The edition of the Bible, which was one of the very first productions of Gutenberg and Fust's press, is called the Mazarin, in consequence of the first known copy having been discovered in the famous library formed by Cardinal Mazarin. It seems to have been printed as early as August 1456, and is a truly admirable specimen of typography; the characters being very clear and distinct, and the uniformity of the printing perfectly remarkable. A copy in the Royal Library at Paris is bound in two volumes, and every complete page consists of two columns, each containing forty-two lines. The reader will recognize the appropriateness of the fact that from the first printing press the first important work produced should be a copy of G.o.d's Word. It sanctified the new art which was to be so fruitful of good and evil results--the good superabounding, and clearly visible--the evil little, and destined, perhaps, to be directed eventually to good--for successive generations of mankind. It was a fitting forerunner of the long generation of books which have since issued so ceaselessly from the printing press; books, of the majority of which we may say, with Milton, that "they contain a potency of life in them to be as active as those souls were whose progeny they are; to preserve, as in a vial, the purest efficacy and extraction of the living intellects that feed them."

Gutenberg's career was dashed with many lights and shadows, but it closed in peace. In 1465, the Archbishop-elector of Mentz appointed him one of his courtiers, with the same allowance of clothing as the remainder of the n.o.bles attending his court, and all other privileges and exemptions. It is probable that from this time he abandoned the practice of his new invention. The date of his death is uncertain; but there is doc.u.mentary evidence extant which proves that it occurred before February 24, 1468. He was interred in the church of the Recollets at Mentz, and the following epitaph was composed by his kinsman Adam Gelthaus:--

"D. O. M. S.

"Joanni Gesnyfleisch, artis impressoriae repertori, de omni natione et lingua optime merito, in nominis sui memoriam immortalem Adam Gelthaus posuit. Ossa ejus in ecclesia D.

Francisci Moguntina feliciter cubant."

II.--WILLIAM CAXTON.

During the last thirty or forty years of the fifteenth century, while printing was becoming gradually more and more practised on the Continent, and the presses of Mentz, Bamberg, Cologne, Strasburg, Augsburg, Rome, Venice, and Milan, were sending forth numbers of Bibles, and various learned and theological works, chiefly in Latin, an English merchant, a man of substance and of no little note in Chepe, appeared at the court of the Duke of Burgundy at Bruges, to negotiate a commercial treaty between that sovereign and the king of England; which accomplished, the worthy amba.s.sador seems to have liked the place and the people so well, and to have been so much liked in return, that for some years afterwards he took up his residence there, holding some honourable, easy appointment in the household of the d.u.c.h.ess of Burgundy. This was William Caxton, who here ripened, if he did not acquire, his love of literature and scholarship, and began, from hatred of idleness, to take pen in hand himself.

"When I remember," says he, in his preface to his first work, a translation of a fanciful "Recueil des Histoires de Troye," "that every man is bounden by the commandment and counsel of the wise man to eschew sloth and idleness, which is mother and nourisher of vices, and ought to put himself into virtuous occupation and business, then I, having no great charge or occupation, following the said counsel, took a French book, and read therein many strange marvellous histories. And for so much as this book was new and late made, and drawn into French, and never seen in our English tongue, I thought in myself, it should be a good business to translate it into our English, to the end that it might be had as well in the royaume of England as in other lands, and also to pa.s.s therewith the time; and thus concluded in myself to begin this said work, and forthwith took pen and ink, and began boldly to run forth, as blind Bayard, in this present work."

While at work upon this translation, Caxton found leisure to visit several of the German towns where printing presses were established, and to get an insight into the mysteries of the art, so that by the time he had finished the volume, he was able to print it. At the close of the third book of the "Recuyell," he says: "Thus end I this book which I have translated after mine author, as nigh as G.o.d hath given me cunning, to whom be given the laud and praise. And for as much as in the writing of the same my pen is worn, mine hand weary and not steadfast, mine eyen dimmed with overmuch looking on the white paper, and my courage not so p.r.o.ne and ready to labour as it hath been, and that age creepeth on me daily, and feebleth all the body; and also because I have promised to divers gentlemen and to my friends, to address to them as hastily as I might, this said book, therefore I have practised and learned, at my great charge and dispense, to ordain this said book in print, after the manner and form you may here see; and is not written with pen and ink as other books are, to the end that every man may have them at once. For all the books of this story, named the 'Recuyell of the Historyes of Troye,' thus imprinted as ye here see, were begun in one day, and also finished in one day" (that is, in the same s.p.a.ce of time).

By the year 1477, Caxton had returned to London, and set up a printing establishment within the precincts of Westminster Abbey; had given to the world the three first books ever printed in England,--"The Game and Play of the Chesse" (March 1474); "A boke of the hoole Lyf of Jason" (1475); and "The Dictes and Notable Wyse Sayenges of the Phylosophers" (1477),--and was fairly started in the great work of supplying printed books to his countrymen, which, as a placard in his largest type sets forth, if any one wanted, "emprynted after the forme of this present lettre whiche ben well and truly correct, late hym come to Westmonster, in to the Almonesrye, at the reed pale, and he shal have them good chepe." From the situation of the first printing office, the term chapel is applied to such establishments to this day.

[Ill.u.s.tration: WILLIAM CAXTON. Page 30.]

Caxton published between sixty and seventy different works during the seventeen years of his career as a printer, all of them in what is called black letter, and the bulk of them in English. He had always a view to the improvement of the people in the works he published, and though many of his productions may seem to us to be of an unprofitable kind, it is clear that in the issue of chivalrous narratives, and of Chaucer's poems (to whom, says the old printer, "ought to be given great laud and praising for his n.o.ble making and writing"), he was aiming at the diffusion of a n.o.bler spirit, and a higher taste than then prevailed.

In 1490, Caxton, an old, worn man, verging on fourscore years of age, wrote, "Every man ought to intend in such wise to live in this world, by keeping the commandments of G.o.d, that he may come to a good end; and then, out of this world full of wretchedness and tribulation, he may go to heaven, unto G.o.d and his saints, unto joy perdurable;" and pa.s.sed away, still labouring at his post. He died while writing, "The most virtuous history of the devout and right renouned Lives of Holy Fathers living in the desert, worthy of remembrance to all well-disposed persons."

Wynkyne de Worde filled his master's place in the almonry of Westminster; and the guild of printers gradually waxed strong in numbers and influence. In Germany they were privileged to wear robes trimmed with gold and silver, such as the n.o.bles themselves appeared in; and to display on their escutcheon, an eagle with wings outstretched over the globe,--a symbol of the flight of thought and words throughout the world. In our own country, the printers were men of erudition and literary acquirements; and were honoured as became their mission.

III.--THE PRINTING MACHINE.

Between the rude screw-press of Gutenberg or Caxton, slow and laboured in its working, to the first-cla.s.s printing machine of our own day, throwing off its fifteen or eighteen thousand copies of a large four-page journal in an hour, what a stride has been taken in the n.o.ble art! Step by step, slowly but surely, has the advance been made,--one improvement suggested after another at long intervals, and by various minds. With the perfection of the printing press, the name of Earl Stanhope is chiefly a.s.sociated; but, although when he had put the finishing touches to its construction, immensely superior to all former machines, it was unavailable for rapid printing. In relation to the demand for literature and the means of supplying it, the world had, half a century ago, reached much the same deadlock as in the days when the production of books depended solely on the swiftness of the transcriber's pen, and when the printing press existed only in the fervid brain and quick imagination of a young German student. Not only the growth, but the spread of literature, was restricted by the labour, expense, and delay incident to the multiplication of copies; and the popular appet.i.te for reading was in that transition state when an increased supply would develop it beyond all bounds or calculation, while a continuance of the starvation supply would in all likelihood throw it into a decline from want of exercise.

Such was the state of things when a revolution in the art of printing was effected which, in importance, can be compared only to the original discovery of printing. In fact, since the days of Gutenberg to the present hour, there has been only one great revolution in the art, and that was the introduction of steam printing in 1814. The neat and elegant, but slow-moving Stanhope press, was after all but little in advance of its rude prototype of the fifteenth century, the chief features of which it preserved almost without alteration. The steam printing machine took a leap ahead that placed it at such a distance from the printing press, that they are hardly to be recognised as the offspring of the same common stock. All family resemblance has died out, although the printing machine is certainly a development of the little screw press.

Of the revolution of 1814, which placed the printing machine in the seat of power, _vice_ the press given over to subordinate employment, Mr.

John Walter of the _Times_ was the prominent and leading agent. But for his foresight, enterprise, and perseverance, the steam machine might have been even now in earliest infancy, if not unborn.

Familiar as the invention of the steam printing machine is now, in the beginning of the present century it shared the ridicule which was thrown upon the project of sailing steam ships upon the sea, and driving steam carriages upon land. It seemed as mad and preposterous an idea to print off 5000 impressions of a paper like the _Times_ in one hour, as, in the same time, to paddle a ship fifteen miles against wind and tide, or to propel a heavily laden train of carriages fifty miles. Mr. Walter, however, was convinced that the thing could be done, and lost no time in attempting it. Some notion of the difficulties he had to overcome, and the disappointments he had to endure, while engaged in this enterprise, may be gathered from the following extracts from the biography of Mr.

Walter, which appeared in the _Times_ at the time of his death in July 1847:--

"As early as the year 1804, an ingenious compositor, named Thomas Martyn, had invented a self-acting machine for working the press, and had produced a model which satisfied Mr. Walter of the feasibility of the scheme. Being a.s.sisted by Mr. Walter with the necessary funds, he made considerable progress towards the completion of his work, in the course of which he was exposed to much personal danger from the hostility of the pressmen, who vowed vengeance against the man whose inventions threatened destruction to their craft. To such a length was their opposition carried, that it was found necessary to introduce the various pieces of the machine into the premises with the utmost possible secresy, while Martyn himself was obliged to shelter himself under various disguises in order to escape their fury. Mr. Walter, however, was not yet permitted to reap the fruits of his enterprise. On the very eve of success he was doomed to bitter disappointment. He had exhausted his own funds in the attempt, and his father, who had hitherto a.s.sisted him, became disheartened, and refused him any further aid. The project was, therefore, for the time abandoned.

"Mr. Walter, however, was not the man to be deterred from what he had once resolved to do. He gave his mind incessantly to the subject, and courted aid from all quarters, with his usual munificence. In the year 1814 he was induced by a clerical friend, in whose judgment he confided, to make a fresh experiment; and, accordingly, the machinery of the amiable and ingenious Koenig, a.s.sisted by his young friend Bower, was introduced--not, indeed, at first into the _Times_ office, but into the adjoining premises, such caution being thought necessary upon the threatened violence of the pressmen. Here the work advanced, under the frequent inspection and advice of the friend alluded to. At one period these two able mechanics suspended their anxious toil, and left the premises in disgust. After the lapse, however, of about three days, the same gentleman discovered their retreat, induced them to return, showed them, to their surprise, their difficulty conquered, and the work still in progress. The night on which this curious machine was first brought into use in its new abode was one of great anxiety, and even alarm. The suspicious pressmen had threatened destruction to any one whose inventions might suspend their employment. 'Destruction to him and his traps.' They were directed to wait for expected news from the Continent.

It was about six o'clock in the morning when Mr. Walter went into the press-room, and astonished its occupants by telling them that 'The _Times_ was already printed by steam! That if they attempted violence, there was a force ready to suppress it; but that if they were peaceable, their wages should be continued to every one of them till similar employment could be procured,'--a promise which was, no doubt, faithfully performed; and having so said, he distributed several copies among them. Thus was this most hazardous enterprise undertaken and successfully carried through, and printing by steam on an almost gigantic scale given to the world."

On that memorable day, the 29th of November 1814, appeared the following announcement,--"Our journal of this day presents to the public the practical result of the greatest improvement connected with printing since the discovery of the art itself. The reader now holds in his hands one of the many thousand impressions of the _Times_ newspaper which were taken off last night by a mechanical apparatus. That the magnitude of the invention may be justly appreciated by its effects, we shall inform the public that after the letters are placed by the compositors, and enclosed in what is called a form, little more remains for man to do than to attend and watch this unconscious agent in its operations. The machine is then merely supplied with paper; itself places the form, inks it, adjusts the paper to the form newly inked, stamps the sheet, and gives it forth to the hands of the attendant, at the same time withdrawing the form for a fresh coat of ink, which itself again distributes, to meet the ensuing sheet, now advancing for impression; and the whole of these complicated acts is performed with such a velocity and simultaneousness of movement, that no less than 1100 sheets are impressed in one hour."

Koenig's machine was, however, very complicated, and before long, it was supplanted by that of Applegath and Cowper, which was much simpler in construction, and required only two boys to attend it--one to lay on, and the other to take off the sheets. The vertical machine which Mr.

Applegath subsequently invented, far excelled his former achievement; but it has in turn been superseded by the machine of Messrs. Hoe of New York. All these machines were first brought into use in the _Times'_ printing office; and to the encouragement the proprietors of that establishment have always afforded to inventive talent, the readiness with which they have given a trial to new machines, and the princely liberality with which they have rewarded improvements, is greatly due the present advanced state of the n.o.ble craft and mystery.

The printing-house of the _Times_, near Blackfriars Bridge, forms a companion picture to Gutenberg's printing-room in the old abbey at Strasburg, and ill.u.s.trates not only the development of the art, but the progress of the world during the intervening centuries. Visit Printing-House Square in the day-time, and you find it a quiet, sleepy place, with hardly any signs of life or movement about it, except in the advertis.e.m.e.nt office in the corner, where people are continually going out and in, and the clerks have a busy time of it, shovelling money into the till all day long. But come back in the evening, and the place will wear a very different aspect. All signs of drowsiness have disappeared, and the office is all lighted up, and instinct with bustle and activity. Messengers are rushing out and in, telegraph boys, railway porters, and "devils" of all sorts and sizes. Cabs are driving up every few minutes, and depositing reporters, hot from the gallery of the House of Commons or the House of Lords, each with his budget of short-hand notes to decipher and transcribe. Up stairs in his sanctum the editor and his deputies are busy preparing or selecting the articles and reports which are to appear in the next day's paper. In another part of the building the compositors are hard at work, picking up types, and arranging them in "stick-fulls," which being emptied out into "galleys,"

are firmly fixed therein by little wedges of wood, in order that "proofs" may be taken of them. The proofs pa.s.s into the hands of the various sets of readers, who compare them with the "copy" from which they were set up, and mark any errors on the margin of the slips, which then find their way back to the compositors, who correct the types according to the marks. The "galleys" are next seized by the persons charged with the "making-up" of the paper, who divide them into columns of equal length. An ordinary _Times_ newspaper, with a single inside sheet of advertis.e.m.e.nts, contains seventy-two columns, or 17,500 lines, made up of upwards of a million pieces of types, of which matter about two-fifths are often written, composed, and corrected after seven o'clock in the evening. If the advertis.e.m.e.nt sheet be double, as it frequently is, the paper will contain ninety-six columns. The types set up by the compositors are not sent to the machine. A mould is taken of them in a composition of brown paper, by means of which a "stereotype"

is cast in metal, and from this the paper is printed. The advertis.e.m.e.nt sheet, single or double, as the case may be, is generally ready for the press between seven or eight o'clock at night. The rest of the paper is divided into two "forms,"--that is, columns arranged in pages and bound together by an iron frame, one for each side of the sheet. Into the first of these the person who "makes up" the paper endeavours to place all the early news, and it is ready for press usually about four o'clock. The other "form" is reserved for the leading articles, telegrams, and all the latest intelligence, and does not reach the press till near five o'clock.

The first sight of Hoe's machine, by several of which the _Times_ is now printed, fills the beholder with bewilderment and awe. You see before you a huge pile of iron cylinders, wheels, cranks, and levers, whirling away at a rate that makes you giddy to look at, and with a grinding and gnashing of teeth that almost drives you deaf to listen to. With insatiable appet.i.te the furious monster devours ream after ream of snowy sheets of paper, placed in its many gaping jaws by the slaves who wait on it, but seems to find none to its taste or suitable to its digestion, for back come all the sheets again, each with the mark of this strange beast printed on one side. Its hunger never is appeased,--it is always swallowing and always disgorging, and it is as much as the little "devils" who wait on it can do, to put the paper between its lips and take it out again. But a bell rings suddenly, the monster gives a gasp, and is straightway still, and dead to all appearance. Upon a closer inspection, now that it is at rest, and with some explanation from the foreman you begin to have some idea of the process that has been going on before your astonished eyes.

The core of the machine consists of a large drum, turning on a horizontal axis, round which revolve ten smaller cylinders, also on horizontal axes, in close proximity to the drum. The stereotyped matter is bound, like a malefactor on the wheel, to the central drum, and round each cylinder a sheet of paper is constantly being pa.s.sed. It is obvious, therefore, that if the type be inked, and each of the cylinders be kept properly supplied with a sheet of paper, a single revolution of the drum will cause the ten cylinders to revolve likewise, and produce an impression on one side of each of the sheets of paper. For this purpose it is necessary to have the type inked ten times during every revolution of the drum; and this is managed by a very ingenious contrivance, which, however, is too complicated for description here.

The feeding of the cylinders is provided for in this way. Over each cylinder is a sloping desk, upon which rests a heap of sheets of white paper. A lad--the "layer-on"--stands by the side of the desk and pushes forward the paper, a sheet at a time, towards the tape fingers of the machine, which, clutching hold of it, drag it into the interior, where it is pa.s.sed round the cylinders, and printed on the outer side by pressure against the types on the drum. The sheet is then laid hold of by another set of tapes, carried to the other end of the machine from that at which it entered, and there laid down on a desk by a projecting flapper of lath-work. Another lad--the "taker-off"--is in attendance to remove the printed sheets, at certain intervals. The drum revolves in less than two seconds; and in that time therefore ten sheets--for the same operation is performed simultaneously by the ten cylinders--are sucked in at one end and disgorged at the other printed on one side, thus giving about 20,000 impressions in an hour.

Such is the latest marvel of the "n.o.ble craft and mystery" of printing; but it is not to be supposed that the limits of production have even now been reached. The greater the supply the greater has grown the demand; the more people read, the more they want to read; and past experience a.s.sures us that ingenuity and enterprise will not fail to expand and multiply the powers of the press, so that the increasing appet.i.te for literature may be fully met.

We have briefly alluded to stereotyping; but some fuller notice seems requisite of a process so valuable and important, without which, indeed, the rapid multiplication of copies of a newspaper, even by a Hoe's six-cylinder machine, would be impossible. If stereotyping had not been invented, the printer would require to "set up" as many "forms" of type as there are cylinders in the machine he uses; an expensive and time-consuming operation which is now dispensed with, because he can resort to "casts." There is yet another advantage gained by the process; "casts" of the different sheets of a book can be preserved for any length of time; and when additional copies or new editions are needed, these "casts" can at once be sent to the machine, and the publisher is saved the great expense of "re-setting."

The reader is well aware that while many books disappear with the day which called them forth, so there are others for which the demand is constant. This was found to be the case soon after the invention of printing, and the plan then adopted was the expensive and c.u.mbrous one of setting up the whole of the book in request, and to keep the type standing for future editions. The disadvantages of this plan were obvious--a large outlay for type, the amount of s.p.a.ce occupied by a constantly increasing number of "forms," and the liability to injury from the falling out of letters, from blows, and other accidents. As early as the eighteenth century attempts seem to have been made to remedy these inconveniences by cementing the types together at the bottom with lead or solder to effect their greater preservation. Canius, a French historian of printing, states that in June 1801 he received a letter from certain booksellers of Leyden, with a copy of their stereotype Bible, the plates for which were formed by soldering together the bottom of common types with some melted substance to the thickness of about three quires of writing-paper; and, it is added, "These plates were made about the beginning of the last century by an artist named Van du Mey."

This, however, was not true stereotyping; whose leading principle is to dispense with the movable types--to set them again, as it were, at liberty--by making up perfect fac-similes in type-metal of the various combinations into which they may have entered. These fac-similes being made, the type is set free, and may be distributed, and used for making up fresh pages; which may once more furnish, so to speak, the punches to the mould into which the type-metal is poured for the purpose of effecting the fac-simile.

The inventor of this ingenious process of casting plates from pages of type was William Ged, a goldsmith of Edinburgh, in 1735. Not possessing sufficient capital to carry out his invention, he visited London, and sought the a.s.sistance of the London stationers; from whom he received the most encouraging words, but no pecuniary a.s.sistance. But Ged was a man not readily discomfited, and applying at length to the Universities and the King's printer, he obtained the effective patronage he needed.

He "stereotyped" some Bibles and Prayer-books, and the sheets worked off from his plates were admitted equal in point of appearance and accuracy to those printed from the type itself.

But every benefactor of his kind is doomed to meet with the opposition of the envious, the ignorant, or the prejudiced. "The argument used by the idol-makers of old, 'Sirs, ye know that by this craft we have our wealth,' and, 'This our craft is in danger to be set at nought,' was, as is usual in such cases, urged against this most useful and important invention. The compositors refused to set up works for stereotyping, and even those which were set up, however carefully read and corrected, were found to be full of gross errors. The fact was, that when the pages were sent to be cast, the compositors or pressmen, bribed, it is said, by a typefounder, disturbed the type, and introduced false letters and words. Poor Ged died, and left the dangerous secret of his art (which he did not disclose during his life-time) to his son, who, after many struggles for success, failed as his father had done before him." There is a tradition current, however, that he joined the Jacobite rebellion, was arrested, imprisoned, tried, and sentenced, but was eventually spared in consideration of the value of his father's admirable invention.

That invention, after being forgotten for nearly half a century, was revived by a Dr. Tilloch, and taken up, improved, and extended by the ingenious Earl Stanhope. It is now practised in the following manner:--

The type employed differs slightly from that in common use. The letter should have no shoulder, but should rise in a straight line from the foot; the s.p.a.ces, leads, and quadrats are of the same height as the stem of the letter; the object being to diminish the number and depth of the cavities in the page, and thus lessen the chances of the mould breaking off and remaining in the form. Each page is corrected with the utmost care, and "imposed" in a small "chase" with metal furniture (or frame-work), which rises to a level with the type. Of course the number of pages in the form will vary according to the size of the book; a sheet being folded into sixteen leaves, twelve, eight, four, or two for 16mo, 12mo, 8vo, quarto, or folio.

Having our pages of type in complete order, we now proceed to rub the surface with a soft brush which has been lightly dipped into a very thin oil. Plumbago is sometimes preferred. A bra.s.s rectangular frame of three sides, with bevelled borders adapted to the size of the pages, is placed upon the chase so as to enclose three sides of the type, the fourth side being formed by a single bra.s.s edge, having the same inward sloping level as the other three sides. The use of this frame is to determine the size and thickness of the cast, which is next taken in plaster-of-paris--two kinds of the said plaster being used; the finer is mixed, poured over the surface of the type, and gently worked in with a brush so as to insure its close adhesion to the exclusion of bubbles of air; the coa.r.s.er, after being mixed with water, is simply poured and spread over the previous and finer stratum.

The superfluous plaster is next cleared away; the mould soon sets; the frame is raised; and the mould comes off from the surface of the type, on which it has been prevented from encrusting itself by the thin film of oil or plumbago.

The next step is to dress and smoothen the plaster-mould, and set it on its edge in one of the compartments of a sheet-iron rack contained in an oven, and exposed, until perfectly dry, to a temperature of about 400.

This occupies about two hours. A good workman, it is said, will mould ten octavo sheets, or one hundred and sixty pages in a day: each mould generally contains a couple of octavo pages.

[Ill.u.s.tration]

In the state to which it is now brought, the mould is exceedingly friable, and requires to be handled with becoming care. With the face downwards it is placed upon the flat cast-iron _floating-plate_, which, in its turn, is set at the bottom of a square cast-iron tray, with upright edges sloping outwards, called the "dipping pan." It has a cast-iron lid, secured by a screw and shackles, not unlike a copying machine. This pan having been heated to 400, it is plunged into an iron pot containing the melted alloy, which hangs over a furnace, the pan being slightly inclined so as to permit the escape of the air. A small s.p.a.ce is left between the back or upper surface of the mould, and the lid of the dipping-pan, and the fluid metal on entering into the pan through the corner openings, _floats_ up the plaster together with the iron plate (hence called the _floating-plate_) on which the mould is set, with this effect, that the metal flows through the notches cut in the edge of the mould, and fills up every part of it, forming a layer of metal on its face corresponding to the depth of the border, while on the back is left merely a thin metallic film.

The dipping-pan, says Tomlinson, is suspended, plunged in the metal, and removed by means of a crane; and when taken out, is set in a cistern of water upon supports so arranged that only the bottom of the pan comes in contact with the surface of the water. The metal thus _sets_, or solidifies, from below, and containing fluid above, maintains a fluid pressure during the contraction which accompanies the cooling.

As it thus shrinks in dimensions, molten metal is poured into the corners of the pan for the purpose of maintaining the fluid pressure on the mould, and thus securing a good and solid cast. For if the pan were allowed to cool more slowly, the thin metallic film at the back of the inverted plaster mould would probably solidify first, and thus prevent the fluid pressure which is necessary for filling up all the lines of the mould.