CATHERINE II., empress of all the Russias, acted so conspicuous a part on the theatre of the world; possessed such uncommon powers of mind, highly cultivated by science and literature; and was such a patroness of science and literature in others—that it cannot be deemed foreign from a work of this nature to give some account of the principal events of her more private life.

SOPHIA AUGUSTA FREDERICA, who, upon her marriage to the grandson of Peter the Great, assumed the name CATHERINA ALEXEIEVNA, was born at Stettin on the 2d of May 1729. Her father was Christian Augustus, prince of Anhalt-Zerbit-Dornburg, at that time major-general in the Prussian service, commander-in-chief of the regiments of infantry, and governor of the town and fortresses of Stettin. Her mother, who was born princess of Holstein-Eutin, was a woman of great parts and beauty, of nearly the same age with the prince royal of Prussia, afterwards Frederic the Great, with whom she kept up a regular correspondence, and who afterwards contributed to the aggrandisement of her daughter. This accomplished princess took upon herself the care of educating the young Sophia, whom she brought up in the simplest manner, and would not suffer to exhibit the least symptoms of that pride to which she had some propensity from her earliest childhood. The consequence of this salutary restraint was, that good humour, Catherine, morn, intelligence, and spirit, were even then the striking features of her youthful character. Being naturally addicted to reading, to reflection, to learning, and to employment, she was taught the French and other fashionable languages; and was instructed to read such books chiefly as might make her acquainted with history and with the principles of science; whilst the doctrines of the Lutheran religion were carefully explained to her by a divine, who little thought how soon his illustrious pupil would embrace another faith.

The Empress Elizabeth, who then swayed the sceptre of Russia, had in early life been promised in marriage to the young prince of Coblenz-Eutin, brother to the princesses of Anhalt-Zerbitz; but at the infant when the marriage was about to be celebrated, the prince fell sick and died. Elizabeth, who loved him to excess, became inconsolable, and in the bitterness of her grief made a vow of celibacy. This vow, though fateful, and even laudable, he kept so far as never publicly to acknowledge any man as a husband; and upon her ascending the throne of her ancestors, she called her nephew the Duke of Holstein-Gottorp to her court, where he was solemnly proclaimed, when fourteen years of age, Grand Duke, with the title of Imperial Highness, and declared successor to the Empress Elizabeth. To secure the succession in the family of Peter the Great, the Empress was very desirous to have her nephew married; and the princesses of Anhalt-Zerbitz, not ignorant of the tender remembrance which she still preferred for her brother, conceived the idea of placing, by means of it, her daughter on the throne of Russia. She communicated her plan to the king of Prussia, who not only applauded it, but lent her his assistance to carry it into execution.

Full of ambitious hopes, therefore, the princess repaired with her daughter to St Petersburg, where she was received with friendship by Elizabeth, and where the young Sophia soon made a considerable impression on the mind of the Grand Duke. As Peter was well made, of a good figure, and, though uneducated, not destitute of natural talents, the attachment became reciprocal; and the princess of Zerbitz, throwing herself at the feet of the empress, assured her, that the two lovers were attached to each other by a passion unconquerable; and calling to her mind the love which she had herself borne to the prince of Holstein, conjured her to promote the happiness of that prince's niece. The stratagem succeeded. The choice of Elizabeth was next day announced to the council and to the foreign ministers; and preparations were made for celebrating the marriage with a magnificence worthy of the heir of the throne of the Russias. In the mean time the Grand Duke was seized with the small-pox, from which, tho' he recovered, it was with such a change of features, as rendered him, from being comely, almost hideous, and converted the love of the young princess of Anhalt, if indeed she ever felt for him that passion, into horror and disgust. She was not, however, of a disposition to let a disfigured countenance frighten her from a throne. She embraced the Greek religion, changed her name from Sophia Augusta Frederica to Catharina Alexeiwna, and with the entire approbation of Elizabeth was married to her nephew the Grand Duke.

For some time this ill-matched pair lived together, though without love, yet on terms apparently decent; but a mutual dislike gradually took place between them, which the courtiers quickly discovered, and were at pains to foment into hatred. Peter was now ugly, and his mind was uninformed. Catharine, if not a beauty, was at least a lovely woman, and highly accomplished. She could find no entertainment in his conversation, and he felt himself degraded by her superiority. A faction was formed at court, headed by the great chancellor Beloshevitch, to exclude the Grand Duke from the throne, and to place Catharine at the head of affairs; and to accomplish this end, every art was employed to fill the feeble mind of the empress with jealousies of her nephew, and with a contempt of his character. He was represented at one time as extremely ambitious, and capable of the most daring enterprises, to get immediate possession of the throne; and at another, as a wretch given up to drunkenness and to every unprincely vice.

The consequence of the first of these accusations was, that he was kept at a distance from his aunt, and a stranger to public affairs; and being wholly unemployed, that time which his education had not fitted him to fill up with reading, reflection, and rational conversation, hung heavy on his mind, that it was no difficult matter for those dissipated young men, who were placed about him for that very purpose, to initiate him in the habits of drunkenness and the other mean practices to which it was pretended he had long been devoted. In such a school, it was no wonder that he became a proficient in grovelling dissipation; or that, being unpolished, and even of rude manners, he chose for his companions some of the lowest of the people.

Catharine, in the mean time, languished for that happiness which she could not find in the society of her husband. She was fond of pleasure; but it was that comparatively refined pleasure which she had enjoyed at the court of Berlin. She loved balls, music, and elegant conversation, and could take no share in the drunken revels of Peter. Among the young men with whom he was surrounded, his chamberlain Solikoff was particularly remarked for the elegance of his taste and the graces of his person; and though yet scarcely more than a boy in years, he was said to have obtained the favours of several ladies of the court. Success had made him confident and ambitious; and his ambition prompted him to aspire at making a conquest even of the Grand Duchesses. By studying her taste, and contriving to amuse her, he was at last successful; and obtained from her Imperial Highness every favour which he could wish; but he enjoyed not his fortune with moderation, and his enemies contrived to get him placed in an honourable office at a distance from the court. He was commissioned to repair to Stockholm, with the title of Envoy Extraordinary, to notify to the king of Sweden the birth of Paul Petrovitch, of whom the Grand Duchesses had just been delivered*. The pretentious Solikoff, proud of the employment, set off with haste to Sweden, and left it with equal speed. But scarcely had he quitted Stockholm, on the wings of love and ambition, when he was flopped on the road by a courier, who put into his hands an order for him to go immediately to Hamburg, and there to reside in the quality of minister plenipotentiary from the court of Russia.

Catharine for some time preserved her attachment to the exiled chamberlain; but all at once the presence of a stranger, whom fortune had brought to the court of Russia, made her forget the lover whom she no longer saw. This person was Stanislaus Poniatowski, the late king of Poland, who first made his appearance at St Petersburg in the train of the British ambassador, and very quickly gained the affections of the Grand Duchess. In carrying on this intrigue, the lovers were not so cautious as to deceive the eyes of the envious courtiers, who reported to the empress not only all that they saw, but whatever they suspected. Elizabeth was incensed, and commanded Poniatowski to quit without delay the dominions of Russia. The accomplished Pole obeyed; but soon returned clothed with a character which made him in some degree independent of the empress.

The Count de Bruhl, then prime minister to the king of Poland, saw of what importance it was to his matter to have a powerful interest at the court of Russia. He was likewise no stranger to the passion which the Grand Duchess entertained for Poniatowski; and having got that nobleman decorated with the order of the White Eagle, he sent him back to St Petersburg in the quality of minister plenipotentiary from the republic and king of Poland. Nor was this all that Bruhl did for the two lovers. Being informed by the chancellor Belscheff, that the Grand Duke and Grand Duchess were languishing in a penury unworthy of their rank, he remitted to Poniatowski 6000 ducats, to be employed in such a manner as he might judge best for securing the favour of the prince and his consort. The ambassador profited by these counsels and benefactions. He was already sure of the Grand Duchess's heart, and he very quickly gained the favour of her husband. He talked English and German with him; drank, smoked, abused the French, and extolled the king of Prussia with unlimited praise.

The Grand Duchess was so blinded by her passion, that she was never without Poniatowski in her company. She devoted to him the whole of her time; and she made this intimacy so little a secret, that public report was loud to her prejudice. In the mean time she was delivered of the Princess Anne, who lived only fifteen months. The Grand Duke was the only person about court who seemed to know nothing of what was passing. His whole time was occupied in copying, with fervent affection, the air, the manners, the tone of the king of Prussia; and in dressing a little army at Oranienbaum in the Prussian uniform. His eyes, however, were at last opened. Some of the courtiers, from hatred to the chancellor, who countenanced the intrigue between the Grand Duchess and the Polish ambassador, roused his jealousy in order to destroy their enemy. They succeeded. He forbade his wife to be seen with Poniatowski, and prevailed with the empress to deprive the chancellor of his office, and to banish him to an estate which he had 120 versts beyond Moscow.

Catherine had now to support at once the aversion of her husband, the indignation of the empress, the insulting disdain of a court, which a few days before was lavish of its affections and smiles; and what afflicted her most of all, the dread of losing for ever her favourite Poniatowski. Her courage, however, did not forsake her. Poniatowski was indeed recalled, and left Russia, after suffering some deserved indignities from the Grand Duke, who about this time formed a connection with one of the daughters of the Senator Vorontzoff, brother Catherine, to the new chancellor. This lady, Elizabeth Romanovna Vorontzoff, was elder sister to the Prince Dashkoff, who acted so conspicuous a part in the revolution which set the crown on the head of Catherine. She was beautiful, but vain; and possessed not either the wit or the understanding of her sister.

In the mean time the health of the empress visibly declining, Catherine was very desirous of being reconciled to her; but the irritated sovereign would listen to no accommodation, except on terms too humiliating for the haughty spirit of the Grand Duchess. Catherine, therefore, absented herself from court, and asked permission to retire into Germany. This, as she had foreseen, was refused. Elizabeth was too fond of the young Paul Petrovitch to permit the departure of his mother, and thereby expose him to the danger of being at some future period declared illegitimate. She took the Grand Duchess again into favour; and it is thought, that had she lived a little longer than she did, she would have excluded Peter from the throne, and declared Paul her immediate successor.

Whilst the empress was meditating the aggrandisement of the young prince and his mother, the Grand Duke had conceived a plan for degrading them both. He had resolved, at the moment his aunt should close her eyes, to assemble his troops, to get himself proclaimed emperor, to repudiate the Grand Duchess, to declare the young Paul Petrovitch illegitimate, and publicly to marry his mistress Elizabeth Romanovna Vorontzoff. We have shown elsewhere (see Russia, p. 72, Encycl.) how this plan, when almost ready to be carried into execution, was betrayed to Catherine, who, ever since her caballing with the Chancellor Belscheff, had resolved, by some means or other, to snatch the sceptre from the feeble hand of her husband. At present, we believe she was not acquainted with it; and though she had, she could not now have turned it to her advantage, as her party, ever since the disgrace of Belscheff, was without a leader of any abilities.

Amid these distractions caused by the prospect of the death of the empress, and the known hatred of the Grand Duke and Duchesses to each other, Count Panin, preceptor to the young prince, devoted himself entirely to Catherine. He wished to see her possessed of all the power of the empire; but he was afraid to proceed to the extremity to which he proposed to go, and to deprive Peter of the name of Emperor. He contrived, therefore to procure an apparent reconciliation between the Grand Duke and his consort, as well as between him and his aunt Elizabeth; and he had almost persuaded the silly prince not to assume the sovereign power on the death of the empress, till he should be solemnly invested with it by a decree of the senate. Could he obtain this point, he knew that the power of Peter would be limited, and the authority secured to his wife and his son. He was, however, disappointed. Catherine herself disapproved of this plan, and concurred with the real friends of her husband in advising him "to conform to established custom in assuming the reins of empire."

He had hardly received this advice when word was brought him that the Empress Elizabeth was dead (a); and

(a) Christmas-day 1761 according to the Russian calendar, or the 5th of January 1762 according to ours. and the courtiers pressed in crowds about him. He accosted them with dignity, received the oaths of the officers of his guard, and seemed at once to have laid aside his weakness. In an hour he got on horseback, traversed the streets of St Petersburg, and distributed money among the multitude and the soldiers. He had been so treated by his aunt, that he could not possibly be grieved at her death; but in paying the last duties to her remains, he betrayed no indecent elation. The first actions of his reign were prudent and patriotic; and such as would have done honour to a greater prince. He appeared to be reconciled to his wife, in whose company he spent much of his time; he recalled from prison and banishment 17,000 persons, some of them of rank and of great talents, who had been the victims of Elizabeth's jealous timidity; he permitted the nobility to bear arms or not at their own discretion, freeing them at the same time from the extreme servitude under which they had been held by his immediate predecessors; and he abolished the secret committee, an infamous inquisitorial tribunal, which ever since the reign of the father of Peter the Great had been the chief engine of Russian despotism.

He neglected, however, one thing; which, among the people over whom he was appointed to reign, would have contributed more to the security of his throne than all the wise and beneficent edicts which he had published. He made no preparations to be crowned at Moscow. Instead of complying with this ancient ceremony, and humouring the prejudices of his superstitious subjects, he thought of nothing but war with Denmark, and of a personal interview with the king of Prussia in Germany. His admiration of that great monarch hurried him indeed into the most extravagant follies. Not contented with giving him peace, and entering into an offensive and defensive alliance with him, he had the means to solicit a commission in his army, and to accept of the rank of major-general. Of this title he seemed more vain than that of Emperor of all the Russias. He constantly wore the Prussian uniform; introduced among his troops the Prussian discipline, which, though better than their own, was disagreeable, because it was new, and much more because it was German; and he raised his uncle, a man of no military talents, and a foreigner, to the dignity of generalissimo of the Russian armies; giving him at the same time the particular command of the horse-guards, a body of men which had never before been under any command but that of the supreme head of the empire. Nor did his infatuated predilection for Germany, a country abhorred by the Russians, stop even here: He disbanded the noble guards which had placed Elizabeth on the throne, dismissed the horse guards from the service which they performed at court, and substituted his Holstein guards in their place.

Whilst he was thus alienating from himself the affections of the army, he contrived to disgust another order of men, whose attachment he should have laboured above all things to retain. He was at pains to shew his preference of the Lutheran faith and worship to the doctrines and ceremonies of the Greek church; he attempted to make some alterations in the dress of the monks; he annexed great part of the possessions of the church to the domains of the crown; and he banished the archbishop of Novgorod, who opposed these innovations; and found himself obliged suddenly to recall Catharine.

He had now returned to his former course. He shut himself up for whole days with his mistress and drunken companions; he compelled the nobility and ladies of the court to sit in company with buffoons and comedians; he insulted every foreign minister but the ministers of Great Britain and Prussia; and he made no secret of his intention to repudiate the empress, declare Paul Petrovitch illegitimate, and marry the Countess Vorontsoff. Convinced, however, as it would seem, that he could not be a father, he resolved to adopt Prince Ivan, the descendant of the elder brother of Peter the Great, whom Elizabeth had deposed and confined in prison, to declare him his successor, and to unite him in marriage with the young princess of Holstein Beck, who was then at St Petersburg, and whom he cherished as his daughter.

This inconsistent and weak conduct of the emperor turned the attention of all orders of men to the empress, who made it her sole employment to gain those hearts which he was losing. Instructed from her infancy in the arts of dissimulation, it was not difficult for her to affect, in the sight of the multitude, sentiments the most foreign to her mind. The pupil of the French philosophers put on the air of a bigot to the most superstitious ceremonies of the Greek religion, and treated the ministers of that religion with the profoundest reverence. And whilst her husband was getting drunk amidst a rabble of buffoons, and disgusting every person of decency who approached him, she kept her court with a mixture of dignity and affability, which attracted to her all who, by capacity, courage, or reputation, were capable of serving her.

Correct, however, as her public conduct appeared, her private life was not less licentious than formerly. While yet Grand Duchess, she had formed a very tender connection with Gregory Orloff, a man of mean birth, and of no education, but possessed at once of personal beauty and the most daring courage. He had an inferior commission in the artillery, while his two brothers were common soldiers in the regiments of guards. The intrigue which she carried on with him was known only to one of her women named Catharine Ivanovna; nor did Orloff himself for some time suspect the rank of the lady who so lavishly conferred upon him her favours in secret. At last finding him intrepid and discreet, she discovered herself, unvailed to him all her ambitious designs, and easily prevailed with him and his brothers to enter with zeal into her conspiracy against the emperor. Orloff likewise gained over Bibikoff his friend, a Lieutenant Pafleck, with other officers; and by their means easily seduced some regiments of the guards. The Princess Dashkoff was strongly attached to Catharine, we believe, from worthy motives, and had frequent meetings with Orloff on the business of the conspiracy, without suspecting that he was so much as known to the empress. Count Panin, too, and the Hetman of the Kofacks, were determined to tumble Peter from the throne; but they were not inclined to go all the lengths proposed by Catharine and her two favourites. Hoping to enjoy the actual power of the empire themselves, they were for declaring Paul Petrovitch emperor in the room of his father, and conferring upon his mother the name and authority only of regent; while the princesses and Orloff, knowing the sentiments and wishes of the empress, were resolved to set her with sovereign power, or to perish themselves in the hazardous attempt.

In the meantime the anniversary of the patron saints of Russia was at hand, when Peter had determined, at the conclusion of the festival, to divorce the empress, shut her up in prison, declare her son illegitimate, and publicly marry his mistress. As they who plan a conspiracy are always more vigilant than those against whom it is directed, the friends of Catherine were carefully informed of all that passed about the emperor, whilst he was kept in total ignorance of their proceedings. It was therefore necessary for them to unite in the same plan, and to carry it quickly into execution; for delay or divisions would involve them all in one common ruin.

The empress contrived to bring over the Hetman entirely to her views; and the Princess Dashkoff, by the sacrifice, it has been said, of her charms, found little difficulty in reconciling Count Panin to the same measures. They now agreed to seize the Tzar on his arrival at Petershoff, an Imperial palace on the shore of the Gulf of Cronstadt, where he proposed to celebrate the approaching festival; and they were waiting impatiently for the moment of action, when all at once their plot was discovered.

Pafick, who has been mentioned among the conspirators, had gained the soldiers of the company of guards in which he was a lieutenant; but one of them, who thought that his captain was in the secret, asked that officer one evening, "When they were to take up arms against the emperor?" The captain, surprised, had recourse to dissimulation, and easily drew from the soldier all that he knew of the conspiracy. It was nine o'clock at night. Pafick was put under arrest; but found means to slip into the hands of a man who had been placed as a spy over him by the Princess Dashkoff, a scrap of paper containing these words, "Proceed to execution this instant, or we are undone." The man was desired to carry it to the Hetman, by whom he would be handsomely rewarded; but he hurried with it to the princess, who instantly communicated the intelligence to the other conspirators. She herself put on man's apparel, and hastened to the place where she was accustomed to meet Orloff and his friends; where she found them, as impatient as herself, to carry their plot into immediate execution.

During this awful crisis the empress was at Petershoff, at the distance of 25 versts from St Petersburg; and one of the brothers of Gregory Orloff, named Alexius, undertook to find her out, whilst he himself, with his other brother and Bibikoff his friend, repaired to the barracks for the purpose of instructing the soldiers of their party how to act on the first signal. Alexius Orloff carried with him a short note from the Princess Dashkoff, but neglected to deliver it; and the empress, being suddenly roused from a sound sleep, was much alarmed, when she saw at the side of her bed a soldier of whom she knew nothing. Her alarm was increased when the stranger said, "Your majesty has not a moment to lose; get ready to follow me;" and instantly disappeared. She rose, however, and calling her woman Ivanovna, they disguised themselves in such a manner that they could not be known by the sentinels about the palace; and the soldier returning, they hurried with him to a coach which was waiting at the gate. Catherine Orloff took the reins, but drove with such fury that the horses soon fell down; and they were obliged to travel part of the way on foot. They had not, however, gone far, when they met a light country cart; and she who was aspiring to the throne of the greatest empire in the world, was glad to enter the capital of that empire in this humble vehicle.

It was seven in the morning when she arrived in St Petersburg; and to the soldiers, who gathered about her in great numbers, she said, that "her danger had driven her to the necessity of coming to ask their assistance; that the Tzar had intended, that very night, to put her and her son to death; and that she had to great confidence in their dispositions, as to put herself entirely into their hands." They immediately shouted, "Long live the empress!" And the chaplain of one of the regiments fetching a crucifix, received their oaths of fidelity.

The troops, however, were not unanimous in this revolt. Though Gregory Orloff was treasurer of the artillery, and well and highly beloved by the soldiers, that corps refused to follow him until he should produce the orders of Villebois their general; and that officer, withheld either by fidelity to the emperor or by fear, presumed to speak to Catherine of the obstacles which yet remained for her to surmount; adding, that she ought to have foreseen them. She haughtily replied, that "she had not sent for him to ask what she ought to have foreseen, but to know how he intended to act." "To obey your majesty," returned Villebois; and putting himself at the head of his regiment, he immediately joined the conspirators. So ripe indeed were the minds of all men for this revolt, that in the space of two hours the empress found herself surrounded by 2000 warriors, together with great part of the inhabitants of Petersburg; and with that numerous train of attendants she repaired to the church of Kazan, where the archbishop of Novgorod, setting the Imperial crown on her head, proclaimed her sovereign of all the Russias, declaring, at the same time, Paul Petrovitch her successor.

Matters had now proceeded by much too far to admit of any compromise between Catherine and her husband; but had the infatuated Tzar put his affairs wholly into the hands of Marshal Munich, that intrepid veteran would have tumbled the empress from her throne almost as quickly as she had got possession of it. He acted, however, a very different part. Upon receiving intelligence of what had been done at St Petersburg, he asked indeed the Marshal's advice, but suffered himself to be guided by his mistress and timid companions. Through their terrors and his own irresolution opportunities were lost which could never be recovered; for though his Holstein guards, with tears in their eyes, swore that they were all ready to sacrifice their lives in his service, and though the old Marshal offered to lead them against the rebels, saying to the emperor, "I will go before you, and their swords shall not reach you till they have pierced my body," he was persuaded to treat with the empress, to acknowledge his misconduct, and to offer to share with her the sovereign power. At last he was weak enough to abandon his troops, and to surrender at discretion to his consort; whose creatures hurried him from Oranienbaum to Petershoff, stripped him of all his clothes, and after leaving him for some time... Catherine, time in his shirt, a butt to the outrages of an insolent soldiery, threw over him an old morning gown, and shut him up alone, with a guard at the door of his wretched apartment. On the 29th of June, O.S.* 1762, Count Panin was sent to him by the empress; and after a long conference, prevailed with him to write and sign a solemn resignation of his crown, and a declaration of his utter incapacity to govern so great an empire.

The revolution was now complete, and Peter seemed to enjoy some composure of mind; but in the evening he was carried a prisoner to Ropetcha, a small Imperial palace, at the distance of 20 versts from Peterhoff, where he was murdered on the 17th of July, just one week after his deposition. Of the manner of his death different accounts have been given. By some he is said to have been poisoned; by others, to have been strangled by one of the Orloffs; and a few have thought that he perished by the same means as Henry V.I. of England. Whether the empress was accessory to his death is not known; though it is certain, that so far from making any inquiry after his murderers, she affected to believe that he had died naturally of the gout.

The first care of Catherine was to reward those who had been the principal actors in the revolt. Panin was made prime minister; the Orloffs received the title of Count; and the favourite Gregory was appointed lieutenant-general of the Russian armies, and knight of the order of St Alexander Nevsky, the second order of the empire. Several officers of the guards were promoted, of whom 24 received considerable estates; and among the soldiers, whom she treated with the greatest affability, brandy and beer were liberally distributed. The chancellor Beluscheff, who had been the most inveterate enemy of Peter, was recalled from his exile, restored to his rank of field-marshal, and had an annual pension settled upon him of 20,000 rubles. To the friends of the emperor she behaved with great moderation. Prince George, whom he had constituted Duke of Courland, was indeed obliged to renounce his title; but the administration of Holstein was committed to him, and he ever after served the empress with zeal and fidelity.

The news of the revolution was soon spread over Europe; and none of the sovereigns, though they knew by what steps Catherine had mounted the throne, hesitated for a moment to acknowledge her title. She was not, however, at perfect ease in her own mind; nor was her right recognised by all her subjects. Tho' she published manifestos, setting forth the intentions of the late emperor towards her and her son, which made resistance necessary; though in these papers she attributed her elevation to the wishes of her people and the providence of God; and though she called upon all who were sincerely attached to the orthodox faith of the Greek church, to consider the sudden death of Peter as the judgment of heaven in favour of the revolution—yet in the distant provinces no exclamations were heard; both soldiers and peasants observed a gloomy silence. Even at Moscow, so great was the disaffection to Catherine's government, that it was some time before she could venture to go to that city to be crowned; and she found in it at last so cold a reception, that she very quickly returned to St Petersburg.

Nor was this the only cause of her uneasiness. The connection between Orloff and her became visible, and gave just offence to her other friends. The princess of Dashkoff first perceived it; and when she presumed to expostulate with the empress on the meanness and imprudence of her passion, she was banished from the court to Moscow. Count Panin and the Hetman saw with indignation that they had dethroned the grandson of Peter the Great, to aggrandise a rude and low born upstart. Cabals and conspiracies were entered into by high and low, both against Catherine and against her favourite; and it required all her abilities and firmness to preserve at once her throne and her lover. On one occasion she hoped to obtain from the Princess Dashkoff sufficient proof that Panin and the Hetman of the Kofacks were concerned in a plot which had just been discovered; and with this view she wrote to her a letter of four pages, filled with the most tender epithets and the most magnificent promises, conjuring her in the name of their long standing friendship, to reveal what she knew of the recent conspiracies. With becoming magnanimity, the princess replied, "Madam, I have heard nothing; but if I had heard anything, I should take good care how I spoke of it. What is it you require of me? That I should expire upon a scaffold? I am ready to mount it."

Catherine, despairing of conquering such a spirit, attempted to attach to her those whom she dared not to punish. Some of the inferior conspirators were banished to Siberia, while Panin and the Hetman, whom she most dreaded, received additional marks of her favour. In the mean time, to gain the affections of the people at large, she paid the utmost attention to the administration of justice; formed magnificent establishments for the education of the youth of both sexes; founded hospitals for orphans, for the sick, and for lying-in women; invited foreigners of all nations, possessed of any merit, to settle in different parts of her vast territories; increased the naval force of the empire; and gave such encouragement to the cultivation of every elegant and useful art, that in the short space of a year and a half from her accession to the throne, the national improvement of Russia was visible.

In the good fortune and glory of Catherine, no one rejoiced more sincerely than Count Poniatowski. He approached towards the confines of Russia, and wrote to her in the tenderest style of congratulation, requesting permission to pay his respects to her in the capital of her empire. It is not improbable that he flattered himself with the hopes that she would give him her hand in marriage, and thus raise him to the throne of the Tsars; but she had promised to the Empress Elizabeth, that she would never again see the count; and to that promise she at present adhered. She wrote to him, however, in the most affectionate terms; and tho' she gave him no encouragement to repair to St Petersburg, she assured him that she had other prospects in view for his aggrandisement, and that he might depend upon her perpetual friendship; and she soon appeared to be as good as her word. On the death of Augustus III., she raised her former favourite to the throne of Poland, in opposition to the wishes of the courts of Vienna and Versailles, as well as of a great majority of the Polish nobles. She defeated the intrigues of the two foreign courts by more skilfully conducted intrigues of her own; and, by pouring her armies into the republic, she so completely overawed the nuncios, that Poniatowski was chosen by the unanimous suffrages of Whilst she was thus disposing of foreign kingdoms, she was kept under perpetual dread of being tumbled from the throne of her own vast empire. Her want of title to that throne was now seen by all ranks of her subjects; the good qualities of Peter the third were remembered, and his failings and faults forgotten. His fate was universally lamented; and, except the conspirators, who may be said to have embraced their hands in his blood, there was hardly a Russian who did not regret that the sovereignty had passed from the ancient family of the Tsars to a foreigner, allied only by marriage to the blood royal. Even the conspirators themselves had lost much of their regard for Catherine.

The princesses of Daskoff were a second time banished to Moscow; and, to magnify her own importance, she spoke freely of the means by which the empress, whom she accused of ingratitude, had been raised to the throne. The inhabitants of Moscow, who never favoured the usurpation, were thus made ripe for a revolt. At St Petersburg, Count Panin felt himself uneasy under the predominant influence of the favourite, and tried in vain to divert Catherine's affections to a new object. She received a few secret visits from a handsome young man, and then appointed him to a lucrative and honourable employment in some distant province of the empire; when Orloff recovered his former ascendancy, which through his own carelessness he had nearly lost. In this state of the public mind, conspiracies were very frequent; and as the general object of them was to place on the throne prince Ivan, who was again languishing in the dungeon from which Peter had taken him, the empress had given to his guard an order, signed by her own hand, to put that unfortunate prince to death, should any attempt be made to liberate him from his prison. An attempt was made by a very inferior officer, as some have supposed, by the instructions of Catherine, and her bloody order was instantly obeyed. The assassins were rewarded, and promoted in the army; but the officer who attempted to rescue the prince was condemned to death, and suffered unexpectedly the sentence of the law. The brothers and sisters of Ivan, who had been kept in a prison different from his, were sent to Denmark; and, to provide them with necessaries suitable to their rank, the empress made them a present of 200,000 rubles, and paid annually to the maintenance of their dignity a pension of thirty thousand.

The throne of Catherine was now firmly established by the death or renunciation of every person who was descended of the imperial family; and she had leisure to turn her thoughts to the aggrandisement of the empire. It was soon seen that this was the object which she had in view when she raised Count Poniatowski to the throne of Poland, and that she was not actuated on that occasion by any remains of her former attachment. We have elsewhere thrown (see Poland, Encyc. n° 98—115) under what pretences she invaded the kingdom of him who had formerly been one of her most favoured lovers, and by what means she annexed great part of it to the territories of Russia. But it is not through her wars that in this article we mean to trace her character: it is not as a sovereign and heroine that her Catherine life is entitled to a place in a general repository of arts, sciences, and miscellaneous literature, but as a patroness of art and of science, and as the legislatrix of a vast empire, who employed all her talents and all her power for the civilization of a great part of the human race.

Under the article Russia (Encyc.), we have mentioned the famous code of laws for a great empire, and the proposed convention of deputies from all the classes, which Catherine and the princess Daskoff so artfully employed as means to bring about the revolution which deposed the former on the throne. The states actually met in the ancient capital of the empire, and the sovereign's instructions for framing a new code of laws was read amidst reiterated bursts of applause. All present extolled the sagacity, the wisdom, the humanity of the empress; but fear and flattery had a greater share in these exclamations than any just knowledge of the subject. The deputies of the Samoiedes alone had the courage to speak freely. One of them stood up, and, in the name of himself and his brethren, said, "We are a simple and honest people. We quietly tend our reindeer. We are in no want of a new code; but make laws for the Russians, our neighbours, that may put a stop to their depredations." The following sittings did not pass to quietly. A debate about the liberation of the boors was carried on with such warmth, that fatal consequences were to be apprehended; and the deputies were dismissed to their respective provinces in the manner which we have elsewhere related. Previous, however, to the dissolution of this assembly, the members were required to signalize the meeting by some conspicuous act of gratitude; and, by a general acclamation, the titles of Great, Wise, Prudent, and Mother of the Country, were decreed to the empress. With assumed modesty she accepted only of the last, "as the most benign and glorious recompense for her labours and solicitudes in behalf of a people whom she loved."

For that people she did indeed labour, and labour most usefully. She introduced into the administration of justice the greatest reformation of which the half civilized state of Russia would perhaps admit. She spared neither trouble nor expense to diffuse over the empire the light of science, and the benefits of useful and elegant arts; and she protected, as far as she could, the poor from the oppressions of the rich. About the middle of 1767, she conceived the idea of sending several learned men to travel through the interior of her vast dominions, to determine the geographical position of the principal places, to mark their temperature, and to examine into the nature of their soil, their vegetable and mineral productions, and the manners of the people by whom they were inhabited. To this employment she appointed Pallas, Gmelin, Euler, and many others of the highest eminence in the republic of letters; from whose journals of these interesting travels large additions have been made to the general stock of useful knowledge. This survey of the empire, and the maps made from it, had Catherine done nothing else, would alone have been sufficient to render her name immortal. Well convinced in her own mind, that it is not so much by the power of arms, as by precedence in science, that nations obtain a conspicuous place in the annals of the world, with a laudable zeal she encouraged CAT

Catherine encouraged artists and scholars of all denominations. She granted new privileges to the two academies of sciences and the arts; encouraged such of the youth as had behaved well in these national institutes, to travel for further improvement over Europe, by bestowing upon them, for three years, large pensions to defray their expense; and, to remove as much as possible the Russian prejudice against all kinds of learning, she granted patents of nobility to those who, during their education, had conducted themselves with propriety, and become proficient in any branch of useful or elegant knowledge. Still farther to encourage the fine arts in her dominions, she assigned an annual sum of 5000 rubles for the translation of foreign literary works into the Russian language.

In the year 1768, the small-pox raged at St Petersburg, and proved fatal to vast numbers of all ranks and of every age. The empress was desirous to introduce the practice of inoculation among her subjects; and resolved to set the example by having herself and her son inoculated. With this view, she applied for a physician from England; and Dr Thomas Dunstable of Hertford being recommended to her, he repaired with his son to the capital of Russia, where he inoculated first the empress, then the grand duke, and afterwards many of the nobility. The experiment proving successful, he was created a baron of the empire, appointed actual counsellor of state, and physician to her imperial majesty, with a pension of L. 500 sterling a-year, to be paid him in England, besides L. 10,000 which he immediately received. So popular was the empress at this period, that, by a decree of the senate, the anniversary of her recovery from the small-pox was enjoined to be celebrated as a religious festival; and it has ever since been observed as such.

She was now engaged in war with the Turks, of which a sufficient account for a work of this nature has been given under the title TURKEY (Encycl.); but there was one transaction of her and her friends, of which no mention was made in that article, though it is of importance to him who would form a just estimate of her personal character.

We have noticed the femalinity of the empress Elizabeth. She bore three children to the grand venuer Alexey Gregorievitch Razumofsky, to whom, indeed, she is said to have been clandestinely married. Of these children the youngest was a girl, brought up under the name of princess Tarakanoff. Prince Radzivill, who has been mentioned in the article POLAND (Encycl.), irritated at Catherine's cruelties to his countrymen, conceived the project of placing the young princess on the throne of her ancestors; and, having gained over the persons to whom her education was entrusted, he carried her off to Rome as a place of safety. Catherine, in return, seized his large estates; and he and the princess were reduced to extreme poverty. Radzivill repaired to Poland in order to learn what could be done to forward his great enterprise; and scarcely had he arrived there when an offer was made to restore to him his possessions, upon condition of his carrying his ward to St Petersburg. This he refused; but had the baseness to promise, that he would give himself no farther concern about the daughter of Elizabeth; and he was put in possession of all his estates.

By the instructions of the empress, Alexius Orloff, who nominally commanded the Russian fleet at the Dardanelles, repaired to Rome, got access to young Tarakanoff, and found means to persuade her that all Russia was ready to revolt from Catherine, and place her on the throne of her mother. To convince her of his sincerity, he pretended to feel for her the tenderest and most respectful passion; and the unsuspicious lady was induced to accept of him as a husband. The Russian who had assassinated the grandson of Peter the Great, did not hesitate to seduce and betray his granddaughter. Under pretence of having the marriage ceremony performed according to the rites of the Greek church, he suborned some furbelower villains to performate priests and lawyers; thus combining profanation with imposture against the unprotected and too confident Tarakanoff.

Having been treated for some days, both at Rome and at Leghorn, with all the respect due to a sovereign, the unsuspecting princess expressed a wish to go on board a Russian ship of war. This was just what Orloff wanted. Attended by a numerous and obsequious train, she was rowed from the shore in a boat with magnificent ensigns, hoisted upon the deck of the ship in a splendid chair, and immediately handcuffed. In vain did she throw herself at the feet of her pretended husband, and conjure him by every thing tender which had passed between them. She was carried down into the hold; the next day the vessel sailed for St Petersburg; where, upon her arrival, the princess was shut up in the fortress; and what became of her since was never known. Such were the means which Catherine employed not to employ in order to get rid of all pretenders to her throne.

Soon after this service rendered to her by Alexius Orloff, she dismissed his brother Gregory from her favour, and connected herself with Vasilchikoff, a lieutenant of the guards. The former favourite had indeed become insolent, and, as Catherine thought, ungrateful. He aspired to nothing less than the throne. From love to himself and to a son which he had borne to him, she offered to enter into a secret marriage; but with this proposal the proud prince (a) was not satisfied, and hoped that his refusal would impel her to receive him publicly as her husband and partner in power. He was mistaken. She divested him of all his employments; but gave him a pension of 150,000 rubles, a handsome service of plate, and an estate with 6000 peasants upon it; and, thus enriched, he set out upon a journey through various parts of Europe. He returned, however, much sooner than was expected; the new favourite was handsomely rewarded, and sent to a distance; Orloff was restored to all his offices, and his baleful influence was again felt.

He attempted to persuade the empress to dismiss Panin from the court; but the grand duke interposed in behalf of his old preceptor; and, for once, Catherine listened to the entreaties of her son. When a dreadful rebellion, under a Kofak of the name of Pugetshoff, who

(a) She had some time before obtained for him a patent, creating him a prince of the Roman empire. CAT

pride who pretended to be Peter III. escaped from his affections, was shaking the throne to its foundation—the influence of Orloff was such as to prevent the empress, for some time, from employing her ablest general against the rebels, because that general was Panin, brother to the minister. Danger, however, at last prevailed over the favourite: Panin was sent against Pugentoff; the rebellion was crushed; and Catharine found leisure to give something like a legal constitution to the empire. In that work, the laws and regulations established for the government of the various provinces, and for the equitable administration of justice through the whole of her vast dominions, evinces the greatest wisdom and sagacity in their author, as well as a proper regard to the practicable liberties and rights of men. In the capital, she established the most perfect police, by which the internal tranquillity of a great city was, perhaps, ever maintained; and whilst her private conduct was far from correct, she was acting in the capacity of sovereign, so as to deserve, indeed, the appellation of Mother of her people.

To follow her through all her wars and intrigues with foreign courts, would swell this article to the size of a volume. Such a narrative, too, belongs rather to the history of Russia than to the memoirs of Catharine; in which it is the business of the biographer to develop the private character of the woman rather than to detail the exploits of the sovereign. Her partition of Poland, and afterwards the annihilation of it as an independent republic; her encroachments on the territories of the grand signior; her formation of the armed neutrality; the influence which she maintained over the courts of Sweden and Denmark; and the art with which she threw the weight of Russia sometimes into the scale of Austria, and sometimes into that of Prussia, just as the interests of her own dominions required the one or the other to preponderate—shew how admirably she was qualified to guide the helm of a great empire in all its transactions with foreign states. We speak not of the equity of her proceedings; for it must be confessed, that equity formed no barrier against her ambition; and that she never failed to subjugate those whom she pretended to take under her protection. Her ruling passion was to enlarge her own territories, already to very extensive; and, for the attainment of that object, she contrived the most judicious plans, which she executed with vigour. In this part of her conduct, however, she has been equalled by other monarchs; but in the zeal and the wisdom with which she endeavoured to introduce among her half savage subjects the blessings of knowledge and industry, she stands unvalued, except, perhaps, by her predecessor Peter the Great. Of this we need bring no other proof, in addition to what has been already stated, than that the founded in St Petersburgh alone thirty-one seminaries, where 6800 children of both sexes were educated at the annual expense to the government of 754,335 rubles. She superintended herself the education of her grandchildren, and wrote for them books of instruction. If it be true, that "every man acquainted with the common principles of human action, will look with veneration on the writer who is at one time combating Locke, and at another making a catechism for children in their fourth year;" with what veneration should we look upon the empress of Russia, could we forget the means by which she obtained that elevation from which the frequently descend-

ed for a similar employment? This she did, not for her Catharine's own descendants alone, but also for the children of others; of whom she had always a great number in her apartments, who shared in the instruction given to her grandchildren, and whose carefree she returned with extreme complaisance.

Her greatest weakness was surely that gross passion which her panegyrists have dignified with the name of love; but to such an appellation it had no claim, if love be anything more than a sexual appetite. Besides Gregory Orloff, she had not fewer than ten favourites after the death of her husband; and of these she seems to have felt a refined affection for none but Lankof, a young Pole of a very ancient family, and of elegant manners, and the famous Potemkin, to whom she is said secretly to have given her hand, and who preserved her friendship, if not her affection, to the end of his life. To Lankof, whose education had been much neglected, she condescended to become preceptrix; and, as he made great progress in the acquisition of useful knowledge, she admired in him her own creation. Potemkin, though not amiable, deserved her favour for the fidelity and abilities with which he served her, both in the council and in the field; and in him, when she had ceased to look on him with the eyes of love, she respected the intriguing politician and intrepid commander, who had formed plans for driving the Turks out of Europe, and setting her on the throne of Byzantium. Her other favourites had nothing to recommend them but masculine beauty and corporeal strength. One of them, however, thought it necessary to have a library in the grand house, of which the empress, upon receiving him into favour, had made him a present; and desired the principal bookseller to fill his shelves. The man asked him what books he would please to have. "You understand that better than I (replied the favourite); that is your business. You know the proper assortments; I have destined a large room to receive them. Let there be large books at the bottom, and smaller and smaller up to the top; that is the way they stand in the empress's library!" In the conversation of such men, the cultivated mind of Catharine could enjoy no interchange of sentiments.

We know not whether that more than Asiatic magnificence, which she displayed on every public occasion, should be considered as an instance of weakness or of wisdom. If she delighted in balls, and masquerades, and sumptuous entertainments, and drest loaded with jewels and every kind of splendid ornament, for their own sakes, she betrayed a weakness unworthy of that sovereign who held in her hand the balance of Europe, and at whose nod the greatest powers of Asia trembled; but if she introduced such splendor into her court merely to divert the attention of the Russians from the means by which she got possession of the throne, and to wean them from their own savage and slovenly manners; even this may perhaps be considered as one of her most masterly strokes in politics.

Her ambition was boundless; but, if such a phrase may be allowed, it was not always true ambition. When the French republic had established itself on the ruins of monarchy, and was propagating new theories of government through all Europe, true ambition would surely have led the autocratix of the north to unite her forces with those of the coalesced powers, in order Catharine, to crush the horrid hydra before its anarchical principles could be introduced among her own barbarous subjects. Such would certainly have been the advice of her favourite Potemkin, who longed to lead a Russian army into France, even before the murder of the unfortunate Louis. That general, however, had died in October 1791; and when Britain, Austria, and Prussia, were league against the new republic, Catharine looked coolly on, in hopes, it is probable, of availing herself of their weakness, when exhausted by a long and bloody war. She gave refuge, indeed, in her dominions to many emigrants from France, and sent a squadron of ships to co-operate with the navy of England; but in this last measure she regarded merely her own immediate interest; for her crazy ships were repaired by British carpenters at the expense of the British government, and her officers had an opportunity of learning the evolutions of the British navy. She had likewise other prospects in view when she lent to the allies this slender aid. She meditated a new war with Turkey; and, depending upon meeting with no opposition, if she should not receive affluence from England and Austria, she flattered herself with accomplishing her darling project of driving the Ottomans out of Europe, and of reigning in Constantinople. But she was disappointed. On the morning of the 9th of November 1796, she was seized with what her principal physician judged a fit of apoplexy; and, at 10 o'clock in the evening of the following day, expired, in the 68th year of her age, leaving behind her the character of one of the greatest sovereigns that ever swayed a sceptre.

After this long detail of the incidents of her life, it is needless to inform the reader that Catharine II. had no religion, and, of course, no principles of morality, which could induce her in every instance to do to others as she would have them do to her. She was a professed disciple of the French philosophers; by some of whom she was ridiculed, and by others cheated. The influence which she paid to the genius of Voltaire did not hinder him from frequently breaking his jeffs upon the autocratix of Russia and her successive favourites; and Diderot, whom she cared for, told her an immense library, when he possessed hardly a book, and was obliged to ransack Germany and France for volumes to enable him to fulfill his bargain. Such is the friendship, and such the gratitude, which subsists among the amiable pupils of nature, and the philanthropic advocates for the rights of man.

**CAUDA Capricorni**, a fixed star of the fourth magnitude, in the tail of Capricorn; called also, by the Arabs, *Dinab Algied*; and by Bayer.

**CAUDA Ceti**, a fixed star of the third magnitude; called also, by the Arabs, *Dinab Kactos*; marked by Bayer.

**CAUDA Cygni**, a fixed star of the second magnitude, in the Swan's tail; called by the Arabs *Dinab Adigege*, or *Eldegizgich*; and marked by Bayer.

**CAUDA Delphini**, a fixed star of the third magnitude, in the tail of the Dolphin; marked by Bayer.

**CAUDA Draconis**, or Dragon's tail, the moon's southern or descending node.

**CAUDA Leonis**, a fixed star of the first magnitude, in the Lion's tail; called also, by the Arabs, *Dinab Eleced*; and marked by Bayer. It is called also *Lucida Cauda*.

**CAUDA Ursae Majoris**, a fixed star of the third magnitude, in the tip of the Great Bear's tail; called also, by the Arabs, *Alalith*; and *Bennuah*; and marked by Bayer.

**CAUDA Ursae Minoris**, a fixed star of the third magnitude, at the end of the Lesser Bear's tail; called also the Pole Star, and, by the Arabs, *Alrakabah*; and marked by Bayer.

CAUSE has been defined, we think, with accuracy in the Encyclopedia, and the doctrine stated which we believe to be true. Objections however have been made to that doctrine, of which we have endeavoured to remove some, under the title ACTION, in this Supplement; and the doctrine itself has been well illustrated (at least such is our opinion) in the supplementary article ASTRONOMY. We have, therefore, very little to add here on the subject of cause, though it is the most important subject which can employ the mind of man. What is the relation between a physical cause and that which is termed its effect—between heat, for instance, and the fusion of metals? Is it a necessary connection, or only a conjunction, discovered by experience to be constant?

If by necessary connection be meant that kind of connection of which the contrary cannot be conceived, we do not think that the connection of any physical cause with its effect can be called necessary. We see no difficulty in conceiving, that fire, instead of fusing gold, might fix mercury. This may indeed be impossible; and we might perhaps see the impossibility, did we as completely know the nature of fire and of metals as we know the relations of pure geometry. We know that the three angles of a plain triangle cannot possibly be either greater or less than two right angles; for in this comparison nothing is hid from our mental view. We do not, however, perceive the impossibility of mercury being fixed, as clay is hardened, by heat; for of heat, and mercury, and clay, we know very little, and that little is the offspring of experience.

But if the connection between cause and effect be not necessary, are we not deprived of the means of demonstrating the great fundamental truth of religion? We have nowhere said, that the connection between cause and effect is not necessary; but only, that we do not perceive the necessary connection between what are called physical causes and their effects. That every event is, and must be, brought about by some cause, or some agency, we hold to be a self-evident truth, which no man can deny who understands the terms in which it is expressed; but what or where the agency is, we can very seldom, if ever, know, except when we think of our own voluntary actions. When a change is observed, we cannot doubt of its being produced by something: either the thing changed is animated and has produced the change by its own agency, just as we move our heads and legs by an act of volition; or if it be inanimated, and of itself incapable of agency, the change has been produced by something external, denominated a cause. But all external causes, which are not likewise agents, in the proper sense of the word, may be traced, we think, as effects up to some agency; and therefore, in our opinion, there is no real, ultimate, efficient, cause but mind, or that which is endowed with power. In proof of this doctrine, if it need any proof, we can only refer to what has been said elsewhere on our notions. It is not our intention to describe the variety of constructions which may be adopted in easy situations, where the arches are of small extent, and where sufficient foundation can be had in every part of it for supporting the frame. In such cases, the frequency of the props which we can set up dispenses with much care; and a frame of very slight timbers, connected together in an ordinary way, will suffice for carrying the weight, and for keeping it in exact shape. But, when the arches have a wide span, and consequently a very great weight, and when we cannot set up intermediate pillars, either for want of a foundation in the soft bottom of a river, or because the arch is turned between two lofty piers, as in the dome of a flatly cathedral—we are then obliged to rely everything on the piers themselves; and the framing which is to support our arch before the keystone is set, must itself be an arch, depending on the mutual abutment of its beams. One should think that this view of the construction of a centre would offer itself at the first, naturally derived from the erection it was to assist; but it has not been so. When intermediate pillars were not employed, it was usual to frame the mould for the arch with little attention to anything but its shape, and then to cross it and recross it in all directions with other pieces of timber, till it was thought so bound together that it could be lifted in any position, and, when loaded with any weight, could not change its shape. The frame was then raised in a lump, like any solid body of the same shape, and set in its place. This is the way still practised by many country artists, who, having no clear principles to guide them, do not stop till they have made a load of timber almost equal to the weight which it is to carry.

But this artist's method, besides leading the employer into great expense, is frequently fatal to the undertaker, from the unskillfulness of the construction. The beams which connect its extremities are made also to support the middle by means of posts which rest on them. They are therefore exposed to a transverse or cross strain, which they are not able to bear. Their number must therefore be increased, and this increases the load. Some of these cross strains are derived from beams which are placed very obliquely, and therefore exert a prodigious thrust on their supports. The beams are also greatly weakened by the mortises which are cut in them to receive the tenons of the crossing beams; and thus the whole is exceedingly weak, in proportion to what the same quantity of timber may be made by a proper disposition of its parts.

The principles from which we are to derive this disposition are the general mechanical principles of carpentry, of which we have given some account in that article. These furnish one general rule: When we would give the utmost strength possible to a frame of carpentry, every piece should be disposed that it is subject to no strain but what either pushes or draws it in the direction of its length; and, if we would be indebted to timber alone for the force or strength of the centre, we must rely all on the first of these strains; for when the straining force tends to draw a beam out of its place, it must be held there by a mortise and tenon, which possesses but a very trifling force, or by iron straps and bolts. Cases occur where it may be very difficult to make every strain a thrust; and the best artists admit of ties; and indeed where we can admit a tie-beam connecting the two feet of our frame, we need seek no better security. But this may sometimes be very inconvenient. When it is the arch of a bridge that we are to support, such a tie-beam would totally stop the passage of small craft up and down the river. It would often be in the water, and thus exposed to the most fatal accidents by freshes, &c. Interrupted ties, therefore, must be employed, whose joint or meetings must be supported by something analogous to the king-posts of roofs. When this is judiciously done, the security is abundantly good. But great judgment is necessary, and a very scrupulous attention to the disposition of the pieces. It is by no means an easy matter to determine whether a beam, which makes a part of our centre, is in a state of compression or in a state of extension. In some works of the most eminent carpenters even of this day, we see pieces considered as struts (and considerable dependence had on them in this capacity), while they are certainly performing the office of tie-beams, and should be secured accordingly. This was the case in the boldest centre (we think) that has been executed in Europe, that of the bridge of Orleans, by Mr. Huppeau. Yet it is evidently of great consequence not to be mistaken in this point; for when we are mistaken, and the piece is stretched which we imagine to be compressed, we not only are deprived of some support that we expected, but the expected support has become an additional load.

To ascertain this point, we may suppose the piers how to yield a little to the pressure of the archstones on the timguitha centre frames. The feet, therefore, fly outwards, and from the shape is altered by the sinking of the crown. We must draw our frame anew for this new state of things, and must notice what pieces must be made longer than before. All such pieces have been acting the part of tie-beams.

But a centre has still another office to sustain; it must keep the arch in its form; that is, while the load on the centre is continually increasing, as the masons lay on more courses of arch-stones, the frame must not yield and go out of shape, sinking under the weight on the hunches, and rising in the crown, which is not yet carrying any load. The frame must not be supple; and must derive its stiffness, not from the closeness and strength of its joints, which are quite insignificant when set in competition with such immense strains, but from struts or ties, properly disposed, which hinder any of the angles from changing its amplitude.

It is obvious, from all that has been said, that the strength and stiffness of the whole must be found in the core flint-triangles into which this frame of carpentry may be resolved. We have seen that the strains which one piece produces on two others, with which it meets in one point, depends on the angles of their intersection; and that it is greater as an obtuse angle is more obtuse, or an acute angle more acute. And this suggests to us the general maxim, "to avoid as much as possible all..." very obtuse angles." Acute angles, which are not necessarily accompanied by obtuse ones, are not so hurtful; because the strain here can never exceed the straining force; whereas, in the case of an obtuse angle, it may surpass it in any degree.

Such are the general rules on this subject. Although something of the mutual abutment of timbers, and the support derived from it, has been long perceived, and employed by the carpenters in roofing, and also (doubtless) in the forming of centres, yet it is a matter of historical fact, that no general and distinct views had been taken of it till about the beginning of this century, or a little earlier. Fontana has preferred the figure of the frames on which the arches of St Peter's at Rome were turned. The one employed for the dome is constructed with very little skill; and those for the arches of the nave and transepts, though incomparably superior, and of considerable simplicity and strength, are yet far inferior to others which have been employed in later times.

It is much to be regretted that no trace remains of the forms employed by the great architect and consummate mechanic Sir Christopher Wren. We should doubtless have seen in them every thing that science and great sagacity could suggest. We are told, indeed, that his centering for the dome of St Paul's was a wonder of its kind; begun in the air at the height of 160 feet from the ground, and without making use of even a projecting corniche whereon to rest it.

The earliest theory of the kind that we have met with, that is proposed on scientific principles, and with the express purpose of serving as a lesson, are two centres by Mr Pitot of the Academy of Sciences, about the beginning of this century. As they have considerable merit (greatly resembling those employed by Michael Angelo in the nave of St Peter's), and afford some good maxims, we shall give a short account of them. We crave the excuse of the artists if we should employ their terms of art somewhat awkwardly, not being very familiarly acquainted with them. Indeed, we observe very great differences, and even ambiguity, in the terms employed.

What we shall describe under the name of a centre is (properly speaking) only one frame, truss, or rib, of a centre. They are set up in vertical planes, parallel to each other, at the distance of 5, 6, 7, or 8 feet, like the trusses or main couples of a roof. Bridging joists are laid across them.—In smaller works these are laid sparingly, but of considerable scantling, and are boarded over; but for great arches, a bridging joist is laid for every course of archstones, with blockings between to keep them at their proper distances. The stones are not laid immediately on these joists, but beams of soft wood are laid along each joist, on which the stone is laid. These beams are afterwards cut out with the chisel, in order to separate the centre from the ring of stones, which must now support each other by their mutual abutment.

The centre is distinguishable into two parts, ALLB Plate XIV. (fig. 1.) and LDL, which are pretty independent of each other, or at least act separately. The horizontal stretcher LL cuts the semicircle ADB half way between the spring and the crown of the arch; the arches AL, LD, being 45° each. This stretcher is divided in the same proportion in the points G and H; that is, GH is one half of LL, and LG, HL are each one-fourth of LL nearly. Each end is supported by two struts EI, GI, which rest below on a sole or bed properly supported. The interval between the heads of the struts GI, HK is filled up by the Straining Beam GH, abutting in a proper manner on the struts (see Carpentry, Supplement). The extremities I, L, are united in like manner by buttress joints, with the heads of the outer struts. The Arch Moulds AP, BP, are connected with the struts by cross pieces PQ, which we shall call Bridles, which come inwards on each side of the struts (being double), and are bolted to them. This may be called the lower part of the frame. The upper part consists of the king post DR, supported on each side by the two struts or braces ML, ON, mortised into the post, and also mortised into the stretcher, at the points L, N, where it is supported by the struts below. The arches LD, LD are connected with the struts by the bridles PQ, in the same manner as below.

There is a great propriety in many parts of this arrangement. The lower parts or launches of the arch of this prefers very lightly on the centres. Each archstone is lying on an inclined plane, and tends to slide down only with its relative weight; that is, its weight is to its tendency to slide down the joint as radius to the line of elevation of the joint. Now it is only by this tendency to slide down the joint that they press on the centering, which in every part of the arch is perpendicular to the joint: But the pressure on the joint, arising from this cause, is much less than this, by reason of the friction of the joints. A block of dry freestone will not slide down at all; and therefore will not press on the centering, if the joint be not elevated 35 degrees at least. But the archstones are not laid in this manner, by sliding them down along the joint, but are laid on the centres, and slide down their slope, till they touch the blocks on which they are to rest; so that, in laying the archstones, we are by no means allowed to make the great deduction from their weight just now mentioned, and which Mr Couplet prefers (Mem. Acad. Sciences, 1729). But there is another cause which diminishes the pressure on the centres; each block slides down the planks on which it is laid, and presses on the block below it, in the direction of the tangent to the arch. This pressure is transmitted through this block, in the same direction, to the next, and through it to the third, &c. In this manner it is plain that, as the arch advances, there is a tangential pressure on the lower archstones, which diminishes their pressure on the frame, and, if sufficiently great, might even push them away from it. Mr Couplet has given an analysis of this pressure, and shows, that in a semicircular arch of uniform thickness none of the arch stones below 30° press on the frame. But he (without saying so) calculates on the supposition that the blocks descend along the circumference of this frame in the same manner as if it were perfectly smooth. As this is far from being the case, and as the obstructions are to the last degree various and irregular, it is quite useless to institute any calculation on the subject. A little reflection will convince the reader, that in this case the obstruction arising from friction must be taken into account, and that it must not be taken into account in estimating the pressure of each successive course of stones as they are laid. It is enough that we see that the pressure of the lower courses of archstones on the frame is diminished. Mr Couplet says, that the whole whole pressure of a semicircular arch is but \( \frac{3}{4} \)ths of its weight; but it is much greater, for the reason just now given. We have tried, with a well-made wooden model (of which the circumference was rubbed with black lead to render it more slippery), whether any part of the wooden blocks representing the archstones were detached from the frame by the tangential pressure of the superior blocks; but we could not say confidently that any were so detached. We perceived that all kept hold of a thin slip of Chinese paper (also rubbed with black lead) between them and the frame, so that a sensible force was required to pull it out. From a combination of circumstances, which would be tedious to relate, we believe that the centres carry more than two-thirds of the weight of the arch before the keystone is set. In elliptical and lower pitched circular arches, the proportion is still greater.

It seems reasonable enough, therefore, to dispose the framing in the manner proposed by Pitot, directing the main support to the upper mass of the arch, which presses most on the frame. We shall derive another advantage from this construction, which has not occurred to Mr. Pitot.

There is an evident propriety in the manner in which he has distributed the supports of the upper part. The struts which carry the king post spring from those points of the stretcher where it rests on the struts below; thus the stretcher, on which all depends, bears no transverse strains. It is stretched by the strut above it, and it is compressed in a small degree between the struts below it, at least by the outer ones. Mr. Pitot proposes the straining beam GH as a lateral support to the stretcher, which may therefore be of two pieces; but although it does augment its strength, it does not seem necessary for it. The stretcher is abundantly carried by the strap, which may and should suspend it from the king post. The great use of the straining piece is to give a firm abutment to the inner struts, without allowing any lateral strain on the stretcher. N.B. Great care must be taken to make the hold sufficiently firm and extensive between the stretcher and the upper struts, so that its cohesion to resist the thrusts from these struts may be much employed.

The only imperfection that we find in this frame is the lateral strains which are brought upon the upper struts by the bridles, which certainly transmit to them part of the weight of the archstones on the curves. The space between the curves and ML should also have been trifled. Mr. Pitot's form is, however, extremely stiff; and the causing the middle bridle to reach down to the stretcher, seems to secure the upper struts from all risk of bending.

This centre gives a very distinct view of the offices of all the parts, and makes therefore a proper introduction to the general subject. It is the simplest that can be in its principle, because all the essential parts are subjected to one kind of strain. The stretcher LL is the only exception, and its extension is rather a collateral circumstance than a step in the general support.

The examination of the strength of the frame is extremely easy. Mr. Pitot gives it for an arch of 60 feet span, and supposes the archstones 7 feet long, which is a monstrous thickness for so small an arch; 4 feet is an abundant allowance, but we shall abide by his construction. He gives the following scantlings of the parts:

- **Suppl. Vol. I. Part I.**

The ring or circumference consists of pieces of oak, 12 inches broad and 6 thick. - The stretcher LL is 12 inches square. - The straining piece GH is also 12 by 12. - The lower struts 10 by 8. - The king post 12 by 12. - The upper struts 10 by 6. - The bridles 10 by 8.

These dimensions are French, which is about \( \frac{1}{4} \)th larger than ours, and the superficial dimensions (by which the section and the absolute strength is measured) is almost \( \frac{1}{4} \)th larger than ours. The cubic foot, by which the stones are measured, exceeds ours nearly \( \frac{1}{4} \)th. The pound is deficient about \( \frac{1}{4} \)th. But since very nice calculation is neither easy nor necessary on this subject, it is needless to depart from the French measures, which would occasion many fractional parts and a troublesome reduction.

The arch is supposed to be built of stone which weighed 160 pounds per foot. Mr. Pitot, by a computation (in which he has committed a mistake), says, that only \( \frac{3}{4} \)ths of this weight is carried by the frame. We believe, however, that this is nearer the truth than Mr. Couplet's assumption of \( \frac{3}{4} \)ths already mentioned.

Mr. Pitot further assumes, that a square inch of sound oak will carry 8640 pounds. By his language we should imagine that it will not carry much more; but this is very far below the strength of any British oak that we have tried; so far, indeed, that we rather imagine that he means that this load may be laid on it with perfect security for any time. But to compensate for knots and other accidental imperfections, he assumes 7200 as the measure of its absolute force.

He computes the load on each frame to be 707520 pounds, which he reduces to \( \frac{3}{4} \)ths, or 555928 pounds. The absolute force of each of the lower struts is 576000 (at 7200 per inch), and that of the curves 518400. Mr. Pitot, considering that the curves are kept from bending outwards by the arch stones which press on them, thinks that they may be considered as acting precisely as the outer struts EI. We have no objection to this supposition.

With these data we may compute the load which the lower struts can safely bear by the rule delivered in the article CARPENTRY. We therefore proceed as follows:

Measure off by a scale of equal parts \( a \times a \), each 576000, and add \( a \times 518400 \). Complete the parallelogram \( a \times s \), and draw the vertical \( x \), meeting the horizontal line \( aC \) in \( c \). Make \( c \) equal to \( a \). Join \( a \times b \), and complete the parallelogram \( a \times b \). It is evident that the diagonal \( xy \) will represent the load which these pieces can carry; for the line \( a \times a \) is the united force of the curve AP and the strut IE, and \( a \times a \) is the strength of IG. These two are equivalent to \( a \times a \). \( a \times b \) is, in like manner, equivalent to the support on the other side, and \( x y \) is the load which will just balance the two supports \( a \times a \) and \( b \times b \).

When \( xy \) is measured on the same scale, it will be found = 285000 pounds. This is more than five times the load which actually lies on the frame. It is therefore vastly stronger than is necessary. Half of each of the linear dimensions would have been quite sufficient, and the struts needed only to be 5 inches by 4. Even this would have carried twice the weight, and would have borne the load really laid on it with perfect safety.

We proceed to measure the strength of the upper part. The force of each strut is 432000, and that of the curve is 518400; therefore, having drawn Mv parallel to the strut ON, make Mv = 432000, and Mr = 432000 + 518400. Complete the parallelogram Mrv. Draw the horizontal line k, cutting the vertical MC in k, and make ky = Ma. It is plain, from what was done for the lower part, that Mr will measure the load which can be carried by the upper part. This will be found = 116000. This is also greatly superior to the load; but not in so great a proportion as the other part. The chief part of the load lies on the upper part; but the chief reason of the difference is the greater obliquity of the upper struts. This shortens the diagonal My of the parallelogram of forces. Mr Pitot should have adverted to this; and instead of making the upper struts more slender than the lower, he should have made them stouter.

The strain on the stretcher LL is not calculated. It is measured by r', when My is the load actually lying on the upper part. Less than the fifth part of the cohesion of the stretcher is more than sufficient for the horizontal thrust; and there is no difficulty of making the foot joints of the struts abundantly strong for the purpose.

The reader will perceive that the computation just now given does not state the proportions of the strains actually exerted on the different pieces, but the load on the whole, on the supposition that each piece is subjected to a strain proportioned to its strength. The other calculation is much more complicated, but is not necessary here.

This centre has a very palpable defect. If the piers should yield to the load, and the feet of the centre fly out, the lower part will exert a very considerable strain on the stretcher, tending to break it across between N and L, and on the other side. HKF of the lower part is firmly bound together, and cannot change its shape, and will therefore act like a lever, turning round the point F. It will draw the strut HK away from its abutment with GH, and the stretcher will be strained across at the place between H and F, where it is bolted with the bridle. This may be rectified in some degree by an iron strap uniting ON and HK; but there will still be a want of proportional strength. Indeed, in an arch of such height (a semicircle), there is but little risk of this yielding of the piers; but it is an imperfection.

The centre (fig. 2) is constructed on the same principle precisely for an elliptical arch (A). The calculation of its strength is nearly the same also; only the two upper struts of a side being parallel, the parallelogram Mrv (of fig. 1) is not needed, and in its stead we measure off on ON a line to represent twice its strength. This comes in place of Mr' of fig. 1.—N.B.

The calculation proceeds on the supposition that the short straining piece MM makes but one firm body with the king post. Mr Pitot employed this piece (we presume) to separate the heads of the struts, that their obliquity might be lessened thereby; and this is a good thought; for when the angle formed by the struts on each side is very open, the strain on them becomes very great.

The stretcher of this frame is scarfed in the middle. Suppose this joint to yield a little, there is a danger of the lower strut ON losing its hold, and ceasing to join in the support; for when the crown sinks by the lengthening of the stretcher, the triangle ORN of fig. 2, will be more distorted than the space above it, and ON will be loosened. But this will not be the case when the sinking of the crown arises from the mere compression of the struts. Nor will it happen at all in the centre, fig. 1. On the contrary, the strut ON will abut more firmly by the yielding of the foot of ML.

The figure of this arch of Mr Pitot's consists of three arches of circles, each of 60 degrees. As it is elegant, it will not be unacceptable to the artist to have a construction for this purpose.

Make BY = CD, and CZ = CY. Describe the Hows semicircle ZAEY, and make ZS = ZA. S is the centre common of the side arches, each of 60 degrees. The centre T of such an arch, which unites these two, is at the angle of an equilateral triangle STS.

This construction of Mr Pitot's makes a handsome oval, and very near an ellipse, but lies a little without it. We shall add another of our own, which coincides with the ellipse in eight points, and furnishes the artist, by the way, a rule for drawing an infinite variety of ovals.

Let AB, DE (fig. 2, No. 2.) be the axes of an ellipse, C the centre, and F, f the two foci. Make CB = CD, and describe a circle AD passing through the three given points A, D, and b. It may be demonstrated, that if from any point P of the arch AD be drawn a chord PD, and if a line PR be drawn, making the angle DPR = PDC, and meeting the two axes in the points R and r, then R and r will be the centres of circles, which will form a quarter APD of an oval, which has AB and DE for its two axes.

We want an oval which shall coincide as much as possible with an ellipse? The most likely method for this is to find the very point P where the ellipse cuts the circle AD. The easiest way for the artist is to describe an arch of a circle am, having AB for its radius, and the remote focus f for its centre. Then set one foot of the compasses on any point P, and try whether the distance PF from the nearest focus F is exactly equal to its distance Pm from that circle. Shifting the foot of the compasses from one point of the arch to another, will soon discover the point. This being found, draw PD, make the angle DPR = PDR, and R and r are the centres wanted. Then make Cs = CR, and we get the centres for the other side.

The geometer will not relish this mechanical construction. He may therefore proceed as follows: Draw Dd parallel to AB, cutting the circle in d. Draw cd, cutting AC in N. Draw CG parallel to A, and make

(a) It is the middle arch of the bridge at Lille Adam, of which Mr Pitot had the direction. It is of 80 feet span, and rises 31 feet. We said, that this centering of Mr Pitot's resembled in principle the one employed by Michael Angelo for the nave and transepts of St Peter's church at Rome. Fontana, who has preferred this, ascribes the construction of it to one of the name of San Gallo. A sketch of it is given in fig. 3. It is, however, so much superior, and so different in principle, from that employed for the cupola, that we cannot think it the invention of the same person. It is, like Pitot's, not only divisible, but really divided into two parts, of which the upper carries by much the greatest part of the load. The pieces are judiciously disposed, and every important beam is amply secured against all transverse strains. Its only fault is a great profusion of strength. The innermost polygon \(a g b b\) is quite superfluous, because no strain can force in the struts which rest on the angles. Should the piers yield outwards, this polygon will be loose, and can do no service. Nor is the triangle \(g i b\) of any use, if the king post above it be strapped to the tie-beam and straining fill. Perhaps the inventor considered the king post as a pillar, and wished to secure the tie-beam against its cross strain. This centering, however, must be allowed to be very well composed; and we expect that the well-informed reader will join us in preferring it to Mr Pitot's, both for simplicity of principle, for scientific propriety, and for strength.

There is one considerable advantage which may be derived from the actual division of the truss into two parts. If the tie-beam \(L L\), instead of resting on the stretcher \(E F\), had rested on a row of chocks formed like double wedges, placed above each other, head to point, the upper part of the centering might be struck independent of the lower, and this might be done gradually, beginning at the outer ends of the stretcher. By this procedure, the joints of the archstones will close on the haunches, and will almost relieve the lower centering, so that all can be pulled out together. Thus may the arch settle and consolidate in perfect safety, without any chance of breaking the bond of the mortar in any part; an accident which frequently happens in great arches. This procedure is peculiarly advisable for low pitched or elliptical arches. But this will be more clearly seen afterwards, when we treat of the internal movements of an arch of masonry.

This may suffice for an account of the more simple construction of trussed centres; and we proceed to such as have a much greater complication of principle. We shall take for examples some constructed by Mr Perronet, a very celebrated French architect.

Mr Perronet's general maxim of construction is to make the truss consist of several courses of separate trusses, independent (as he thinks) of each other, and thus to employ the joint support of them all. In this construction it is not intended to make use of one truss, or part of one truss, to support another, as in the former set, and as is practised in the roofs of St Paul's church, Covent Garden, and in Drury Lane theatre. Each truss spans over the whole distance of the piers, and would stand alone (having, however, a tottering equilibrium). It consists of a number of struts, set end to end, and forming a polygon. These struts are arranged, that the angles of one are in the middle of the sides of the next, as when a polygon is inscribed in a circle, and another (of the same number of sides) is circumscribed by lines which touch the circle in the angles of the inscribed polygon. By this construction the angles of the alternate struts lie in lines pointing towards the centre of the curve. King posts are therefore placed in this direction between the adjoining beams of the trusses. These king posts consist of two beams, one on each side of the truss, and embrace the truss-beams between them, meeting in the middle of their thickness. The abutting beams are mortised, half into each half of the post. The other beam, which makes the base of the triangle, passes through the post, and a strong bolt is driven through the joint, and secured by a key or a nut. In this manner is the whole united; and it is expected, that when the load is laid on the uppermost truss, it will all butt together, forcing down the king posts, and therefore pressing them on the beams of all the inferior trusses, causing them also to abut on each other, and thus bear a share of the load. Mr Perronet does not assume the invention to himself; but says, that it was invented and practised by Mr Manfard de Sagonne at the great bridge of Moulins. It is much more ancient, and is the work of the celebrated physician and architect Perrault; as may be seen in the collection of machines and inventions of that gentleman published after his death, and also in the great collection of inventions approved of by the Academy of Sciences. It is this which we propose to examine.

Fig. 4 represents the centering employed for the centering bridge of Cravant. The arches are elliptical, of 60 feet employed span and 20 feet rise. The archstones are four feet thick, and weigh 176 pounds per foot. The truss-beams were from 15 to 18 feet long, and their section was 9 inches by 8. Each half of the king posts was about 7 feet long, and its section 9 inches by 8. The whole was of oak. The five trusses were 5½ feet under. The whole weight of the arch was 135000 lbs., which we may call 600 tons (it is 558). This is about 112 tons for each truss. We must allow near 90 tons of this really to press the truss. A great part of this pressure is borne by the four beams which make the feet of the truss, coupled in pairs on each side. The diagonal of the parallelogram of forces drawn for these beams is, to one of the sides, in the proportion of 360 to 285. Therefore say, as 360 to 285, so is 90 to 71½ tons, the thrust on each foot. The section of each is 144 inches. We may with the utmost safety lay three tons on every inch for ever. This amounts to 43½ tons, which is more than six times the strain really pressing the foot beams in the direction of their length; nay, the upper truss alone is able to carry much more than its load. The absolute strength of its foot-beam is 216 tons. It is much more advantageously placed; for the diagonal of the parallelogram of forces corresponding to its position is to the side as 438 to 285. This gives 68½ tons for the strain on each foot; which is not much above the fourth part of what it is able to carry for ever. No doubt can therefore be entertained of the superabundant strength of this centering. We see that the upper row of struts is quite sufficient, and all that is wanted is to procure flues for it; for it must be carefully kept in mind, that this upper row is not like an equilibrated arch. It will be very unequally loaded as the work advances. The haunches of the frame will be pressed down, and the joints at the crown raised up. This must be resisted.

Here then we may gather, by the way, a useful lesson. Let the outer row of struts be appropriated to the carriage of the load, and let the rest be employed for giving stiffness. For this purpose let the outer row have abundant strength. The advantages of this method are considerable. The position of the beams of the exterior row is more advantageous, when (as in this example) the whole is made to rest on a narrow foot; for this obliges us to make the last angle, at least of the lower row, more open, which increases the strain on the strut; besides, it is next to impossible to distribute the compressing thrusts among the different rows of the truss beams; and a beam which, during one period of the mason work, is acting the part of a strut, in another period is bearing no strain but its own weight, and in another it is stretched as a tie. A third advantage is, that, in a case like this, where all rests on a narrow foot, and the lower row of beams are bearing a great part of the thrust, the horizontal thrust on the pier is very great, and may push it aside. This is the most ruinous accident that can happen. An inch or two of yielding will cause the crown of the arch to sink prodigiously, and will instantly derange all the bearings of the abutting beams: but when the lower beams already act as ties, and are quite adequate to their office, we render the frame perfectly stiff or unchangeable in its form, and take away the horizontal thrust from the piers entirely. This advantage is the more valuable, because the very circumstance which obliges us to rest all on a narrow foot, places this foot on the very top of the pier, and makes the horizontal thrust the more dangerous.

But, to proceed in our examination of the centering of Cravant bridge, let us suppose, that the king posts are removed, and that the beams are joined by compass joints. If the pier shall yield in the smallest degree, both rows of struts must sink; and since the angles (at least the outermost) of the lower row are more open than those of the upper row, the crown of the lower row will sink more than that of the upper.

The angles of the alternate rows must therefore separate a little. Now refore the king posts; they prevent this separation. Therefore they are stretched; consequently they no longer butt on their mortises, and must be held in their places by bolts. Thus it appears that, in this kind of faggoting, the original distribution of the load among the different rows of beams is changed, and the upper row becomes loaded beyond our expectation.

If the faggoting of the whole truss proceed only from the compression of the timbers, the case is different, and we may prefer the original distribution of mutual abutment more accurately. But in this case the stiffness of the frame arises chiefly from cross strains. Suppose that the frame is loaded with archstones on each side up to the posts HC, BC; the angles E and F are pressed down, and the beams EOF, EOF push up the point F. This cannot rise without bending the beams EOF, EOF; because O and S are held down by the double king posts, which grasp the beams between them. There is therefore a cross strain on the beams. Observe also, that the triangle EHF does not preserve its shape by the connection of its joints; for although the strut beams are mortised into the king post, they are in very shallow mortises, rather for steadying them than for holding them together. Mr Perronet did not even pin them, thinking that their abutment was very great. The triangle is kept in shape by the base EF, which is firmly bolted into the middle post at O. Had these intersections not been strongly bolted, we imagine that the centres of some of Mr Perronet's bridges would have yielded much more than they did; yet some of them yielded to a degree that our artists would have thought very dangerous. Mr Perronet was obliged to load the crown of the centering with very great weights, increasing them as the work advanced, to prevent the frames from going out of shape: in one arch of 120 feet he laid on 45 tons. Notwithstanding this imperfection, which is perhaps unavoidable, this mode of framing is undoubtedly very judicious, and perhaps the best which can be employed without depending on iron work.

Fig. 5. represents another, constructed by Perronet for an arch of 90 feet span and 28 feet rise. The truss-bridge fees were 7 feet apart, and the arch was 4½ thick; so that Nogent, the unreduced load on each frame was very nearly 235 tons. The scantling of the struts was 15 by 12 inches. The principle is the same as that of the former. The chief difference is, that in this centre the outer truss-beam of the lower row is not coupled with the middle row, but kept nearly parallel to the outer beam of the upper row. This adds greatly to the strength of the foot, and takes off much of the horizontal thrust from the pier.

Mr Perronet has shewn great judgment in causing the polygon of the inner row of truss beams gradually to approach the polygon of the outer row. By this disposition, the angles of the inner polygon are more acute than those of the outer. A little attention will shew, that the general faggoting of all the polygons will keep the abutments of the lower one nearer, or exactly, to their original quantity. We must indeed except the foot-beam. It is still too oblique; and, instead of converging to the foot of the upper row, it should have diverged from it. Had this been done, this centre is almost perfect in its kind. As it is, it is at least five times stronger than was absolutely necessary. We shall have occasion to refer to this figure on another occasion.

This maxim is better exemplified by Mr Perronet in the centering of the bridge of St Maxence, exhibited in fig. 5, n° 2, than in that of Nogent, fig. 5, n° 1. But we think that a horizontal truss-beam ab should have been inserted (in a subordinate manner) between the king posts next the crown on each side. This would prevent the crown from rising while the haunches only are loaded, without impairing the fine abutments of cd, cd, when the arch is nearly completed. This is an excellent centering, but is not likely to be of much use in these kingdoms; because the arch itself will be considered as ungraceful and ugly, looking like a huge lintel. Perronet says, that he preferred it to the ellipse, because it was lighter on the piers, which were thin. But the failure of one arch must be immediately followed by the ruin of all. We know much better methods of lightening the piers.

Fig. 6. represents the centering of the bridge of Neuilly, The arch has 120 feet span, and 30 feet rise, and is 5 feet thick. The frames are 6 feet apart, and each carries an absolute (that is, not reduced to \( \frac{4}{7} \) or to \( \frac{9}{7} \)) load of 350 tons. The strut beams are 17 by 14 inches in scantling. The king-pots are of 15 by 9 each half; and the horizontal bridles, which bind the different frames together in five places, are also 15 by 9 each half. There are eight other horizontal binders of 9 inches square.

This is one of the most remarkable arches in the world; not altogether on account of its width (for there are several much wider), but for the flatness at the crown; for about 26 feet on each side of the middle it was intended to be a portion of a circle of 150 feet radius. An arch (semicircular) of 300 feet span might therefore be easily constructed, and would be much stronger than this, because its horizontal thrust at the crown would be vastly greater, and would keep it more firmly united.

The bolts of this centre are differently placed from those of the former; and the change is judicious. Mr Perronet had doubtless found by this time, that the stiffness of his framing depended on the transverse strength of the beams; and therefore he was careful not to weaken them by the bolts. But notwithstanding all his care, the framing sunk upwards of 13 inches before the keystones were laid; and during the progress of the work, the crown rose and sunk, by various steps, as the loading was extended along it. When 20 courses were laid on each side, and about 16 tons laid on the crown of each frame, it sunk about an inch. When 46 courses were laid, and the crown loaded with 50 tons, it sunk about half an inch more. It continued sinking as the work advanced; and when the keystone was set it had sunk 13½ inches. But this sinking was not general; on the contrary, the frame had risen greatly at the very haunches, so as to open the upper part of the joints, many of which gripped an inch; and this opening of the joints gradually extended from the haunches towards the crown, in the neighbourhood of which they opened on the under side. This evidently arose from a want of stiffness in the frame. But these joints closed again when the centres were struck, as will be mentioned afterwards.

We have taken particular notice of the movements and twisting of this centre, because we think that they indicate a deficiency, not only of stiffness, but of abutment among the truss beams. The whole has been too flexible, because the angles are too obtuse: This arises from their multiplicity. When the intercepted arches have so little curvature, the power of the load to press it inward increases very fast. When the intercepted arch is reduced to one half, this power is more than doubled; and it is also doubled when the radius of curvature is doubled. The king-pots should have been farther apart near the crown, so that the quantity of arch between them should compensate for its diminished curvature.

The power of withstanding any given inequality of load would therefore have been greater, had the centre consisted of fewer pieces, and their angles of meeting been proportionally more acute. The greatest improvement would have been, to place the foot of the lower tier of truss-beams on the very foot of the pier, and to have also separated it at the head from the rest with a longer king-post, and thus to have made the distances of the beams on the king-pots increase gradually from the crown to the spring. This would have made all the angles of abutment more acute, and would have produced a greater pressure on all the lower tiers when the frame sagged.

Fig. 7 represents the centering of the bridge of Orleans. The arch has 100 feet span, and rises 30, and the arch-flues are 6 feet long. It is the construction of Mr Huppeau, the first architect of the bridge. It is the boldest work of the kind that we have seen, and is constructed on clear principles. The main abutments are few in number. Because the beams of the outer polygon are long, they are very well supported by straining beams in the middle; and the struts or braces which support and butt on them, are made to rely on points carried entirely by ties. The inventor, however, seems to have thought that the angles of the inner polygon were supported by mutual compression, as in the outer polygon. But it is plain that the whole inner polygon may be formed of iron rods. Not but that both polygons may be in a state of compression (this is very possible); but the smallest sagging of the frame will change the proportions of the pressures at the angles of the two polygons. The pressures on the exterior angles will increase, and those on the lower or interior angles will diminish most rapidly; so that the abutments in the lower polygon will be next to nothing. Such points could bear very little pressure from the braces which support the middle of the long bearings of the upper beams, and their pressures must be borne chiefly by the joints supported by the king-pots. The king-pots would then be in a state of extension. It is difficult, however, to decide what is the precise state of the pressure at these interior angles.

The history of the erection of this bridge will throw much light on this point, and is very instructive. Mr Huppeau died before any of the arches were carried farther than a very few of the first courses. Mr Perronet succeeded to the charge, and finished the bridge. As the work advanced, the crown of the frame rose very much. It was loaded; and it sunk as remarkably. This showed that the lower polygon was giving very little aid. Mr Perronet then thought the frame too weak, and inserted the long beam DE, making the diagonal of the quadrangle, and very nearly in the direction of the lower beam ab, but falling rather below this line. He now found the frame abundantly strong. It is evident that the truss is now changed exceedingly, and consists of only the two long sides, and the short straining beam lying horizontally between their heads. The whole centering consists now of one great truss a E e b, and its long sides a E, e b, are trifled up at B and f. Had this simple idea been made the principle of the construction, it would have been excellent. The angle a DE might have been about 170°; and the polygon D e g h employed only for giving a flight support to this great angle, so as not to allow it to exceed 180°. But Mr Perronet found, that the joint e, at the foot of the post E c, was about to draw loose, and he was obliged to bolt long pieces of timber on each side of the joint, embracing both beams. These were evidently acting the same part as iron traps would have done; a complete proof that whatever may have been the original pressures, there was no abutment now at the point \( e \), and that the beams that met there were not in a state of compression, but were on the stretch. Mr Perronet says that he put these checks to the joints to stiffen them. But this was not their office; because the adjoining beams were not struts, but ties, as we have now proved.

We may therefore conclude, that the outer polygon, with the affluence of the pieces \( a b \), DE, were carrying the whole load. We do not know the distance between the frames; but supposing them seven feet apart, and the arch 6 feet thick, and weighing 170 pounds per foot, we learn the load. The beams were 15 inches square. If we now calculate what they would bear at the same very moderate rate allowed to the other centres, we find that the beams A.B and \( a b \) are not loaded to one-sixth of their strength.

We have given this centre as a fine example of what carpentry is able to perform, and because, by its simplicity, it is a sort of text on which the intelligent artist may make many comments. We may see plainly that, if the lower polygon had been formed of iron rods, firmly bolted into the feet of the king-polls, it would have maintained its shape completely. The service done by the beam DE was not so much an increase of abutment as a discharge of the weight and of the pull at the joint \( e \). Therefore, in cases where the feet of the trusses are necessarily confined to a very narrow space, we should be careful to make the upper polygon sufficient to carry the whole load (say by doubling its beams), and we may then make the lower polygon of slender dimensions, provided we secure the joints on the king-polls by iron straps which embrace a considerable portion of the tie on each side of the joint.

We are far from thinking that these centres are of the best kind that could be employed in their situation; but they are excellent in their kind; and a careful study of them will teach the artist much of his profession. When we have a clear conception of the state of strain in which the parts of a frame really are, we know what should be done in order to draw all the advantage possible from our materials. We have said in another place, that where we can give our joints sufficient connection (as by straps and bolts, or by cheeks or fishes), it is better to use ties than struts, because ties never bend.

We do not approve of Mr Perronet's practice of giving his trusses such narrow feet. By bringing the foot of the lower polygon farther down, we greatly diminish all the strains, and throw more load on the lower polygon; and we do not see any of Mr Perronet's centres where this might not have been done. He seems to affect a great span, to show the wonders of his art; but our object is to teach how to make the best centre of a given quantity of materials; and how to make the most perfect centre, when we are not limited in this respect, nor in the extent of our fixed points.

We shall conclude this series of examples with one where no such affectation takes place. This is the centering of the bridge at Blackfriars, London. The span of the arch is 100 feet, and its height from the spring is about 43. The drawing fig. 8. is sufficiently minute to convey a distinct notion of the whole construction. We need not be very particular in our observations, after what has been said on the general principles of construction. The leading maxim, in the present example, seems to be, that every part of the arch shall be supported by a simple truss of two legs resting, one on each pier. H, H, &c., are called apron pieces to strengthen the exterior joints, and to make the ring as stiff in itself as possible. From the ends of this apron-piece proceed the two legs of each truss. These legs are 12 inches square: They are not of an entire piece, but of several, meeting in firm abutment. Some of their meetings are secured by the double king-polls, which grasp them firmly between them, and are held together by bolts. At other intersections, the beams appear halved into each other; a practice which cannot but weaken them much, and would endanger their breaking by cross strains, if it were possible for the frame to change its shape. But the great breadth of this frame is an effectual stop to any such change. The fact was, that no sinking or twisting whatever was observed during the progress of the mason work. Three points in a straight line were marked on purpose for this observation, and were observed every day. The arch was more than six feet thick; and yet the sinking of the crown, before setting the key-stones, did not amount to one inch.

The centre employs about one-third more timber than Perronet's great centre in proportion to the span of the arch; but the circumference increases in a greater proportion than this, because it is more elevated. In every way of making a comparison of the dimensions, Mr Mylne's arch employs more timber; but it is beyond all comparison stronger. The great elevation is partly the reason of this. But the disposition of the timbers is also much more advantageous, and may be copied even in the low pitched arches of Neuilly. The simple truss, reaching from pier to pier for the middle point of the arch, gives the strong support where it is most of all wanted; and in the lateral points H, although one leg of the truss is very oblique, the other compensates for it by its upright position.

The chief peculiarity of this centre is to be seen in its base. This demands a more particular attention; but we must first make some observations on the condition of an arch, as it rests on the centering after the keystones are all set, and on the gradual transference of the pressure from the boards of the centering to the joints of the archstones.

While all the archstones lie on the centering, the lower courses are also leaning pretty strongly on each other. But the mortar is hardly compressed in the joints; and least of all in the joints near the crown. Suppose the arch to be Catenarian, or of any other shape that is perfectly equilibrated: When the centering is gradually withdrawn, all the archstones follow it. Their wedge-like form makes this impossible, without the middle ones squeezing the lateral ones aside. This compresses the mortar between them. As the stones thus come nearer to each other, those near the crown must descend more than those near the haunches, before every stone has lessened its distance from the next by the same quantity; for example, by the hundredth part of an inch. This circumstance alone must cause a sinking in the crown, and a change of shape. But the joints near the crown are already more open than those near the haunches. This produces a still greater change of form before all is settled. Some nations encourage to remedy, or at least to diminish, this, by using no mortar mortar in the joints near the crown. They lay the stones dry, and even force them together by wedges and blocks laid between the stones on opposite sides of the crown: they afterwards pour in fine cement. This appears a good practice. Perronet rejects it, because the wedging sometimes breaks the stones. We should not think this any great harm; because the fracture will make them close where they would otherwise lie hollow. But, after all our care, there is still a flanking of the crown of the arch. By gradually withdrawing the centering, the joints close, the archstones begin to butt on each other, and to force aside the lateral courses. This abutment gradually increases; the pressure on the haunches of the centering is gradually diminished by the mutual abutment, and ceases entirely in that course, which is the lowest that formerly precluded it: it then ceases in the course above, and then in the third, and so on. And, in this manner, not only the centering quits the arch, gradually, from the bottom to the top, by its own retiring from it, but the arch also quits the centering by changing its shape. If the centering were now pushed up again, it would touch the arch first at the crown; and it must lift up that part gradually before it come again in contact with the haunches. It is evident, therefore, that an arch, built on a centre of a shape perfectly suited to equilibration, will not be in equilibrium when the centering is removed. It is therefore necessary to form the centering in such a manner (by raising the crown), that it shall leave the arch of a proper form. This is a very delicate task, requiring a previous knowledge of the ensuing change of form. This cannot be ascertained by the help of any theory we are acquainted with.

But, suppose this attained, there is another difficulty: While the work advances, the centering is warped by the load laid on it, and continually increasing on each side. The first pressure on the centering forces down the haunches, and raises the crown. The arch is therefore less curved at the haunches than is intended: the joints, however, accommodate themselves to this form, and are close, and filled with mortar. When the masons approach the middle of the arch, the frame sinks there, and rises up at the haunches. This opens all the joints in that place on the upper side. By the time that the keystones are set, this warping has gone farther; and joints are opened on the under side near the crown. It is true we are here speaking rather of an extreme case, when the centering is very flexible; but this occurred to Mr Perronet in the two great bridges of Neuilly and of Mantz. In this last one, the crown sunk above a foot before the key was set, and the joints at the haunches opened above an inch above, while some nearer the crown opened near a quarter of an inch below.

In this condition of things, it is a delicate business to strike the centering. Were it removed in an instant, all would probably come down; for the archstones are not yet abutting on each other, and the joints in the middle are open below. Mr Perronet's method appears to us to be very judicious. He began to detach the centering at the very bottom, on each side equally, where the pressure on the centering is very slight. He cut away the blocks which were immediately under each archstone. He proceeded gradually upwards in this way with some speed, till all was detached that had been put out of shape by the bending of the centering. This being no longer supported, sunk inward, till it was stopped by the abutment which it found on the archstones near the crown, which were still resting on their blocks. During part of this process, the open joints opened still more, and looked alarming. This was owing to the removal of the load from the haunches of the centering. This allowed the crown to sink still more, by forcing out the arch stones at the haunches. He now paused some days; and during this time the two haunches, now hanging in the air, gradually pressed in toward the centering, their outer joints closing in the meanwhile. The haunches were now pressing pretty hard on the archstones nearer the crown. He then proceeded more slowly, destroying the blocks and bridgings of these upper archstones. As soon as he destroyed the support of one, it immediately yielded to the pressure of the haunch; and if the joint between it and the one adjoining toward the crown happened to be open, whether on the under or the upper side, it immediately closed on it. But in proceeding thus, he found every stone sank a little while it closed on its neighbour; and this was like to produce a ragged soffit, which is a deformity. He therefore did not allow them to sink too much. In the places of the blocks and bridgings which he had cut away, he set small billets, standing on their ends, between the centering and the archstones. These allowed the pendulous arch to push toward the crown without sensibly descending; for the billets were pushed out of the perpendicular, and some of them tumbled down. Proceeding in this way, he advanced to the very next course to the keystone on each side, the joints closing all the way as he advanced. The last job was very troublesome; we mean the detaching the three uppermost courses from the centering: for the whole elasticity of the centering was now trying to unbend, and pressing hard against them. He found that they were lifted up; for the joints beyond them, which had closed completely, now opened again below; but this job was finished in one day, and the centre sprung up two or three inches, and the whole arch sunk about six inches. This was an anxious time; for he dreaded the great momentum of such a vast mass of matter. It was hard to say where it would stop. He had the pleasure to see that it dropped very soon, settling slowly as the mortar was compressed, and after one or two days settling no more. This settling was very considerable both in the bridge at Neuilly and in that at Mantz. In the former, the sinking during the work amounted to 13 inches. It sunk six inches more when the blocks and bridgings were taken out, and 1½ when the little standards were destroyed, and 1½ more next day; so that the whole sinking of the pendulous arch was 9½ inches, besides what it had sunk by the bending and compression of the centering.

The crown of the centering was an arch of a circle described with a radius of 150 feet; but by the sinking of the arch its shape was considerably changed, and about 60 feet of it formed an arch of a circle whose radius was 24¾ feet. Hence Mr Perronet infers, that a semicircle of 500 feet span may be erected. It would no doubt be stronger than this arch, because its greater horizontal thrust would keep the stones firmer together. The sinking of the arches at Mantz was not quite so great, but everything proceeded in the same way. It amounted in all to 20½ inches, of which 12 inches were owing owing to the compression and bending of the centering.

In fig. 5, no 1, may be observed an indication of this procedure of the masonry. There may be noticed a horizontal line \(a_c\), and a diagonal \(a_b\). These are supposed to be drawn on the masonry as it would have stood had the frames not yielded during the building. The dotted line \(A b c\) shows the shape which it took by the sinking of the centering. The dotted line on the other side was actually drawn on the masonry when the keystone was set; and the wavy black line on the same side shows the form which the dotted line took by the striking of the centering. The undulated part of this line cuts its former position a little below the middle, going without it below, and falling within it above. This shows very distinctly the movement of the whole masonry, distinguishing the parts that were forced out and the parts which sunk inward.

We presume that the practical reader will think this account of the internal movements of a stupendous arch very instructive and useful. As Mr Perronet observed it to be uniformly the same in several very large arches which he erected, we may conclude that it is the general process of nature. We by no means have the confidence in the durability or solidity of his arches which he prudently professes to have. We have conversed with some very experienced masons, who have also erected very great arches, and in very difficult situations, which have given universal satisfaction; and we have found them uniformly of opinion, that an arch which has settled to such a proportion of its curvature as to change the radius from 120 to 244 feet, is in a very hazardous situation. They think the hazard the greater, because the span of the arch is so great in proportion to its weight (as they express it very emphatically) or its height. The weight, say they, of the haunches is too small for forcing together the keystones, which have scarcely any wedgelike form to keep them from sliding down. This is very good reasoning, and expresses very familiar notions. The mechanician would say, that the horizontal thrust at the crown is too small. When we questioned them about the propriety of Mr Perronet's method of removing the centering, they unanimously approved of its general principle, but said that it was very ticklish indeed in the execution. The cases which he narrates were new to them. They should have almost despaired of success with arches which had gone so much out of shape by the bending of the centres; because, said they, the slope of the centering, to a great distance from the crown, was so little, that the archstones could not slide outwards along it, to close even the under side of the joints which had opened above the haunches; so that all the archstones were at too great a distance from each other; and a great and general subsiding of the whole was necessary for bringing them even to touch each other. They had never observed such bendings of the centerings which they had employed, having never allowed themselves to contract the feet of their trusses into such narrow spaces. They observed, that nothing but lighters with their mails down can pass under the trusses, and that the sides must be so protected by advanced works from the accidental shock of a loaded boat, that there cannot be left room for more than one. They added, that the bridges of communication, necessary for the expeditious conducting of the work, made all this supposed roominess useless; besides, the business can hardly be so urgent and crowded anywhere, as to make the passage through every arch indispensible necessary. Nor was the inconvenience of this obstruction greatly complained of during the erection of Westminster or Blackfriars bridges. Nothing should come in competition with the undoubted solidity of the centering and the future arch; and all boasting display of talent and ingenuity by an engineer, in the exhibition of the wonders of his art, is misplaced here.

There appeared to us good reasons for preferring the more cautious, and incomparably more secure, construction of Mr Mylne, in which the breadth given to each base of the trusses permitted a much more effective disposition of the abutting timbers, and also enabled the engineer to make it incomparably stiffer; so that no change need be apprehended in the joints which have already closed, and in which the mortar has already taken its set, and commenced an union that never can be restored if it be once broken in the smallest degree, nor even by greater compulsion.

Here we beg leave to mention our notions of the connection that is formed by mortar composed of lime needles or gypsum. We consider it as consisting chiefly, if not that is solely, in a crystallization of the lime or gypsum and by means of water. As much water is taken up as is necessary for the formation of the crystals during their gradual conversion into mild calcareous earth or alabaster, and the rest evaporates. When the free access of air is absolutely prevented, the crystallization never proceeds to that state, even although the mortar becomes extremely dry and hard. We had an opportunity of observing this accidentally, when passing through Maastricht in 1779, while they were cutting up a masonry revetment of a part of the fortifications more than 300 years old. The mortar between the bricks was harder than the bricks (which were Dutch clinkers, such as are now used only for the greatest loads); but when mixed with water it made lime water, seemingly as strong as if fresh lime had been used. We observed the same thing in one small part of a huge mass of ancient Roman work near Romney in Kent; but the rest, and all the very old mortar that we have seen, was in a mild state, and was generally much harder than what produces any lime water. Now when the mortar in the joints has begun its first crystallization, and is allowed to remain in perfect rest, we are confident that the subsequent crystals, whether of lime, or of calcareous earth, or of gypsum, will be much larger and stronger than can ever be produced if they are once broken; and the farther that this crystallization has been carried, that is, the harder that the mortar has become, less of it remains to take any new crystallization. Why should it be otherwise here than in every other crystallization that we are acquainted with?

We think therefore that it is of great consequence to keep the joints in their first state if possible; and that the strength (as far as it depends on the mortar) joins is greatly diminished by their opening; especially when the mortar has acquired considerable hardness, which it will do in a month or six weeks, if it be good. The cohesion given by mortar is indeed a mere trifle, when opposed to a force which tends to open the joints, acting, as it generally does, with the transverse force of a lever; but in situations where the overload on any particular not be of long duration. The bridge at Mantz is still Center- more exceptionable, because its piers are tall and slender. If any one of the arches fails, the rest must fall in a moment. An arch of Blackfriars Bridge might be blown up without disturbing its neighbours.

Mr Perronet mentions another mode of striking the A bad me- centering, which he says is very usual in France. Every third of second bridging is cut out. Some time after, every second of the second of the remainder; after this, every second of the remainder; and so on, till all are removed. This is nev- er practised in this country, and is certainly a very bad method. It leaves the arch hanging by a number of di- flant points; and it is wonderful that any arch can bear this treatment.

Our architects have generally proceeded with extreme caution. Wherever they could, they supported the cen- tering by intermediate pillars, even when it was a trussed centre, having a tie-beam reaching from side to side. The centre was made to rest, not immediately on these The com- pillars, but on pieces of timber formed like acute wedges, men me- placed in pairs, one above the other, and having the third point of the one on the thick end of the other. These British wedges were well foaped and rubbed with black lead, to make them slippery. When the centres are to be struck, men are stationed at each pair of the wedges with heavy mauls. They are directed to strike together on the op- posite wedges. By this operation, the whole centering defends together; or, when any part of the arch is ob- served to have opened its joints on the upper side, the wedges below that part are slackened. The framing may perhaps bend a little, and allow that part to sub- side. If any part of the arch is observed to open its joints on the under side, the wedges below that part are allowed to stand after the rest have been slackened. By this process, the whole comes down gradually, and as slowly as we please, and the defects of every part of the arch may be attended to. Indeed the caution and mo- deration of our builders have commonly been such, that few defects have been allowed to show themselves. We are but little acquainted with joints opening to the ex- tent of two inches, and in such a case would probably lift every stone of the arch again (n). We have not employed trussed centerings so much perhaps as we should have done; nor do we see their advantage (speaking as mere builders) over centres supported all over, and unchangeable in their form. Such centres must bend a little, and require loading on the middle to keep them in shape. Their compulsion and their elasticity are very troublesome in the striking of the centres in Mr Perronet's manner. The elasticity is indeed of use when the centres are struck in the way now described.

These observations on the management of the inter- nal movements of a great arch, will enable the reader to appreciate all the merit of Mr Mylne's very ingenious construction. We proceed therefore to complete our description.

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(n) The writer of this article can only say, that, after much inquiry, he has no information of any arch being received from the builder as sufficient that had suffered half the change of shape mentioned by Mr Perronet. The arch of Dublin bridge, built by an excellent, but a very private, mason, Mr Steven, is 105 feet wide, with only 22 feet of rise. It was erected (but not on a trussed centering) without changing one full inch in its ele- vation; and when the centering was removed, it sunk only 1/3 inches, and about half an inch more when the parapets were added and the bridge completely finished. The gradual enlargement of the base of the piers of Blackfriars bridge enabled the architect to place a series of five posts \( e_1, e_2, e_3, e_4 \) on each flap of the pier; the ingenious construction of which made it like one solid block of stone (see Arch, Supplement). These struts were gradually more and more oblique, till the outer one formed an obtuse angle with the lowest side of the interior polygon of the truss. On the top of these posts was laid a sloping seat or beam D of stout oak, the upper part of which was formed like a zig-zag scarfing. The posts were not perpendicular to the under side of the seat. The angles next the pier were somewhat obtuse. Short pieces of wood were placed between the heads of the posts (but not mortised into them), to prevent them from slipping back. Each face of the scarf was covered with a thick and smooth plate of copper. The feet of the truss were mortised into a similar piece F, which may be called the sole of the truss, having its lower side notched in the same manner with the upper side of D, and like it covered with copper. Between these two lay the striking wedge E, the faces of which corresponded exactly with the flint faces of the seat and the sole. The wedge was so placed, that the corresponding faces touched each other for about half of their length. A block of wood was put in at the broad end or base of this wedge, to keep it from slipping back during the laying the archstones. Its outer end E was bound with iron, and had an iron bolt several inches long driven into it. The head of this bolt was broad enough to cover the whole wood of the wedge within the iron ferrule.

We presume that the reader, by this time, foresees the use of this wedge. It is to be driven in between the sole and the seat (having first taken out the block at the base of the wedge). As it advances into the wider spaces, the whole truss must descend, and be freed from the arch; but it will require prodigious blows to drive it back. Mr Mylne did not think so, founding his expectation on what he saw in the launching of great ships, which slide very easily on a slope of 10 or 12 degrees. He rather feared, that taking out the block behind would allow the wedge to be pushed back at once, so that the descent of the truss would be too rapid. However, to be certain of the operation, he had prepared an abundant force in a very ingenious manner. A heavy beam of oak, armed at the end with iron, was suspended from two points of the centre like a battering ram, to be used in the same manner. Nothing could be more simple in its structure, more powerful in its operation, or more easy in its management. Accordingly the success was to his wish. The wedge did not slip back of itself; and very moderate blows of the ram drove it back with the greatest ease. The whole operation was over in a very few minutes. The spectators had suspected, that the space allowed for the recess of the wedge was not sufficient for the settlement of the arch; but the architect trusted to the precautions he had taken in its construction. The reader, by turning to the article Arch in this Supplement, will see, that there was only the arch LY which could be expected to settle; accordingly, the recess of the wedge was found to be much more than was necessary. However, had this not been the case, it was only necessary to take out the pieces between the posts below the seat, and then to drive back the heads of the struts; but this was not needed (we believe) in any of the arches. We are well assured that none of the arches sunk an inch and a half. The great arch of 100 feet span did not sink one inch at the crown. It could hardly be perceived whether the arch quitted the centering gradually or not, so small had been the changes of shape.

We have no hesitation in saying, that (if we except the profuse waste of great timber by uncommon joggling) the whole of this performance is the most perfect of any of the kind that has come to our knowledge. We doubt not but that several have equalled it, or may have excelled it; but we do not know of them; and we think, that the bringing forward such performances is no less serviceable to the public than it is honourable to the inventor. Nor do we suppose that any views of interest can be so powerful as to prevent an ingenious architect from communicating to the public such honourable specimens of his own talents. We should be happy to communicate more of this kind; for we consider it as a very important article of practical mechanics, and think that it is of consequence to the nation that it should be very generally understood. In every corner of the country bridges are to be built—we have everywhere good masons, who are fully able to execute any practicable project, but too little acquainted with principle to invent, or to accommodate even what they know to local circumstances, and are very apt to be duped by appearances of ingenuity, or misled by erroneous notions of the strains which are excited. We profess more science, and to treat the subject with the affluence of accurate principles. But while we are certain that every circumstance is susceptible of the most accurate determination, we must acknowledge that we have by no means attained an accurate knowledge of all the strains which are produced and excited in a frame of carpentry, which is settling and changing its shape, even though it be not very complicated; far less are we possessed of a clear view of what happens in a mass of masonry in similar conditions. Therefore, though we speak with the strong belief of our being right, we speak with a sense of our fallibility, and with great deference to the judgment of eminent and experienced architects and engineers. We should consider their free and candid criticisms as the highest favour; and we even solicit them, with assurances of thanks, and that we will take some opportunity, before the close of this work, to acknowledge and correct our mistakes. We even presume to hope, that the liberal minded artist will be pleased with this opportunity which we give him of increasing the national stock of knowledge. Let mutual jealousy and rivalry reign in the breasts, and prompt the exertions of our reflex neighbours on the continent—let them think that the dignity of man consists in perpetual warfare, in which every individual feels himself indebted only to himself, freed from all the sweet ties of domestic partiality, of friendship, and of patriotic attachment. We hope that the hearts of Britons will long continue to be warmed and fortified by the thoughts of mutual affilience, mutual co-operation, mutual attachment, and a patriotic preference of their countrymen to all other men. While these sentiments are regulated by unshaken honesty, by candour, and by Christian charity, we shall be secured from the errors of partial attachments, and yet enjoy all the pleasures of unpolished nature. Families will still be bound together by the affectionate ties. ties of blood; and the whole frame of British society will be in harmony with the bonds which connect the members of each family, by their endless crossings and intermixings. In this state, the state of social nature, the man of talents will not look up all the fruits of his exertions in his own breast, but will feel a pleasure in imparting them to a society that is dear to him, and on which he depends for all his best enjoyments. Nothing will hold the good man back when this is in his power, but the virtuous use which he can make of his superiority in the discharge of his own little circle of duties. This is all that is required of true patriotism; and it is not too much to be expected from Britons, who feel a pleasure in viewing their country as the great school of the arts, under the patronage of a sovereign who has done more for their improvement than all the other princes of Europe, and who (we are well assured) is now meditating a plan which must be highly gratifying to every eminent professor of the arts.

The subject which we have been considering is very closely connected with the construction of wooden bridges. These are not always constructed on the sole principles of equilibrium, by means of mutual abutment. They are stiff frames of carpentry, where, by a proper disposition, beams are put into a state of extension, as well as of compression, so as to stand in place of solid bodies as big as the spaces which the beams inclose; and thus we are enabled to couple two, three, or four of these together, and set them in abutment with each other like mighty archstones. We shall close this article, therefore, with two or three specimens of wooden bridges, disposed in a series of progressive composition, so as to serve as a sort of introduction to the art in general, and furnish a principle which will enable the intelligent and cautious artist to push it with confidence as far as it can go.

The general problem is this. Suppose that a bridge is to be thrown over the space AB (fig. 9.), and that this is too wide for the strength of the size of timber which is at our command; how may this beam AB be supported with sufficient effect? There are but two ways in which the middle point C (where the greatest strain is) can be supported: 1. It may be suspended by two ropes, iron rods, or wooden ties, DC, EC, made fast to two firm points D, E, above it; or it may rest on the ridge of two rafters dC, eC, which rest on two firm points d, e, below it. 2. It may be supported by connecting it with a point to support; and this connection may be formed, either by suspending it from this point, or by a pole resting on it. Thus it may hang, by means of a rod or a king-post FC, from the ridge F of two rafters AF, BF; or it may rest on the first C, whose lower extremity f is carried by the ropes, rods, or wooden ties Af, Bf.

Whichsoever of these methods we employ, it follows, from the principles of carpentry, that the support given to the point C is so much the more powerful, as we make the angle DCE, or dCe, or the equivalent angles AFB, or A/B, more acute.

Each of these methods may be supposed equally strong. Our choice will depend chiefly on the facility of finding the proper points of support D, E, d, e; except in the second case, where we require no fixed points but A and B. The simple forms of the first case require a great extent of figure. Very rarely can we suspend it from points situated as D and E. It is even seldom that we have depth enough of bank to allow the support of the rafters dC, eC; but we can always find room for the simplest truss AFB. This therefore is the most usually employed method of construction.

In the construction, we must follow the maxims and directions preferred in the article Carpentery of this volume, and the article Roof of the Barge. The beams FA, FB must be mortised into AB, in the firmest manner, and there secured with straps and bolts; and the middle must hang by a strap attached to the king post FC, or to the iron rod that is used for a king post. No mortising in the point C must be employed; it is unnecessary, and it is hurtful, because it weakens the beam, and because it lodges water, and soon decays by rot. The best practice is not to suspend the beam immediately by this strap, but to let it rest, as in fig. 10., on a beam C, which crosses the bridge below, and has its other end supported in the same manner by the other truss.

It is evident that the length of the king post has no effect on the support of C. We may therefore contract everything, and preserve the same strength of support, by finding two points a and b (fig. 11.) in the banks, at a moderate distance below A and B, and setting up the rafters aF, bF, and suspending C from the shortened king post. In this construction, when the beam AB rests on a cross bearer, as is drawn here, the struts aF, bF are kept clear of it. No connection between them is necessary, and it may be hurtful, by inducing cross strains on both. It will, however, greatly increase the stiffness of the whole. This construction may safely be loaded with ten times the weight that AB can carry alone.

Suppose this done, and that the scantling of AB is too weak for carrying the weight which may be brought upon the parts AC, CB. We may now truss up each of that half, as in fig. 12., and then the whole will form a hand-rail of the simplest bridges, of the simplest construction possible. The intersections of the secondary braces with those of the main trusses will form a hand-rail of agreeable figure.

We are not confined to the employment of an entire piece AB, nor to a rectilineal form. We may frame the bridge as in fig. 13., and in this form we displace from allowing any connection with the middle points of the main braces. This construction also may be followed till each beam AC and CB is loaded to ten times what it can safely bear without the secondary trussing.

There is another way by which a bridge of one beam may be supported beyond the power of the first and simplest construction. This is represented in fig. 14., and fig. 15. The truss beam FG should occupy one-third of AB. The advantage of this construction is very considerable. The great elevation of the braces (which is a principal element of the strength) is preserved, and the braces are greatly shortened.

This method may be pushed still farther, as in fig. 16.

And all these methods may be combined, by joining the constructions of fig. 14 and fig. 15., with that of fig. 16.

In all of them there is much room for the display of skill, in the proper adjustment of the scantling of the timber. timber, and the obliquity of the braces to the lengths of the different bearings. A very oblique strut, or a slender one, will suffice for a small load, and may often give an opportunity to increase the general strength; while the great timbers and upright supports are reserved for the main pressures. Nothing will improve the composition so much as reflecting progressively, and in the order of these examples, on the whole. This alone can preserve the great principle in its simplicity and full energy.

These constructions are the elements of all that can be done in the art of building wooden bridges, and are that can be found more or less obviously and distinctly in all done in this attempts of this kind. We may assert, that the more obviously they appear, the more perfect the bridge will be. It is astonishing to what extent the principle may be carried. We have seen a bridge of 42 feet span formed of two oak trusses, the biggest timber of which did not exceed six inches square, bearing with perfect steadiness and safety a wagon loaded with more than two tons, drawn by four stout horses. It was framed as fig. 16, nearly, with the addition of the dotted lines, and was near thirty years old; protected, however, from the weather by a wooden roof, as many bridges in Germany are.

We recollect another in the neighbourhood of Stettin, which seemed constructed with great judgment and spirit. It had a carriage road in the middle about 20 feet (we think) wide, and on each side a foot way about five feet wide. The span was not less than 60 feet, and the greatest scantling did not appear to exceed 10 inches by 6.

This bridge consisted of four trusses, two of which formed the outside of the bridge, and the other two made the separation between the carriage road and the two foot ways. We noticed the construction of the trusses very particularly, and found it similar to the last, except in the middle division of the upper truss, which, being very long, was double trussed, as in fig. 17.

The reader will find in that volume of Leopold's Theatrum Machinarum, which he calls Theatrum Pontificum, many specimens of wooden bridges, which are very frequent in the champaign parts of Germany. They are not, in general, models of mechanic art; but the reflecting reader, who considers them carefully, will pick up here and there subordinate hints, which are ingenious, and may sometimes be useful.

What we have now exhibited are not to be considered as models of construction, but as elementary examples and lessons, for leading the reader systematically into a thorough conception of the subject.

We cannot quit the subject without taking notice of a very wonderful bridge at Wittingen in Switzerland, slightly described by Mr. Coxe (Travels, vol. I. 132.) It is of a construction more simple still than the bridges we have been describing. The span is 230 feet, and it rises only 25. The sketch (fig. 18.) will make it sufficiently intelligible. ABC is one of two great arches, approaching to a Catenarian shape, built up of seven courses of solid logs of oak, in lengths of 12 or 14 feet, and 16 inches or more in thicknesses. These are all picked of a natural shape, fitted to the intended curve; so that the wood is nowhere cut across the grain to trim it into shape. These logs are laid above each other, so that their abutting joints are alternate, like those of a brick wall; and it is indeed a wooden wall, simply built up, by laying the pieces upon each other, taking care to make the abutting joints as close as possible. They are not fastened together by pins or bolts, or by scarfings of any kind. They are, however, held together by iron straps, which surround them, at the distance of five feet from each other, where they are fastened by bolts and keys.

These two arches having been erected (by the help, we presume, of pillars, or a centering of some kind), and well buttressed against the rock on each side, were freed from their supports, and allowed to settle. They are so placed, that the intended road a b c intersects them about the middle of their height. The roadway is supported by cross joists, which rest on a long horizontal summer beam. This is connected with the arches on each side by uprights bolted into them. The whole is covered with a roof, which projects over the arches on each side to defend them from the weather. Three of the spaces between these uprights have struts or braces, which give the upper work a sort of trussing in that part.

This construction is simple and artless; and appears, by the attempt to truss the ends, to be the performance of a person ignorant of principle, who has taken the whole notion from a stone arch. It is, however, of a strength much more than adequate to any load that can be laid on it. Mr. Coxe says, but does not explain how, that it is so contrived that any part of it can be repaired independent of the rest. It was the last work of one Ulrich Grubenhamm of Tuffen, in the canton of Appenzell, a carpenter without education, but celebrated for several works of the same kind; particularly the bridge over the Rhine at Schaffhausen, consisting of two arches, one of 172 and the other of 193 feet span, both resting on a small rock near the middle of the river.

While writing this article, we got an account of a wooden bridge erected in North America, in which this simple notion of Grubenhamm's is mightily improved. The span of the arch was said to exceed 250 feet, and its rise exceedingly small. The description we got is very general, but sufficient, we think, to make it perfectly intelligible.

In fig. 19. DD, EE, FF, are supposed to be three beams of the arch. They consist of logs of timber of North small lengths, suppose of 10 or 12 feet, such as can be found of a curvature suited to its place in the arch without trimming it across the grain. Each beam is double, consisting of two logs applied to each other, side to side, and breaking joint, as the workmen term it. They are kept together by wedges and keys driven through them at short intervals, as at K, L, &c.

The manner of joining and strongly binding the two side pieces of each beam is shown in fig. 20. The mortise a i b and d e i o, which is cut in each half beam, is considerably longer on the outside than on the inside, where the two mortises meet. Two keys, BB and CC, are formed, each with a notch b e d, or a i o, on its side; which notch fits one end of the mortise. The inner side of the key is straight, but so formed, that when both keys are in their places, they leave a space between them wider at one end than at the other. A wedge AA, having the same taper as the space just mentioned, is put into it and driven hard. It is evident that this must hold the two logs firmly together. This is a way of uniting timber not mentioned in the article Carpentry; and it has some peculiarities worthy of notice. In the first place, it may be employed so as to produce a very strong lateral connection, and would then cooperate finely with the other artificial methods of tearing and tabling that we described in the article referred to. But it requires nice attention to some circumstances of construction to secure this effect. If the joints are accurately formed to each other, as if the whole had been one piece divided by an infinitely thin saw, this manner of joining will keep them all in their places. But no driving of the wedge AA will make them firmer, or cause one piece to press hard on the other. If the abutment of two parts of the half beam is already close, it will remain so; but if open in the smallest degree, driving of the wedge will not make it tighter. In this respect, therefore, it is not so proper as the forms described in Carpentry.

In order that the method now described may have the effect of drawing the halves of the beams together, and of keeping them hard squeezed on each other, the joints must be made so as not to correspond exactly. The prominent angle a i o (fig. 21), formed by the ends of the two half mortises, must be made a little more obtuse than the angle a f o of the notch of the key which this prominence is intended to fill up. Moreover, the opposite side e t of this key should not be quite straight, but a very little convex. With these precautions, it is easy to see that, by driving the wedge AA, we cause the notch a f o to take hold, first at the two points a and o, and then, by continuing to drive the wedge, the sides a f , o f of the notch gradually compress the wood of the half beams, and press them on each other. By continuing to drive the wedge, the mutual compression of the key and the beam squeezes all together, and the space a f o i is completely filled up. We may see, from this process, that the mutual compression and drawing together of the timber will be greater in proportion as we make the angle a i o more prominent, and its corresponding angle a f o more deep; always taking care that the key shall be thick enough not to break in the narrow part.

This adjustment of the keys to the mortise is necessary on another account. Supposing the joints to fit each other exactly before driving the wedge, and that the whole shrinks a little by drying—by this the angle a i o will become more prominent, and the angle a f o will become more shallow; the joint will open at a and o, and the mutual compression will be at an end.

We may also observe, that this method will not give any additional firmness to the abutments of the different lengths employed to piece out the arch-beam; in which respect it differs materially from the other modes of joining timber.

Having shown how each beam is pieced together, we must now show how a number of them are united, so as to compose an arch of any thickness. This is done in the very same way. The beams have other mortises worked out of their inner sides, half out of each half of the beam. The ends of the mortises are formed in the same way with those already described. Long keys BB, CC, (fig. 19.) are made to fit them properly, the notches being placed so as to keep the beams at a proper distance from each other. It is now plain that driving in a long wedge AA will bind all together.

In this manner may an arch be extended to any span, and made of any thickness of arching. The bridge over Portsmouth river, in North America, was more than 250 feet in length, and consisted of several parallel arches of beams. The inventor (we think that his name is Blodget) said that he found the strength so great, that he could with perfect confidence make one of four times the span.

We admire the ingenuity of this construction, and think it very effectual for bringing the timbers into firm and uniform abutment; but we imagine that it requires equilibration, because it is extremely flexible. There is nothing to keep it from bending, by an inequality of load, but the transverse strength of the beams. The keys and wedges can have very little power to prevent this bending. The distance between the beams will also contribute little or nothing to the stiffness; nay, we imagine that a great distance between them will make the frame more flexible. Could the beams be placed so near each other that they could be somehow joggled on each other, the whole would be stiffer; but at present they will bend like the plates of a coachspring. But nothing hinders us from adding diagonal pieces to this construction, which will give it any degree of stiffness, and will enable it to bear any inequality of loading. When completed in this manner, we imagine that it will be at least equal to any construction that has yet been thought of. One advantage it possesses that is very precious: Any piece that fails may be taken out, and replaced by another, without disturbing the rest, and without the smallest rift. On the whole, we think it a very valuable addition to British carpentry. The method here practised, both for joining the parts of one beam and for framing the different beams together, suggests the most firm and light constructions for dome-roofs that can be conceived; incomparably superior to any that have yet been erected. The whole may be framed, without a nail or a spike, into one net-like shell that cannot even be pulled in pieces. We may perhaps consider this in another article; at present we return to the consideration of trussed bridges.

When the width of the river exceeds what is thought practicable by a single truss, we must then combine, combined either by simple addition, or by composition, different by simple trusses together. We compose a bridge by simple addition or dition when we make a frame of carpentry of an unchangeable and proper shape, to serve as one of the archstones of a bridge of masonry. This may easily be comprehended by looking at fig. 22. Each of the frames A, B, C, D, must be considered as a separate body, and all are supported by their mutual abutment. The nature of the thing is not changed, although we suppose that the rails of the frame B, instead of being mortised into an upright b b, unconnected with the frame C, is mortised into the upright c c of that frame, the direction and intensity of the mutual pressures of the two frames are the same in both cases; accordingly this is a very common form of small wooden bridges. It is usual, indeed, to put diagonal battens into each; but we believe that this is more frequently done to please the eye than to produce an unalterable shape of each frame.

To an unskilled carpenter this bridge does not seem essentially different from the centering of Mr Hupeau for the bridge of Orleans; and indeed, in many cases, it requires reflection, and sometimes very minute reflection, to distinguish between a construction which is only an addition of frame to frame till the width be covered, from a construction where one frame works on the adjoining one transversely, pushing it in one part, and drawing it in another. The ready way for an unlettered artist to form a just notion of this point, is to examine whether he may pass through the connecting piece b' from one end to the other, and make them two separate frames. Whenever this cannot be done without that part opening, it is a construction by composition. Some of the beams are on the stretch; and iron straps, extending along both pieces, are necessary for securing the joint. The bridge is no longer a piece of masonry, but a performance of pure carpentry, depending on principles peculiar to that art. Equilibration is necessary in the first construction; but, in the second, any inequality of loading is made ineffectual for hurting the edifice, by means of the stretch that is made to operate on some other piece. We are of opinion, that this most simple employment of the distinguishing principle of carpentry, by which the beams are made to act as ties, will give the most perfect construction of a wide bridge. One polygon alone should contain the whole of the abutments; and one other polygon should consist entirely of ties; and the beams which form the radii, connecting the angles of the two polygons, complete the whole. By confining the attention to these two simple objects, the abutments of the outer polygon, and the joints of the inner one, may be formed in the most simple and efficient manner, without any collateral connections and dependencies, which, divide the attention, increase the complication, and commonly produce unexpected and hurtful strains. It was for this reason that we have so frequently recommended the centering of the bridge of Orleans. Its office will be completely performed by a truss of the form of fig. 23.; where the polygon ABCDEF, consisting of two layers of beams (if one is not sufficient), contains the whole abutments, and the other A b c d e F is nothing but an iron rod. In this construction, the obtuseness of the angles of the lower polygon is rather an advantage. The braces G r, G d, which are wanted for trifling the middle of the outer beams, will effectively secure the angles of the exterior polygon against all risk of change. The reader must perceive that we have now terminated in the construction of the Norman roof. We indeed think it the best general form, when some moderate declivity is not an insuperable objection. When this is the case, we recommend the general plan of the centering of the bridge of Orleans. We would make the bridge (we speak of a great bridge) consist of four trusses; two to serve as the outsides of the bridge, and two inner trusses, separating the carriage-way from the foot-paths. The road should follow the course of the lower polygon, and the main truss should form the rails. It might look strange; but we are here speaking of strength; and evident, but not unwieldy, strength, once it becomes familiar, is the surest source of beauty in all works of this kind.

Centers of Friction, is that point in the base of a body on which it revolves; into which, if the whole surface of the base, and the mass of the body, were collected, and made to revolve about the centre of the base of the given body, the angular velocity destroyed by its friction would be equal to the angular velocity destroyed in the given body by its friction in the same time. See Friction in this Supplement.

Centers of Gyration, is that point in which, if the whole mass be collected, the same angular velocity will be generated in the same time, by a given force acting at any place, as in the body or system itself. This point differs from the centre of oscillation, in as much as in this latter case the motion of the body is produced by the gravity of its own particles; but, in the case of the centre of gyration, the body is put in motion by some other force acting at one place only.

Centers of Oscillation, is that point in the axis or line of suspension of a vibrating body, or system of bodies, in which, if the whole matter or weight be collected, the vibrations will still be performed in the same time, and with the same angular velocity, as before. Hence, in a compound pendulum, its distance from the point of suspension is equal to the length of a simple pendulum whose oscillations are isochronal with those of the compound one.

Centre of Pressure, of a fluid against a plane, is that point against which a force being applied equal and contrary to the whole pressure, it will just sustain it, so that the body pressed on will not incline to either side.—This is the same as the centre of percussion, supposing the axis of motion to be at the intersection of this plane with the surface of the fluid; and the centre of pressure upon a plane parallel to the horizon, or upon any plane where the pressure is uniform, is the same as the centre of gravity of that plane.