PER
PER
little hooks at the ends i and the upper lamina of the cover- ings of the gills is ferrated at the edges. There are fome- times two fins on the back, fometimes only one. Of the former fort, or thofe which have two fins on the back, the fpecies are, I. The pearch with fix tranfverfe black lines, and with the fins of the belly red. This is our com- mon pearcb. 2. The pale fpotted parch* with two teeth in the jaws, on each fide, larger than the others. This is the lucioperca or bar}. 3- The P earcb with ei S ht or mne black tranverfe lines on each fide. Tins is the a/per pifaculus of authors.
Of thofe pearch which have only one fin on the back, the following are the (becies : i. The pearcb with one back-fin, and a cavernous head. This is the cemua of authors. 2. The pearcb with one back-fin, with black longitudinal lines on each fide. This is the fifh, called by the Germans fcbra'n- f er 3, The pearcb with feven tranfverfe black lines on each fide, and with red and blue marks on the head and belly- This is the pe>ca min't?na auclorum. 4. The pearcb with thirteen rays in the back-fin, and forty in the pinna arii. This is the labrax of authors, by fome called the lupus ma- rinui; and by others fpigo/a ; we call it zbajfe. Ariedi Gen. Pifc. p. 38.
The name perca is of Greek origin, and is derived from the word 7T£pxoc, which fignifies variegated with black or dufky fpots ; a fcliarafter common to muft of the fpecies of this fifh.
The common pearch affords good fport for the angler. The beft time for their biting is, when the fpring is over, and before the heats of fummer come on. At this time they are very greedy, and the angler, with good management, may take at one ftanding all that are in the hole, be they ever fo many.
The proper baits are a minow or young frog; but the worm called the branding* well fcowered, is alfo excellent at all times of the year. When the pearcb bites, he fhould always have a great deal of time allowed him to fwailow the bait. The pearcb will bite all day long, if the weather be cloudy ; but the beft time is from eight to ten in the morning, and from three till fix in the afternoon. The pearcb is very ab- ftemious in winter, and will fcldom bite in this feafon of the year ; if he does at all, it is in the middle of the day ; at which time indeed all fifh bite beft at that feafon. If the bait be a minow, which is the bait that affords moft diverfion to the angler, it muft be faftened to the hook alive, by putting the hook thro' the upper lip or the back-fin ; it muft be kept at about midwater, and the float muft be a quill and a cork, that the minow alone may not be able to fink it.
The line muft be of filk, and ftrong ; and the hook armed with a fmall and fine wire, that if a pike ftiould take the bait, as is not unfrequently the cafe, he may be taken. The way to carry the minows or fmall gudgeons alive for baits, is this : A tin pot is to be provided with holes in the lid, and filled with water, and the fifh being put in this, the water is to be changed once in a quarter of an hour by the holes, without taking off the lid at any time, except when the bait is to be taken out.
A fmall cafting-nct made for thefe little fifh fh6uld be taken out with the pearch tackle, and one or two cafts of this will take baits enough for the day, without any farther trouble. When the bait is a frog, the hook is to be faftened to the upper part of the leg. The beft place for the fifhing for pearcb is, in the turn of the water near fome gravelly fcour. A place of this kind being pitched upon, it fhould he baited over night with lobworms choped to pieces ; and in the morning on going to it, the depth is'to be regularly plumbed, and then the hook is to be baited with the worm or other bait ; and as it drags along, the pearcb will foon feize upon it.
Perxa amhoinenfn* in zoology, the name of a frefli water fifh, fomewhnt refembling our pearch, but differing in that it is of a brown colour, and has fevera! blue lines under its fnout, and fome bluenefs in the adjoining fins, but its back and helJy fins are green. It is caught in the lakes and rivers of frefli water, at Amboina, and is a very delicately tailed fifh. Kay's Ichthyogr. Append, p. 1.
Perca aurata* the gilded pearch* a fifh of the fhape of the pearch, but remarkable for a gold colour about its gills : it is common in our rivers, and is called in Englifh a ruffe ; its more common name among authors is ccrnua. Gefner de Pifc. p. 825. See Cernua.
Perca marina* the fea pearch* in zoolo£v, the name of a fea fifh, much refembling the common freih water pearch in fhape, fize, and colours, but fomething thinner in propor- tion to its length, and more variegated. Its bark is variegated with fix or feven blackifh tranfverfe lines, like thofe of the pearch, and the intermediate fpace is of a fomewhat reddifh hue, efpecially toward the head. Its whole head, and part of its belly, are very elegantly va- riegated with red and blue ftreaks. Its back fin is of a pale yellow, with feveral reddifh yellow fpots. and is very long, and has its anterior rays terminated in (pines. Its mouth is extremely wide, and almoft always gapes open ; its teeth are fharp, and its eyes very large. It is very common in the Suppl. Vol. II.
Mediterranean, and is a well-tafted fifh. Rondelet. de Pifc, p. t?o. Gefnerde Pifc. p. R 19.
PERCOLATION {CycQ— Many have attempted the conden- fing of wines by percolation* or feparating from them that fuperfiuous moifture or water, which dilutes them below the 1 trueftandard of vinous liquors, in order to make them richer and fitter for keeping ; but it does not appear that fuch at- tempts have as yet fucceeded. See Stably de concentr. vin.
PERCUNOS, the name of an antient Pruftian idol. See the article Potrimpos.
PERCUSSION (Cycl.)— If an elaftic body ftrike a larger ela ffic body at reft, the fmaller body will always rebound, and move in a contrary direction to what it did at firft, arid alfo give the larger body a greater quantity of motion than what was originally in the fmaller body And this quan- tity of motion communicated, may be greater than the ori ■ ginal motion, in any ratio lefs than that of 2 to 1. For fuppofing the fmail body and its velocity, to be denoted by unity, and the larger body at reft by x, the velocity of x
after the fhock will be
1+*,
and its quantity of motion
will be
.+*.
But tho' the proportion of 2x to i-\-x
creafes when x increafes, yet its limit will be the propor- tion of 2 to i ; which is the limit to which the motion of x may approach indefinitely near.
Hence it follows, that if the force of moving bodies be mea- fured by their quantity of motion, a' body may, by -percuffion* immediately communicate a greater force than itfelf has to another, and this in any proportion lefs than double. But if other elaftic bodies be interpofed, a body may me- diately communicate a greater force than its own to another larger body x, in any proportion lefs than that of x^/x to 1. Thus if x = ioooooo, the boay 1 with the velocity j, may, by the means of intermediate bodies, give it the force 999000000 ; and the fum of the abfolute forces of all thefe bodies taken together, arifing from the percuffion, may ex- ceed the force of the firft or unity, in any ratio lefs than 2Xa/x to 1, or 2COC0OOO0O to 1, altho' the relative force in all cafes remains as at firft — r.
Thefe are paradoxes which cannot be urged againft thofe who maintain the forces of bodies to be meafured by their maffes and the fquares of their velocities.
It is alfo to be obferved, that thofe who meafure force by the quantity of motion, muft deny that there is any force loft by the impxeflions made on foft bodies, or by the bending of elaftic bodies : for if any were loft from fuch caufes, the fum of the quantities of motion of bodies moving the fame way, could not remain the fame before, during, and after the ftiock, which is an umverfal rule admitted by the New- tonians. But they admit that a fpring bent between two bo- dies may, by unbending itfelf, generate motion in thefe bo- dies. Hence it follows, that according to this doctrine, tho* a fpring may give motion to bodies, it cannot, by a contrary action, take it away; which feems no fmall paradox. If a body A move towards the body C, at reft, and an in- termediate bodv B of a mean magnitude be placed at reft between them, fo that A firft impels B, and then B impels C, a greater motion will be given to C than if A had ftruck it directly. And if B be a mean proportional between A
and C, the velocity communicated to C will be the great- eft that can be. Huygem* de mot. Corp. ex Percufl". Prop. 12.
If more bodies be interpofed between A and C, the motion of C will be greater ; and it will be the greateft when the bodies interpofed conftitute a feries of mean proportionals between A and C. Ibid. Prop, j 3.
If there be a feries of an hundred bodies in the proportion of r, 7* 4, 8, 16, bV. and the motion begins by the greateft body, the velocity communicated to the fmalleft will be to that with which the greateft moved nearly as 2,338,50c, oco,ooo to 1. And if the motion begins by the leaft body, the quantity of motion will be increafed on the whole near- ly as 1 to 4,677,000,000,000. Huyg- ib. But it is to be obferved, that there is a miftake in the im- preffion of Huygens's book, as to the firft number, which is thereprinted 14,760,000,000, infteadof 2,338, iffc. fo that the velocity thus given to the fmaileftbody is 150 times grea- ter than what the printed copy of Huygens afligns. Bernoulli* Difc. furle Mouv. Oper. Tom. 3. p. 34. and S. Grave/and^, Pref. ad Huygen. Oper. Pofth. Tom. 2. Amft. 1728. This ftupendous augmentation of the quantity of motion, is a remarkable inftance of the falfily of the Cartefian prin- 1 D d ciple,
