ON BURNING GLASSES. II.

here? But their fierce looks did not alarm us, for we knew that we were under the protection of our good Master. We then inquired in the same language, if they spoke French: they replied that they knew no language but Basque. 'Go for Gracieuse,' said the woman to her husband.—' Not only for Gracieuse,' said I, 'but for all the Bohemiaus that you can bring with you.' The husband ran off, but soon came back, accompanied by several men who could hardly recover their breath: they had put themselves to such speed, doubtless supposing that the family by which we were surrounded wanted help. Not one of them entered, but they placed themselves before the door in such a way as to prevent our escape had we attempted it. To us they all seemed to look very decided; but without giving them too much time to think, I asked which of them could speak French. The eldest of the party, a man seemingly about fifty, then passed the threshold, and said in Basque-Not French only, but you may talk to us in Gascon or in Spanish; it matters not, we are ready to reply to anything you may have to say.' 'Well then,' said I, laying my hand on his arm, 'we have very important things to speak about,' and drawing him towards me, I added, 'come in and let us be seated.'

"There was but one chair and a small wooden stool. I took the chair, and pointed to him to take the stool. I then told him the object of our journey; of the wish felt by some of God's servants to do something for the present comfort, and still more for the everlasting welfare of the Basque Bohemians. Think you,' exclaimed the Bohemian with great force, 'that we know nothing about God?'-'You may know that He is a terrible being, but our wish is that you should know Him as a God of love.' I then with much earnestness spoke to them of the good news of salvation, when I was interrupted by one of them who exclaimed-Alas, we are much greater sinners than you are.'-'Yes, that is very true,' said the others; 'we cannot expect forgiveness but on much harder terms than you.' 'Undeceive yourselves, my friends,' said I; if you go to Jesus who is the source of foreness, you, no less than we, will be graciously fece I then asked whether, in case of a schoolmaster coming to instruct both them and their children; to teach them to read the word of God, and to know more of its author, they would give him a welcome reception, intrust their children to him, and go themselves to receive his instructions? The eldest of the party replied-'Assuredly we would; we ask for nothing better; let him but come, and he will be sure of a welcome.' 'I want you all, one by one,' said I, 'to answer;' on which they all exclaimed, 'Yes, Sir, yes.'

PARKER'S BURNING GLASS, NOW IN THE POSSESSION OF
THE EMPEROR OF CHINA.

In our former paper we traced the progress of burning instruments down to the construction, by Buffon, of one consisting of a great number of flat silvered plates of glass.

Buffon next constructed. instruments that would focalize at the distance of a few inches, instead of one or two hundred feet. He took a circular plate of glass, about two feet in diameter, perforated it in the centre, and fixed it into an iron frame. A very fine screw, fixed at the back, passed through the central hole, and worked in a nut on the other side, so that by turning the screw, the flat glass became drawn into the form of a concave mirror. The experiment was then tried, of giving it various degrees of curvature, in order that it might focalize at various distances; but, in the attempt to bend it more than it would bear, it broke; as did also another, three feet in diameter; but a third one, about eighteen inches in diameter, was preserved as a model of Buffon's ingenuity.

The cause of the fracture of these mirrors seems to have been through the perforation made in their centres. Buffon, therefore, proposed to get rid of this perforation in a singular way. He caused a circular glass plate to be fixed at the end of a cylindrical drum, made of iron or copper, and completely airtight. The cavity of the drum being exhausted of air, by means of the air-pump, the glass was pressed in by the weight of the external atmosphere, and became a concave mirror. He afterwards proposed another mode of effecting this, without the aid of the air-pump; which was, to grind a small portion of the glass into the form of a convex lens, and to place a sulphur match behind it; the heat of the sun would kindle the sulphur, if the latter were in the focus of the little lens, and the heat would rarify the air within the drum, and make a partial vacuum. Other modifications of this contrivance have been proposed; but they are not sufficiently practicable to be mentioned

After this, Buffon constructed concave mirrors in a

different way, by bending plates of glass, aided by heat, on moulds of a spherical form, and having them afterwards ground, polished, and silvered on the convex side. But so delicate were all these processes, that twenty-one mirrors were broken out of twentyfour. One of the three successful ones was forty-six inches in diameter, and was regarded as the most powerful burning mirror in Europe.

Mirrors of a peculiar kind were made by Hoesen and Ehrard. They were built up of several pieces of solid wood, made concave on one side. This side was then covered with copper plate, highly polished, and in front of it was an iron arm, so placed as to receive any substance precisely in the focus of the mirror. The curve was a parabola, much more difficult to construct than a spherical curve. This form is so calculated for concentrating the parallel rays of the sun into a point, that although these mirrors were not coated with any more reflecting substance than copper, metals were rapidly perforated, vegetables and bones were immediately burnt to a cinder and vitrified, and the hardest stones resisted but a few seconds.

Newton's attention was directed to the subject of burning glasses, by some remarks of Gregory's, and he procured seven concave glass mirrors, each about a foot in diameter. One of these was placed in the centre, and the other six ranged round them at such angles, that the reflected rays were all focalized to one spot. The focal length was a little less than two feet, and the heat was sufficient to melt gold in half a minute, and to vitrify brick in one second.

Leiher, of St. Petersburgh, made many burning mirrors, the most curious of which was this: he made a concave frame of wood, and covered the concave surface with a paste, made of flour, chalk, &c., and on this paste he fixed a number of pieces of silvered glass, each about half an inch square; thus forming a compound mirror of numerous pieces.

After this, some elaborate calculations were made by M. Peyrard, as to the most effectual mode of combining a number of plane mirrors together, to produce a powerful effect. Where flat pieces are used, it is extremely difficult to arrange them in such a manner that the reflected images shall combine in one focus. Peyrard proposed to employ as many persons as there are mirrors, each person to have the management of a mirror, and, by means of some rather complicated apparatus, to hold or fix it in such a position as to assist in producing the desired effect. His arguments, however, though they might appear good in theory, were not of a nature to be easily put in practice.

We have stated that one distinguished modern writer conceives it to be not improbable that the effects attributed to the burning apparatus of Archimedes, were really produced, though not, perhaps, to the fullest extent. Another modern writer (in the Encyclopædia Britannica) comes to an opposite conclusion, from the results of Buffon's experiments. He says:

A distinguished philosopher, in the end of the eighteenth century, with all the advantages of the amazing progress of science and the arts up to that period, has, after a laborious research, and numerous experiments, with all the leisure of philosophical inquiry, at last succeeded in constructing a combination of mirrors, which inflames combustible materials at a distance, and in a convenient situation. But when we consider the low state of the arts in the time of the Syracusan philosopher, the inferior reflecting power of any mirror then in use, the difficulty and expense of procuring such a number as would be necessary, and of combining them together, so as to act with facility and effect on an enemy's fleet,-seeing that even in Buffon's apparatus, it took half an hour to bring the mirrors

We shall now direct the reader's attention to those burning instruments which act by focalizing light transmitted through glass lenses, instead of reflected from mirrors.

Although it seems probable that the ancients were, to a certain extent, acquainted with burning glasses of such a form, yet the first one of any importance, of which we have an account, was made by Tschirnhausen. This was a convex glass lens, between three and four feet in diameter, with a focal distance of twelve feet, and the diameter of the focal image about an inch and a half. In order to reduce this image, the refracted rays were received on a second lens, behind the larger one, with a focal length of eight inches. The large lens weighed 160 pounds, and produced powerful effects. Wood, even when wet, was burnt instantly; small pieces of metal were easily melted; tile, slate, delft ware, pumice-stone, and talc, grew red, and then vitrified; sulphur, pitch, and resin, were melted, even when under water; the ashes of wood, vegetables, paper, and cloth, were converted into a kind of glass.

Buffon did not confine himself to burning instruments acting on the catoptric, or reflecting principle but he also made some on the dioptric, or transmissive plan. He first constructed a water lens; he combined two circular segments of a glass sphere, so as to form a lens-shaped cavity to be filled with water; these glass segments were first moulded into their proper shapes, and then regularly ground on both sides, so that the convex and concave sides were exactly parallel. The lens thus formed was thirtyseven inches in diameter; but with all the care that Buffon took in the construction, he found that he could not obtain results adequate to the labour bestowed, principally on account of the small refractive power of water.

Buffon proposed to construct a glass burning lens, with a smaller quantity of material than that of Tschirnhausen, although of the same diameter, and of greater power. In a large lens, the thickness at the centre is great, and the absorption of the rays of light is also great. He therefore proposed to form a lens of three concentric circular pieces, or zones, resting upon each other. Supposing the required lens to be twenty-four inches in diameter, this would require a central thickness of three inches, if the lens were one solid piece; but it might be built up piecemeal thus: let the central part be a lens, eight inches in diameter, and one inch thick; let this be inserted in the middle of a circular zone, whose internal and external diameters are respectively eight and sixteen inches; lastly, let this zone be inserted in an outer one, whose diameters are sixteen and twenty-four inches. If the curvatures of all the pieces were ground to the same radius, the solar rays would be refracted to a focus just as completely as if the lens were made in one solid piece, while the weight of material would be less, and the absorption of light less. We do not believe that Buffon actually constructed a lens of this kind; but the suggestion rests on sound principles.

The next lens we have to record was a modification

of Buffon's liquid lens. M. Bernieres constructed, for M. Trudaine de Montigny, a lens, consisting of two spherical segments of glass, eight feet radius, and

three-quarters of an inch thick.

The

When these were placed edge to edge, they enclosed a lens-shaped cavity, four feet in diameter, and six and a half inches thick in the middle. This cavity was filled with 140 pints of spirit of wine; and a lens was thus produced which would melt a farthing in half a minute. focus was, however, very large, and, in order to concentrate it, the rays were received upon a second lens, by which they were brought to such a narrow focus, that pieces of forged iron were fused in fifteen seconds, and hardened steel melted in five minutes. Its effect on platina was curious; some grains of this metal appeared to be drawn together, diminished in bulk, and prepared for fusion; a little afterwards, it bubbled up and smoked; all the grains were united into one mass, but without forming a spherical button, like other melted metals.

The most powerful burning apparatus ever made was constructed by Mr. Parker, a London optician, about the beginning of the present century. It consisted of two convex glass lenses, connected together by the apparatus represented in our cut. There is a conical-shaped frame-work, at the upper and broad end of which is fixed the larger of the two lenses, and at the lower end the smaller lens. The large lens was double convex; when fixed in the frame it exposed a surface of thirty-two and a half inches; it was three and a quarter inches thick in the middle; focal distance, six feet eight inches; diameter of focus, one inch; and weight of the lens, 212 pounds. The smaller lens exposed a clear surface of thirteen inches diameter when in the frame; it was about one and a half inches thick in the middle; focal distance, twentynine inches; diameter of focus three-eighths of an inch; and weight of the lens twenty-one pounds. The combined focal length of the two lenses was five feet three inches; and the diameter of the focus, half an inch. The conical frame-work was intended merely to keep the lenses in their proper relative positions. A rack and pinion was placed beneath the machine, so that it could be elevated or depressed, according to the altitude of the sun. A little platform or stage was fixed behind the smaller lens, and the stage was capable of being so adjusted, that any substance placed on it would be precisely in the focus of the instrument. With this apparatus experiments were made before the Royal Society. Gold, platina, nickel, and cast-iron, were fused in three seconds; a cube of steel, weighing ten grains, melted in twelve seconds; minerals, gems, lavas, and bodies of similar refractory natures, were fused in times varying from sixteen to eighty seconds. A diamond of ten grains, when exposed to the lens for thirty minutes, was reduced, by evaporation, to six grains. Ten cut garnets were taken from a bracelet, and exposed to the lens; in a few seconds, they all fused together into a globular form. It is remarkable, as showing how little we yet know of the connexion between light and heat, that although the lunar rays are so beautifully clear and bright, not the slightest effect could be produced on a very delicate thermometer, when the lunar rays were concentrated on it by Parker's lens. A similar experiment was tried on the lunar rays, by Buffon and Tschirnhausen, but with a similar want of success; though millions of lunar rays were congregated in one spot, no sensible heat could be perceived,

Sir David Brewster, thirty years ago, proposed many improvements in the constructing of burning apparatus, chiefly by combining transmission and reflexion to produce joint effects; and there is no doubt that, in an optical point of view, these suggestions were worthy of the distinguished author. But

so much has been done of late years, in the production of an intense heat by chemical means, especially by the oxy-hydrogen blowpipe; that it is not very probable that burning glasses, acting by solar rays, will occupy so much attention hereafter, as they have hitherto. The relation between a burning lens and the oxy-hydrogen blowpipe, is somewhat analogous to that between a sun-dial and a chronometer: the lens and the sun-dial can only be used while the sun is shining: the blowpipe and the chronometer are available in spite of weather, or of temperature, or of the darkness of night.

THE SMOKE-JACK,

"I give you joy of the report
That he's to have a place at court.
Yes! and a place he will grow rich in-
A turnspit in the royal kitchen."

A WORK of considerable learning and extent might be written on the application of self-acting machinery to the performance of processes which at one time were executed by the human hand, or by animal power. It is not many years ago that the turnspit was to be met with in every kitchen, almost as com.. monly as the cook; but now the smoke-jack performs, in a far more effectual and less expensive manner, the office which a little boy or a poor little dog once held. Indeed, one of the strongest incentives to the adoption of automatic machinery is the superiority and regularity of the work performed by it, compared with the productions of human or animal machines, that have a will of their own, not always controllable by a master's voice.

It appears that boys were employed to turn the spit, both before and after dogs were trained to that office. Du Cange speaks of a boy-turnspit so early as 1201. We also meet with an anecdote of Louis the Eleventh, (about 1460) to the following effect, A whim of the monarch led him to visit the royal kitchen, to see what was going forward. He found a little boy turning the spit. The boy was handsome, and his appearance so engaging, that the king thought him deserving of some better office than the one he filled. Louis accosted him, inquired whence he came, who he was, and what he earned by his employment. The turnspit, not knowing the king, replied to his queries, without the least embarrassment, "I am from Berny, my name is Stephen, and I earn as much as the king." "What then does the king earn?" asked Louis. "His expenses," replied Stephen, "and I mine." The king, pleased with the smartness of the reply, promoted the turnspit to the situation of groom of the chamber.

The employment of dogs as turnspits has given a distinctive name to a peculiar class of those animals. The turnspit is described as a spirited, active, and industrious kind of dog, once considered as an indispensable attendant on the spit, which, by a peculiar contrivance, and the aid of its own exertions, it was enabled to turn at an even pace. The body of this animal is long, the legs very short, and the tail curled on the back; its usual colour is grayish, with black spots. Gmelin notices three varieties of this family of dogs; one of which has straight feet, another curved feet, and the third has the body covered with long curly hair. The terrier is the Gmelinian Canis vertagus.

The method of teaching these dogs their duty was summary rather than humane. The dog was put into a hollow wheel, or drum, and a few live coals with him; thus situated, the poor animal could not pause without burning his legs; he therefore kept up a

6. SARACENIC ARCHITECTURE. WHEN Constantine, in A.D. 328, founded the city of Constantinople, he enriched it with the treasures of Rome: all that could be removed was taken from the "imperial city," to adorn his new capital. At a subsequent period, when the successor of Constantine removed from the city every valuable specimen of art, (which Alaric had spared in the sacking of Rome,) he loaded several ships therewith, which were driven by a tempest upon the coast of Sicily; the commander was killed, and the Saracens, then in possession of the country, carried their spoils to Alexandria.

The Arabian, Saracenic, or Moorish architecture is another illustration of the remark already made of the influence of religion upon architecture. The religion of Mohammed was diffused from Indus to the Nile, by Arabs, whose fanatical zeal led them to put to the sword all who refused to own the doctrines of the Koran. The Roman power had declined, and the eastern countries, once subject to it, were so enfeebled by luxury or misrule, as to fall easily under the dominion of the arch impostor and his immediate successors. In a very few years Syria, Persia, and Egypt were VOL. XVI.

totally subdued; more than four thousand Christian churches were destroyed, and rude shrines, called maschiads, afterwards mosques, were substituted. But as the religion of Mohammed became more securely settled, its followers acquired a love of luxury and splendour, accompanied by a taste for learning and the arts of life. The literature of the Greeks was translated into Arabic, and schools were established for the study of science, especially mathematics, in which the Arabians greatly excelled.

Thus, while Europe was involved in darkness,-while the rude tribes of the north subjected the fairer portion of the Roman empire to devastation and ruin,-the Arabians were advancing in civilization, and obtaining that efficiency in the arts of peace and war, which made them so long celebrated.

The earlier style of the Saracenic architecture bears some resemblance to that which prevailed in the Byzantine empire. It is probable that the cupolas of their earlier buildings were suggested by those of Sancta Sophia, and similar structures; and the columns which they, employed, if not actually taken from existing Roman structures, were rude imitations of them. But as they advanced in science and

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