existence. We may conceive things that 

 have no reality, but we are never said to perceive such things. Per- 

 ception differs from cotacioumea in that it takes cognisance only of 

 objecto without the mind. We perceive a man, a horse, a tree ; when 

 we think or feel, we are conscious of our thoughts and emotions. It is 

 further supposed in perception that the objects of it are present. We 

 can remember former objects of perception, but we do not perceive 

 them again until they are once more present. Besides the sense which 

 has been explained, the term perception is sometimes analogically 

 employed in common speech in reference to truths the evidence of 

 which is certain. Thus we may perceive the truth of a mathematical 

 proposition. But Mr. Hume is perhaps the only writer of eminence 

 who designedly applies the word in a metaphysical disquisition with a 

 meaning different from that which has been here assigned to it. By 

 him it is applied indiscriminately to all the operations and states of the 

 mind ; passions being designated perceptions, and the acts of memory 

 and imagination converted into so many acts of perception. Such 

 latitude of phraseology confounds under one general name things 

 essentially distinct, and tends to introduce vagueness and inaccuracy 

 into a department of philosophical investigation where definiteness and 

 precision are peculiarly indispensable. 



The distinction between things perceived (cuVflTjTci) and things con- 

 ceived (vorira) was familiar to the Greek philosophep and to their Latin 

 expositors, of whom Cicero expresses the former class of things by the 

 phrases "qute sunt," " qusa cerni tangive possunt ;" and the latter by 

 "qute tangi demonstrative non possunt, cerni tamen ammo atque 

 intelligi possunt," and gives examples of each. (' Top.' v.) 



The perceptive faculty is exercised through the instrumentality of 

 the senses. We see by means of the eye, and hear by means of the 

 ear, and so in reference to the other senses. An individual in whom 

 these organs are wanting or defective, will either not perceive at all, or 

 perceive imperfectly. In order to perception it is requisite that an 

 impression should be made on the organ of sense, either by the direct 

 application of the object, or through some medium that communicates 

 with the object and the organ. Thus an immediate application is 

 necessary with regard to the senses of taste and touch ; but only an 

 intermediate one with regard to those of sight, hearing, and smell. 

 The impression made on the organs of sense affects the nerves, and is 

 by them conveyed to the brain. The necessity of this communication 

 is ascertained by observation. If the nerve appropriated to any organ 

 be cut or tied hard, no perception takes place ; and the same result is 

 noticed in certain disordered conditions of the brain, even though tha 

 organs of sense and the nerves perform their respective functions. 

 When however the conditions that have been specified are complied 

 with, perception ensues. 



Various theories have been formed to explain the functions of the 

 nerves and brain in connection with perception. It was imagined by 

 the ancient* that the nervous fibres are tubular, and filled with a 

 subtile vapour named animal spirits ; that the brain is a gland by 

 which this ethereal fluid is secreted ; and that by means of it the 

 nerves perform their office. (Reid, ' Essay ' ii., ch. iii.) Des Cartes, 

 who adopted this hypothesis, has described with great minuteness how 

 all mental operations and movements are accomplished through the 

 agency referred to. Dr. Briggs, Newton's instructor in anatomy, was 

 the first who proposed a new doctrine on this point. He maintained 

 that the nerves operate by vibrations, like musical chords, and thus 

 conduct impressions to the brain. Newton himself (' Opt.,' qu. 23) 

 appears to have been inclined to a notion of this kind, and the sugges- 

 tions relating to it thrown out by him as a query were afterwards 

 amplified and defended by Hartley. The latter supposed that " external 

 objects impressed on the senses occasion, first in the nerves on which 

 they are impressed and then in the brain, vibrations of the small and, 

 as one may say, infinitesimal medullary jparticles ; " and that these 

 vibrations " are excited, propagated, and kept up partly by the ether, 

 that is, by a very subtile elastic fluid: partly by the uniformity, 

 continuity, softness, and active powers of the medullary substance of 

 the brain) spinal marrow, and nerves." (' Observations on Man/ part I., 

 prop. 4, 5.) Both Des Cartes and Hartley believed that by the action 

 of the nerves In the manner described by them, images of external 

 objects were formed in the brain. 



It Is scarcely necessary to remark concerning these hypotheses that 



