Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/172

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tricula, all diftinct in them, as in the cartilaginous fifhes of many kinds. In others, as in the cetaceous kinds, they are very i'mall and fimple, and have no diftinct appearance either of white, yoik, or of the cicatricula. Hence it is evident, that the fcetus of cetaceous fifhes, while included in the egg, receives its nourifhment from the womh itfelf ; but in the others, as the cartilaginous, and other fifh, the white of the egg ferves as nourifhment to the fcetus while it remains in it. Finally, in the generality of other fifhes, the eggs are very fmall, and probably do contain a white, and yolk, tho' their fize makes it not eafy to fee their parts diftincifly. All the fpinofe fifhes have thefe fmall forts of eggs, and among the cartilaginous ones, the accipenfer and petromyzum. It has been fuppofed by fome, that the eggs of thefe fifhes were fimple bodies, and only anfwered to what Harvey, and other authors on thefe fubjecls call the ciartruu'a; but this cannot be the cafe, for if fo, the young fifh mult pcrifh; for the femen of the male fifh is only fcattered over the eggs while they lie in the water, and ferves only to the rendring them prolific. The figures of all the eggs of fifhes, fo far as is yet known, is round. Artcdi, Ichthyology.

OVATED-Aa/, among botanifts. See Leaf.

OUCH, in our old writers, a collar of gold, or fuch like or- nament, worn by women about their necks. Stat. 24 Hen. VIII. c. 13 Blount, Ccwel.

OVEN, or Maying Ovkn, in metallurgy, is the particular fort of furnace, ufed by the afiayers in their operations on metals.

It is moft conveniently conftructed in the following manner : Make with iron plates a hollow quadrangular prifm, eleven inches broad, and nine inches high, ending at top in a hollow quadrangular pyramid of feven inches in height; this prifm muft be clofed at bottom with fuch another iron plate, which ferves as a bottom to it. Near the bottom make a door three inches high, and five inches broad, to lead to the afh-hole. Above this door, and at the height of fix inches from the bafis, make another door of the figure of the fegment of a circle, four inches broad at its bafis, and three inches and a half high in the middle; then faftcn three iron plates on the forepart of this furnace; let the firft of them, which muff he eleven inches long, and half an inch high, be placed with its lower edge againft the bottom of the furnace, and faftened there with three or four rivets in fuch a manner, that there may be, between the upper edge of the faid plate, and the fide of the furnace, a groove fo wide, that the Aiders of the lower door may be put into it, and freely move back- ward and forward therein : thefe mult be made of a thicker iron plate.

The fecond iron plate eleven inches long, and three inches high, muff be placed perfectly parallel to the foregoing plate ; and in the fpace between the two doors in fuch a manner, that both the upper and lower edge of it may, with the fide of the furnace, form a hollow groove: one of thefe grooves which looks downward, ferves to receive the upper edges of the Aiders that fhut the lower door ; and the other that looks upward, is to receive the inferior edges of the Aiders of the door above.

The third plate, which is to be of the fame dimenfions with the firft, muff be riveted elofe above the upper door, in fuch a manner, that it may form a groove looking downward, and contiguous to the upper edge of the upper door. In order to fhut both doors, you muff adapt to each of them two Aiders made of iron plates, that niay move within the above- mentioned grooves ; but the two Aiders belonging to the up- per door, mull have each a hole near the top ; that is, one a fmall hole a fifth of an inch broad, and an inch and a half long ; and the other a femicircular aperture, one inch high, and two inches broad : befides this, let each Aider have a handle, that it may be laid hold of when it is to be moved.

Befide thefe, let five round holes, each of an inch in dia- meter, be bored in the furnace ; two of thefe muff be made in the fore part of the furnace, two in the back part of it, but all at the height of five inches from the bottom, and three inches and a half diffant from each fide of the furnace : and, finally, a fifth hole muff be made at the height of one inch above the upper edge of the upper door. The infide of the furnace muff be lined throughout with iron hooks, Hand- ing out about half an inch, and placed at about three inches diftance from one another, to hold on, and fallen the matter of the lute, with which the whole inner furface of the fur- nace is to be coated.

Let an iron moveable hollow quadrangular pyramid, of three inches in height, be next adapted to the upper aperture of the furnace ; this mufi he feven inches broad at the bafis, and end upwards in a hollow tube three inches in diameter, two inches high, and nearly cylindrical, but a little conver- gent at the top : this prominent tube ferves to fupport a fun- nel, or flue, which muff be nearly cylindrical, hollow, and two foot high, and made of iron plates. This, when a very ftrong fire is required, mud be put perpendicularly upon the fljortcr tube, in fuch a manner, that it enters clofely and evenly into it an inch and a half, or two inches deep ; and

may be taken off at pleafure when the fire is not required to be fo very violent. This pyramidal cover muff, however, have befide, two handles fitted to it, that it may convenient- ly alfo be taken off or put on at pleafure; and that this when put on the aperture of the furnace may not be fiibjeQ to be eafily thrown down, let an iron plate be riveted to the right and left upper edge of the furnace, and be turned down to- ward the inlide, fo as to make a furrow open before and be- hind, into which the lateral edges of this cover may enter, and be faftened, and at pleafure moved backward and for- ward when it is to be put on or removed. Laftly, let a fquare ledge, made of a thick iron plate, be faftesed on the top of the upper edge of the lower door which will conveniently fupport the grate and the lute; but this muff be made of two pieces, that it may eafily be in- troduced into the cavity of the furnace; and thus will you have an ajfay oven compleat, and ready for a great variety of operations.

When this is to be ufed, that the fire may be the better con- fined, and the iron not be deft-roved by growing red-hot, the whole infide of the furnace muft be covered over with'lute a finger, or a finger and a half thick. The matter for the making the afiaying metals, l$e. or a mixture of clav with fand, powder of calcined flints, or broken crucibles 'cither mixed with water, or with blood diluted with' water will ferve very well for this purpofe: but the moft ready of all lutes, and at the fame time one of the beft in the world is the coarfe earth called Windfor loam : this muft be mixer! up with water pretty ftiff, and preffed on the infide of "the furnace, firft wetted alfo with water; and when this lute begins to dry, it muft be beat down clofe to the fides with a wooden mallet ; and then the unevennedes and cracks, filled up with clay fomewhat moifter, fo as to be made i'moorh and even with a trowel, and then left to dry gently ; and if any cracks happen, they muft again be filled up'. ' If a nv pieces of this lute are broken off by the fire, let it be quite cold, and wet it after the operation is over, and the edges of the old lute and frefh clay will unite very well, and fill up the holes. If the crack is but fmall you may ufe the furnace again immediately, even before the frefh clay is dry. This is the method of coating the ajay furnace or oven; but before this is done there muft be put within the furnace fmall iron bars, equal in length to the diameter of the oven ; thefe muft be prifmatical in fhape, quadrangular, and half an inch thick : their extremities will be fupported by the ledge be- fore defcribed within the oven ; and they muft be placed at three quarters of an inch diftance from one another. Thefe muft alfo be fo placed, that their flat fides may be oblique with regard to the tranfverfe feflion of the furnace, and that the two oppofite angles may look one upwards and the other downwards. The bars being thus not laid flat, but edge- ways, you hinder the afhes of the fuel of the fire from be- ing detained too long between the interfaces of the bars and from making any obflruclion that would oppofe the fr-4 draught of the air. " After the placing thefe, the furnace being coated over with lute, and dried by a gentle heat, is fit for ufc.

When an operation is to be performed in this, two iron bars of an inch in thicknefs, and fomewhat more than the dia- meter of the furnace in length, muft be put thro' the four holes before defcribed, ftanding oppofite to one another • and the ends of the bars muft jut out a little beyond the edges of the holes on each fide ; thefe ferve to fupport the muftfewith its bottom, in which the coppcl or teft, with the matter to be worked on is to be placed : thefe are then to be introduced thro' the upper aperture of the furnace, and let on thefe bars in fuch a manner, that the open forefide of the muffle be contiguous to the inward border of the upper door. The fuel is to be introduced thro' the top of the furnace and the cover on this account muft be moveable, and not too ' heavy. The beft fuel is charcoal made of hard wood and broken into pieces of an inch big, that the muffle may be covered regularly over fome inches high ; large pieces of coal are to be avoided at this time, becaufe they cannot fall into the narrow mterftices between the fides of the muffle and thofc of the furnace ; and cannot of courfe fur- round every way this circumference of the muffle ; for which reafon if fuch were ufed, there muft be fome places void of fuel, and the fire, confequently, either not ftrono enough or unequal : and if, on the other hand, the pieces of coal were too fmall, they would fall immediately thro' the intcr- ftices of the grate into the afh hole, and the tendereft par- ticles of them turn too foon into afhes, and increafe the heap of afhes, and obftrucf the free draught of the air. Scotch coal, or Kennel coal, but not Newcaftle or fea coal broken into fmall pieces, may be mixed with an equal quan- tity of charcoal ; or if any ores require long roaftin", the fire may be firft kindled with Scotch coal alone, but th^'ope- r,ition muft be finifhed with charcoal alone. Cramer's Art of Aff. p. 75. feq. OVER (Cycl.) — Over-Mow, in the fea language. Thoyfaylt over-blows when the wind blows fo very hard that the ftiip can bear no top fails.

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