a mountain chain in the S.W. of Europe, separating France from Spain, and extending from Cape Creux on the Mediterranean, westwards to the neighbourhood of Fuenterrabia on the Bay of Biscay. It lies between N. Lat. 42° 26' and 43° 23', E. Long. 3° 10' and W. O. 48'. Its length is about 270 miles; and in the centre, where its width is greatest, it has a breadth of 60 miles. It does not consist of a single chain of heights, but of two parallel ranges about 20 miles distant, connected near the centre by a transverse ridge. It is near the middle of the range that the highest elevations occur; from this point it gradually slopes downwards to either extremity. From the central ridge numerous spurs project, both to the north and to the south; between which lie the principal valleys of the Pyrenees. These valleys do not extend, like those of the Alps, in the direction of the principal chain, but at right angles with it; and they terminate in what are called necks (cofs) or gates (portes), where there are frequently passes over the mountains. The length of the valleys varies from 10 to 40 miles, and many of them terminate in vast circular basins called cirques or oules, surrounded on three sides by steep precipices. The slope of the mountains on the side of France is much more gradual than on the other side. On the former side there are gentle declivities and terraces, which lead down to smooth and verdant meadows; while on the Spanish side the scenery is wild and rugged, and the precipices steep.

The principal summits and ridges of the Pyrenees are the following, beginning from the Mediterranean:

| Height in feet | Height in feet | |---------------|---------------| | Le Camigon | 9,651 | Port Viel d'Estaube | 8,373 | | Pic Pedroux | 9,511 | Port de Pinede | 8,122 | | Col de Puymore | 6,240 | Mont Perdu | 10,591 | | Pic du Port de Siguiel | 9,523 | Le Cylindre | 10,798 | | Montcalm | 10,663 | Port de Gavarsle | 7,882 | | Nestos | 11,063 | Passage de Tourmalet | 7,073 | | Maladetta | 10,764 | Vignemale | 10,718 | | Port d'Ox | 9,756 | Pic du Midi | 9,350 |

The chain of the Pyrenees has very much the appearance of a huge wall between the two countries which it separates, as the ridge has almost everywhere a height little less than that of the lofty summits. The passes of the mountains are at a much greater height than many of those across the Alps, and they are in consequence much less accessible. Although there are as many as seventy-eighty passes in the whole range, many of them are both difficult and dangerous, and there are but few practicable for carriages, such as that of the Bidassoa near the Bay of Biscay, and that of the Col de Pertus along the shore of the Mediterranean. The snow-line of the Pyrenees has an elevation of about 9000 feet on the northern slope, and 8000 on the southern; being considerably below that of the Alps. There are numerous glaciers in these mountains, but they are in general of small size and widely removed from each other. They only occupy the higher slopes of the mountains, and not like those of the Alps the deep glens and valleys. The most important of the glaciers are those of Maladetta, Cabridou, Mont Perdu, Vignemale, and Nouvielle. The most of the glaciers, as well as of the lakes, which are generally of a small size, lie on the French side of the mountains. Numerous streams take their rise in the Pyrenees, but the most of them are small and insignificant. Each of the valleys is traversed by a brook, called in French Gore, in Spanish Gala, and these again unite to form larger rivers. Those on the north side form the Adour, the Ariège, and the Garonne, flowing into the Bay of Biscay; and the Aude, Gly, Tet, and Tech, into the Mediterranean. Those on the Spanish side, with the exception of the Bidassoa, which falls into the Atlantic, and a few small streams into the Mediterranean join the Ebre on its left bank. There are many mineral springs, both cold and hot, especially on the French side of the mountains. The principal of these, which are much frequented by visitors, are Bagnères de Luchon, Bagnères de Bigorre, Barèges, Cauterets, St Sauveur, Eaux Bonnes, and Eaux Chaudes. In geological formation, the nucleus of the mountains is granitic, and the highest summits of the chain are of this nature. Micaceous schist, limestone, sandstone, oolite, and calcareous strata also occur on the lower slopes; and trap, basalt, porphyry, &c., are scattered about in different places. Iron, copper, zinc, and lead are among the mineral riches of the Pyrenees; but iron is the only one of these that has been profitably worked. The climate of the mountains varies considerably in different parts; towards either extremity the lower elevation of the range, and the vicinity of the sea, renders it more mild than it is in the centre. Vegetation ascends to a higher altitude here than in the Alps; and the immense forests which cover the sides and sometimes the tops of the mountains, not only of firs, but of oaks and beeches, form one of the most characteristic and beautiful features of the Pyrenees. The timber has, however, suffered much from the carelessness and waste of the people of the country. These vast forests are filled with wild animals of many different kinds, the bear, the boar, the wolf, the lynx, and the fox, being among the number. The izard, an animal like the chamois, but of smaller size, and the wild goat, form a great attraction for the sportman. The rivers abound in trout, and those that flow into the Atlantic in salmon. The inhabitants of the Pyrenees include several races that are remarkable for their antiquity or peculiarities of dress and manners. At the western extremity, partly in France and partly in Spain, dwell the Basques, the descendants of the ancient Cantabrians, who so stubbornly resisted the Roman arms, and have since kept their position in these mountain fastnesses against all invaders. Further east, on the French side, are the simple and primitive people of Béarn, from among whom Henri IV. sprung; and in the eastern part of the mountains the inhabitants of the French side have much resemblance to the Catalans of Spain. The people on the Spanish side of the Pyrenees are a bold hardy race, living by smuggling and the chase, with an invincible hatred to the French, and forming excellent guerrilla soldiers, as has often been proved. The Pyrenees have been the scene of several important historical events. Hannibal crossed them by the Col de Pertus, just before his more celebrated Pyrenees, passage of the Alps; and Caesar afterwards did so at the same place. In 778, Charlemagne, advancing into Spain, crossed by the Pass of Roncesvalles, where he suffered a defeat and lost many of his peers at the hands of the Basque mountaineers. The same pass was again surmounted by an English army under the Black Prince, invading Navarre; and in 1813, after the victory of Vittoria, the British army under Wellington drove the French across the Pyrenees into their own country. It was on this occasion that the battle of Roncesvalles, the assault and capture of St. Sebastian, the passage of the Bidassoa and of the Nivelle, and other actions, took place.

Pyrenees, Basses, a department of France, in the south-western corner of the country, lying between N. Lat. 42° 47' and 43° 35', E. Long. 0° 2' and W. 1° 45'; bounded on the N. by the department of Landes, E. by those of Gers and Hautes-Pyrénées, S. and S.W. by Spain, and N.W. by the Bay of Biscay; length, from E. to W., 88 miles; greatest breadth, 44; area, 2900 square miles. It lies in the lower slopes and at the foot of the Pyrenees, from which it derives its name; and a part of its surface is occupied by the offsets and valleys which extend northwards from these mountains. The department is watered by the streams which rise in the Pyrenees, and flow down these valleys; most of them, except the Nivelle, and the Bidassoa which rises in Spain, and forms the frontier between the two countries, flow into the Adour. This river, which only washes the department at its N.E. corner, and again at the N.W. near its mouth, receives, among other affluents from Basses-Pyrénées, the Gave de Pau, the Gave d'Oloron, and the Nive. Of these rivers, the Adour, Nive, and Nivelle, are navigable for some distance above their mouths; but all are employed for floating down timber and other articles almost from their very sources. Though the soil is in general not very remarkable for fertility, the lower valleys are richly productive, and the loftier slopes and mountains, besides their vast and valuable forests, afford excellent pasturage. The hillsides are covered with excellent vineyards, and with plantations of fruit trees. There are, however, some tracts of barren or marshy ground, especially in the N.W. and near the Adour. The climate of the lower regions is temperate and healthy, though very variable; but in the more elevated parts the goutte is not unfrequent. The department is calculated to contain 386,072 acres of arable land, 321,671 acres of wood, 57,266 of vineyards, 163,720 of meadows, and 841,995 of heaths and waste land. Agriculture is in a very backward state, and the produce of corn is not at all adequate to supply the wants of the people. Besides maize, which forms a principal article of food here, wheat and flax are the chief crops raised; while rye, barley, and millet are also grown. The quantity of wine produced in the department is upwards of 6,500,000 gallons annually. Horses, mules, and cattle are raised here in considerable numbers, as well as sheep and pigs; the latter of which supply the much-esteemed hams of Pau and Bayonne. The mineral riches of the country are very great, including iron, salt, marble, alabaster, slate, limestone, and potter's clay, which are worked to some extent. Manufactures have made considerable progress in the country, especially those of linen and woollen stuffs; as well as leather, paper, pottery, hardware, chocolate, and brandy. Ship-building is carried on along the coast, the timber of the forests being excellent for that purpose. The commerce is very active; horses, cattle, hams, hides, wool, wines, brandy, timber, and other produce of the country are exported; while colonial wares, whale and seal oil, &c., are imported. A great deal of trade, chiefly contraband, is carried on with Spain. The department forms the diocese of Bayonne; and besides the places of worship of the established church, contains a Calvinistic church at Orthez, and several Jewish synagogues. There is at Pau a court Pyrénées, of appeal for the three departments of Landes, Hautes, and Basses-Pyrénées; five tribunals of the first instance, and two of commerce, being the other courts of justice in the department. For educational purposes there are a normal school, a college, a lycéeum, six superior communal schools, and 914 elementary schools. The capital is Pau, and there are five arrondissements as follows:

| Canton | Communes | Pop. (1866) | |--------|----------|------------| | Pau | 11 | 185 | 127,771 | | Oloron | 8 | 80 | 73,675 | | Orthez | 7 | 135 | 78,929 | | Bayonne| 8 | 52 | 86,996 | | Mauléon| 6 | 108 | 69,071 | | Total | | 560 | 436,442 |

Pyrenees, Hautes, a department of France, lying between N. Lat. 42° 39' and 43° 34', E. Long. 0° 30' and W. 0° 20'; bounded on the W. by the department of Basses-Pyrénées, N. by that of Gers, E. by that of Haute-Garonne, and S. by Spain; length, from N. to S., 48 miles; greatest breadth, 45; area, 1790 square miles. The surface is very mountainous, being almost entirely occupied with the Pyrenees and their branches. In many of the glens and valleys the scenery is of the most sublime and beautiful character. The country slopes gradually towards the north; and a chain of hills running in the same direction separates between the valley of the Adour on the W. and of the Garonne on the E. These rivers receive all the numerous brooks which flow down from the mountains; the chief affluents of Adour being the Gave de Pau, which waters the south-west of the department, and the Arros, which flows northward, and joins it from the E. The Garonne, rising in the valley of Arau, which belongs to Spain, receives from this department the Neste in the south-east, the Gers, and the Baise, which water its north-eastern portion. The principal plain in the Hautes-Pyrénées is that of Bigorre, which lies between two branches of the Pyrenees, and slopes gradually towards the north. With the exception of this plain, there is very little good soil, from the rugged and mountainous character of the country. The pasture grounds, however, are good, and the forests valuable. The extent of arable land is estimated at 235,000 acres; of meadow land, 123,000 acres; of vineyards, 37,000 acres; of wood, 200,000 acres; of waste land, not less than 396,000 acres. The principal crops are maize and wheat, but the quantity produced is insufficient for domestic consumption; of the wine grown, however, there is a surplus for exportation. The climate varies with the varying elevation of the country; but it is on the whole salubrious. On the plain of Bigorre it is mild; but in the higher regions changeable and inclement weather prevails. Besides agriculture, the peasantry are actively employed in pastoral pursuits. Large numbers of horses, which are much valued for cavalry, as well as horned cattle, mules for export into Spain, sheep and pigs, are raised here. Much attention is also paid to poultry, especially geese, and to bees. The mineral wealth of the department is great, including iron, copper, zinc, lead, &c.; but these are only worked to a small extent. Granite, marble, chalk, limestone, and slate, are quarried. Manufacturing industry is not very active here. It consists chiefly in the working of iron, and producing woollen and cotton fabrics, the stuffs called barèges (from the town of that name), paper, and leather. There is a considerable trade in timber for ship-building, cattle, salt provisions, cheese, &c. Hautes-Pyrénées forms the diocese of Tarbes, and contains three courts of the first instance, 4 colleges, 4 upper schools, and 768 elementary schools. The capital is Tarbes; and there are three arrondissements as follows: Pyrometer (from πῦρ, fire, and μέτρον, a measure), an instrument for estimating high degrees of temperature, such as the heat of furnaces and the fusing-points of many of the metals. The scale of a common thermometer is graduated on the supposition that equal increments of heat produce equal amounts of expansion; but this is only true within moderate ranges of temperature; for as we increase the heat, most fluids leap forward, as it were, to meet their boiling points, and cease to be trustworthy as measurers of temperature. Thus, according to Regnault, the total expansion of mercury for three progressive intervals of 180° Fahr., is as follows:—Between 32° and 212° it is 1 part in 55.08 parts; between 212° and 392° it is 1 in 54.61; and between 392° and 572° it is 1 in 54.01. Thus, at various times, instruments have been contrived for measuring temperatures above the boiling-point of mercury (662° Fahr.), the term pyrometer having been first used by Muschenbroek about the year 1730. His instrument consisted of a metallic bar, about six inches long, one end of which was fixed, while the other was free to move as the metal increased in length, from the effect of a number of spirit-lamps placed beneath it, and charged with a known quantity of highly-rectified spirits of wine. As the bar increased in length, it moved a pinion and wheel, and the latter moved an index over a graduated circle, each degree of which corresponded to a linear expansion of \( \frac{1}{12} \) inches of an inch. The instrument was improved by Desaguliers, who substituted fine cords and friction rollers for the wheel and pinion.

Pyrometers were also contrived by Ellicott, Graham, Smeaton, Ferguson, and others. These do not differ from Muschenbroek's instrument, in which the minute expansion of a bar of metal is multiplied by means of a succession of levers, or a system of wheels and pulleys, a method which must be very liable to error, in consequence of the bending of the parts, obliquity of action, and other causes. Besides this, the substance itself, if liable to be softened by heat, would undergo compression in moving the machinery, an objection which, to a certain extent, applies to the more refined apparatus employed by Lavoisier and Laplace, in which the expansion of the metal bar deflected a telescope from the position that it had at the beginning of the experiment, and the absolute expansion was deduced from the extent of this deflection, which was read off upon a graduated scale placed at a considerable distance in front of the telescope. In 1794 Troughton contrived an apparatus somewhat similar; only, instead of a telescope, he used a spirit-level, the deviations of which from the horizontal determined the expansion of the metal.

Pouillet's method may also be referred to for measuring directly the linear expansion of solids, and it has the advantage of being applicable to very high temperatures. It consists of a solid plate of metal, on which is placed a radius turning on a centre, and traversing a graduated arc, the divisions of which are read off by a microscope. The radius carries a telescope of short focus, fixed at right angles to its direction, while a similar telescope is fixed to the Pyrometer plate itself, allowing the radius to traverse under it. The bar which is the subject of experiment is placed in a copper trough furnished with parallel plates of glass, through which its ends can be seen. Now, if one extremity of the bar be kept opposite the fixed telescope, while the moveable telescope is directed to the other extremity at the commencement of the observation, any expansion of the bar caused by raising its temperature may be estimated by the arc through which the radius must be turned, in order to bring the moveable telescope to bear on the other extremity in its new position, the distance of the radius from the bar being accurately known. For very high temperatures, the bar may be placed in a furnace, and when raised to the required temperature, apertures may be opened in the furnace walls, so as to give a view of the ends of the bar, and allow its expansion to be measured as before. By means of this apparatus an expansion in the bar of \( \frac{1}{1000} \)th of a millimetre, or about \( \frac{1}{3285} \)th of an inch, can be appreciated.

In the measurement of the base-line for the great French survey, Borda used rods consisting of a rule of brass placed upon a somewhat longer rule of platinum, and attached at one extremity. The portion of the platinum rule not covered by the brass one was divided into millionths of the entire length of the rule, and further subdivided by means of a vernier and microscope adjusted to the extremity of the brass rule. The value of each of these divisions was first ascertained by surrounding the compound rule with melting ice, and afterwards with boiling water, when it was only required to observe the indications of the vernier to apply the requisite correction for reducing the length of the rod to the standard temperature. Ramsden's contrivance, used by General Roy in determining the expansion of the rods employed in measuring the base-line on Hounslow Heath was as follows:—The rod was immersed in water, and over each extremity was placed a microscope, to which a slow motion could be given in the direction of the length of the bar by means of a micrometer-screw. The lines of collimation of the microscopes having been adjusted so as to coincide with two points near the ends of the rod, the temperature of the water was gradually raised any required number of degrees, as indicated by a thermometer, when the elongation of the rod destroyed the coincidence of its extremities with the lines of collimation of the microscopes, which was re-established by turning the micrometer-screws, and noting the number of turns and fractions of a turn required for the purpose. In this way a direct measure of the expansion was obtained, free from the errors of levers, wheels, and pinions.

In the measurement of the base-line of Loch Foyle for the ordnance survey of Ireland, General Colby employed a compound bar of iron and brass, so arranged that their different powers of expansion and contraction should preserve exactly the same distance between two points at the extremities of the bars, instead of the usual method of allowing for the change in length according to the temperature at which each rod was laid. The two bars, one of iron, the other of brass, each ten feet long, were placed parallel to each other, and riveted together at their centres, and coated with a non-conducting substance, to equalize the susceptibility of the two metals to change of temperature. It was ascertained, by numerous experiments, that the iron and the brass bars expanded and contracted, in their transitions from cold to heat and from heat to cold, in the proportion of 3 to 5. Across each extremity, therefore, of these combined bars was fixed a tongue of iron, with a minute dot of platinum, so situated that under every degree of expansion and contraction of the rods the dots at each end always remained at the constant distance of 10 feet.

In the Philosophical Transactions for 1782, 1784, and 1786, is a description of Wedgwood's pyrometer. This Pyrometer was based upon the property possessed by clay to contract and harden by exposure to a high temperature, the clay, under such circumstances, losing a portion of its combined water, exactly proportioned, as it was thought, to the intensity of the heat. The clay was punched out into small cylinders, with one side flattened, and having been exposed to the temperature which it was desired to measure, the amount of the contraction was determined by sliding the cylinders along a metallic groove or gauge, the sides of which gradually converged until they arrived at a point beyond which they would not descend. The gauge was divided into 240 parts or degrees, each of which was calculated to be equal to 130° Fahr., while the zero of the scale, indicating a red heat, corresponded, according to Wedgwood's experiments, to 1077°. The difficulty of obtaining clay of uniform composition is one objection to this method of measuring high temperatures; different kinds of clay afford very different results, as does also the same kind of clay prepared with a little more or less of mechanical force; but the most fatal objection is, that clay will contract as much by the long continuance of a comparatively low heat as by the short continuance of a high one. This accounts for the enormous exaggeration of Wedgwood's degrees, and the disuse into which the instrument has fallen.

Achard contrived a pyrometer in the form of the common thermometer, consisting of a bulb and graduated tube of semi-transparent porcelain highly baked, and containing a fusible alloy of bismuth, lead, and tin, which liquefied at about 212°, and by its expansion noted higher temperatures, the semi-transparent material allowing the rise of the metal to be seen.

The method contrived by Messrs Dulong and Petit for measuring the absolute expansions of different substances deserves notice. By observing the difference in height at which mercury stood in the two limbs of a U tube, when of different temperatures, they obtained the absolute expansion of the mercury, and by comparing this with the apparent expansion of mercury in a glass tube, they deduced the absolute expansion of the glass. When the expansion of a metal was to be estimated, a cylinder of it was put into a glass tube, closed at one end, and drawn to a capillary opening at the other, while the rest of the tube was filled with mercury. On heating the tube, a portion of the mercury was expelled, equal to the excess of the absolute expansions of the mercury and the metal above that of the glass; and as the expansions of the mercury and of the glass had already been determined, the weight of the mercury expelled determined the expansion of the metal.

In 1803 M. Guyton de Morveau submitted to the National Institute a pyrometer, consisting of a bar of platinum, about 2 inches long, placed in a porcelain groove, one end resting against the solid end of the groove, while the other, pressed upon the short arm of a lever, the longer arm of which moved a vernier over a graduated circular arc. The indications of the vernier at the beginning and end of an experiment furnished the means for determining an expansion. This instrument, however, was not applied to the determining of higher temperatures than that of the melting point of antimony, in consequence of the softening which platinum undergoes at a red heat.

In the Philosophical Transactions for 1830 is a description of Professor Daniell's pyrometer, which deserves special notice, as being the first instrument constructed for determining high temperatures with anything like accuracy. It consists of two parts, the register and the scale; the register is a solid bar of black-lead earthenware highly baked; in this is drilled a hole, into which a bar of any metal 6 inches long may be dropped, so as to rest upon its solid end. A cylindrical piece of porcelain, called the Pyrometer index is next placed on the top of the bar, and confined in its place by a ring or strap of platinum passing round the top of the register, which is partially cut away at the top for the reception of a wedge of porcelain. When this arrangement is exposed to a high temperature, the expansion of the metallic bar forces the index forwards to the amount of the excess of its expansion over that of the black lead, and on cooling, the index will be left at the point of greatest elongation. It is the function of the scale to measure the distance which the index has been thrust forward from its first position. The scale, which is independent of the register, consists of two rules of brass joined together at a right angle by their edges, and fitting square upon two sides of the black-lead bar. At one end of this double rule projects at a right angle a small plate of brass, which may be brought down upon the shoulder of the register, which is formed by the notch cut away for the reception of the index. Attached to this frame is a moveable arm turning near one end upon a centre, and carrying at its other end an arc of a circle, the radius of which is 5 inches, and divided into degrees and thirds of a degree. Upon this arc, at the centre of the circle, turns another lighter arm, to the further end of which is attached a vernier moving upon the face of the arc, and subdividing it into minutes of a degree; the other end passes beyond the centre, and terminates in a steel point turned inwards at a right angle.

The various parts of the instrument are represented in figs. 1 and 2. Fig. 1 represents the register, in which A


is the bar of black lead, aa' the cavity for the reception of the metal bar, cc' is the porcelain index, d the platinum band, with its wedge e. Fig. 2 represents the scale by which the expansion is measured, ff' is the greater rule upon which the smaller one g is fixed square. The projecting arm h is also fitted square to the ledge under the platinum band d. D is the arm which carries the graduated arc attached to the rule ff', moving on the centre i. C is the lighter bar moving on the centre k; H is the vernier, m the steel point. The rule g can be adjusted upon ff', so that the arm h may be adjusted to the centre i, in order that at the commencement of an experiment the vernier may rest at the beginning of the scale.

When an observation is to be made, the metallic bar is placed in the cavity of the register at the ordinary temperature; the porcelain index is pressed down upon it, and firmly fixed in its place by means of the platinum strap and the porcelain wedge. The scale is then applied by carefully adjusting the brass rule to the sides of the register, and fixing it by pressing the cross piece upon the shoulder, and placing the moveable arm so that the steel point of the radius may drop into a small cavity made for its reception, and coinciding with the axis of the metallic bar. The minute of the degree which the vernier indicates upon the arc must be noted; the scale is then removed, and the register exposed to the temperature which is to be measured. After it has been withdrawn and allowed to cool, the porcelain index retains the position corresponding to the maximum temperature of the rod; the scale being now again applied to the register in exactly the same position as before, the arc through which the radius must be moved to bring the point in the arm of the lever to bear against the index, measures the quantity by which the latter has been protruded. This quantity is the excess of the expansion of the metal rod over the black-lead envelope, or rather that excess diminished by the contraction of the index in cooling. The scale of this pyrometer is connected with that of the thermometer by observing the amount of expansion between two fixed points, such as the freezing of water and the boiling of mercury. The amount of expansion for a known number of degrees is thus determined, and the value of all other expansions may be considered as proportional. By means of this instrument, the melting-point of cast-iron has been ascertained to be 2786°, and the highest temperature of a good wind-furnace about 3300°, points which were estimated by Wedgwood at 20,577° and 32,277° respectively.

M. Breguet has a pyrometer consisting of a compound ribbon of three metals, platinum, gold, and silver, rolled out into a very thin lamina, and coiled into a cylindrical spiral, to the lower extremity of which is attached an index, while the upper end of the spiral is fixed. The silver expands much more than the platinum, so that the coil twists and untwists as the temperature rises and falls. The degrees on this instrument are marked upon a horizontal circle, and their value is found by comparison with a standard thermometer.

In the Philosophical Transactions for 1828, Mr Prinsep describes a method of measuring high temperatures based upon the fixed value of the fusing-points of pure metals. The noble metals alone embrace a range from the low melting-point of silver to the very high one of platinum, and although there may be only three fixed points in the scale thus furnished, intermediate links may be supplied by alloying the three noble metals together in different proportions. When such a series has been prepared, the heat of a furnace may be expressed by the alloy of least fusibility which it is capable of melting. As the melting-points of silver and of gold are comparatively near to each other, ten intermediate gradations were assumed, the lowest of which corresponded to the fusing-point of pure silver, and the others to the fusing-points of silver alloyed with 10, 20, 30, &c., per cent. of gold. From the melting-point of gold to that of platinum 100 gradations were assumed, which were the melting-points of pure gold, and of gold alloyed with 1, 2, 3, &c., per cent. of platinum. The only apparatus required for this method is a small cupel, containing in separate cells 8 or 10 pyrometric alloys, each about as large as a pin's head, and when the specimens had been once used, they could be used again by simply flattening them under a hammer. The notation is equally simple, since two letters and the decimal of alloy would express the maximum heat; thus S . 3 G gives the temperature of the fusing-point of silver when alloyed with gold in the proportion of 7 to 3, and G . 23 P expresses the fusing point of gold when alloyed with platinum in the

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1 An able article on pyrometers contained in the Penny Cyclopaedia has a mathematical investigation of the correct formula to be used with Professor Daniell's instrument, and shows that the formula employed by him, "though probably sufficiently correct for all practical purposes, gives the expansions one per cent. too great without exception, and in many cases much more... The error thus introduced is perhaps within the limits of the error to which the instrument itself is liable." Pyrometer proportion of 77 to 23. Mr Prinsep endeavoured to connect the fusing-points of his alloys with the thermometric scale by employing the expansion of air on the principle of the differential thermometer, assuming that the increase of temperature is proportional to the expansion of the air, which, however, is not the case.

A trial of Mr Prinsep's method was made some years ago at Sèvres for the purpose of determining the temperatures of the ovens used for baking porcelain. It was found necessary to expel silver from the short list of available metals, on account of the peculiar property possessed by this metal of absorbing oxygen during its fusion, and spitting it out again during the cooling. The small tubes or globules of the melted metal, which are forcibly expelled by the escaping oxygen, are more fusible than the principal alloy, and disturb its fusing-point. The presence of 1 or 2 per cent. of copper might have neutralized this property if it did not interfere with the other results. The experiments were therefore limited to the alloys of platinum and gold in various proportions. It was found, however, to be very difficult to determine in a close furnace the exact moment when the alloys fused; moreover, it was found that similar alloys did not always melt at the same moment, although apparently placed under precisely the same circumstances. It was also found impossible to use the same bead of alloy more than once, the action of the fire effecting some molecular change which led to an alteration in the fusing-point; still, however, M. Laurent (whose results were subsequently confirmed by M. Salvétat) was able to determine that the highest temperature attained in the kiln at Sèvres was represented by the alloy G. 47, 100, when a well-fused button was formed, while, with 54 parts platinum, there was only a softening perceived. M. Brongniart remarks on this result—“When we succeed in being able to determine exactly and promptly the moment when complete fusion takes place in these alloys, we shall have comparable measures of high temperatures, but not a method fit to be employed habitually in industrial operations, in which it is not required to know the temperature in one single spot, or at the end of an operation, but in various places and at various times in order to manage the fire as equably as possible, and to arrest it at the proper moment.”

A good pyrometer is still a desideratum in the useful arts for regulating the heat of furnaces, &c. It should be easy of application in the hands of common workmen, prompt in its results, exact in its indications, so as not only to serve as a guide to the particular process in hand, but to give information to manufacturers in all places and at all times of the temperature required for the successful conducting of a particular process. The unalterable nature of platinum would seem to point out this metal as a desirable pyrometer, especially since it is more equable in its expansion than any of the other metals, but is not free from that increasing rate of expansion with increasing temperatures which belongs to most substances. The small amount of dilatation of platinum is an objection to its use, especially when that dilatation has to be communicated to a distance through rods of various kinds which act upon the measuring apparatus, all of which are liable to error. Herr Wurm has endeavoured to employ platinum in the form of a moderately strong wire stretched across a strong massive frame of iron, furnished with a handle; the wire is kept in a state of tension by a spring or otherwise, and when it becomes relaxed by expansion, an index in the handle connected with it shifts and marks the amount of extension. When it is required to ascertain the temperature of a furnace or oven, the whole frame is introduced into it, the wire instantly acquires the temperature of the furnace, but the massive frame does so slowly; relative expansion of the wire, therefore, takes place, and when this has attained its maximum, the index is read off.

As long since as the year 1805 M. Brongniart constructed a pyrometer for measuring the comparatively moderate heat of the glass oven or muffle furnace (see Pottery and Porcelain) by means of the dilatation of a bar of silver. The advantages of this metal are stated to be its resistance to the highest temperature of the muffle furnace, the facility with which it may be obtained pure, so that the results of two instruments will be comparable; it resists an incandescent heat, and is more dilatable than any other metal possessing all these advantages. The only inconvenience accompanying its use is the impossibility of applying it to measure higher temperatures. The bar of silver is two decimetres in length; it is placed in the muffle in the midst of the articles which are being fired, and is inserted through one of the sight-holes used for watching the progress of the vitrification; its dilatation serves to note the rise of temperature and the point at which the heat ought to be arrested. In order to measure this dilatation, the bar is placed in a groove of hard porcelain, one end of which is turned up to serve as a fulcrum to the inserted end of the bar. The near end of the bar presses against a porcelain rod which gives motion to a needle over a graduated arc, so graduated as to multiply a hundred-fold the dilatation of the bar of silver. This arc is divided into 300 parts; from 27° to 30° of this arc are equivalent to 100° of the centigrade thermometer. The firing of the colours upon porcelain varies from 200° to 280°C. Silver fuses between 300° and 325° of this pyrometer, so that the maximum heat for vitrifiable colours is not far short of the fusing-point of silver. M. Brongniart modestly remarks that this instrument is imperfect, since it does not register absolute measures of temperature, but only the difference between the dilatation of a bar of silver two decimetres long, and that of a bar of hard porcelain of the same length. The dilatation of the porcelain at this temperature is not known, except that it is very small; we have therefore only the difference between two quantities, of which one only has been ascertained.

In the year 1835 Mr Adie of Edinburgh read a paper before the Royal Society of Edinburgh (Trans. xiii.) on the expansion of different kinds of stone. His pyrometer consisted of a vertical metal cylinder about 2 inches in diameter, and about 27 or 28 inches long, for containing the stone rod which was to be measured. The cylinder was surrounded by a steam jacket, through which was passed a current of steam for the purpose of raising the temperature of the cylinder and the rod. The case contained windows of plate glass, through which both ends of the rod could be seen; the lower end rested on a support which could be adjusted in height by means of a screw passing through the bottom of the cylinder, and steadied against the sides by means of springs and friction-rollers. Two silver studs were fixed in each rod that was to be operated on, at the exact distance of 23 inches, at the ordinary temperature. The cylinder and case were attached to a vertical oaken beam, to which were fixed two microscopes, with their axes horizontally directed to the studs. The lower microscope kept the lower stud stationary in view, while the upper one measured the expansion of the bar by means of a micrometer. The oaken beam was screened from the radiation of the heat of the steam jacket, and the current of steam was maintained therein until the rod ceased to increase in length, when its expansion was measured. The rods were generally raised... Pyrotechny to about 207° or 208° Fahr., and about four hours were required to raise a rod from 50° to this temperature, the section of the rods varying from a square half-inch to an inch. From the amount of expansion for the observed changes of temperature, a table of expansions for 180° Fahr. was calculated. All the stones operated on contained moisture, the effect of which, in the case of greenstone and some marbles, was to increase the amount of expansion; in other instances no such effect was observed. In white Sicilian marble a permanent increase in length was produced every time its temperature was raised, the amount of increase diminishing each time.

In the Great Exhibition of 1851 Mr Ericsson exhibited in the United States department a pyrometer for measuring temperatures from the freezing-point of water to the melting-point of iron, as indicated by the tension of a permanent volume of air or of nitrogen gas, which is measured by the reading of a column of mercury under a vacuum. The instrument is intended for the regulation of processes in the useful arts in which great heat is required, and an evenly-regulated temperature is of importance. In the formation of the scale, 32° and 212° have been taken for the points of freezing and of boiling water. The instrument consists of a chamber containing mercury, with a flexible bottom composed of a steel spring, or of india-rubber held between steel plates, and capable of being raised or lowered by means of a screw. Into this chamber a glass tube filled with mercury is plunged to within 1/4th of an inch of the base. Into the mercurial cistern is inserted a short glass tube connected with a platinum bulb by a small passage, the base of which is nearly filled by a silver wire and a stop-cock. A coupling-joint is affixed to the bulb, so that it may be removed at pleasure. The top and sides of the mercurial chamber are surrounded by a cistern for the reception of pounded ice; the whole being encircled by double plates of iron to be filled with clay or some other badly-conducting substance, for the purpose of shielding and supporting the instrument. The screen itself is supported upon a base-plate. Two scales are graduated for reading off the height of the mercury in the tube, as determined by the temperature of the medium in the platinum bulb. The graduation of the smaller of these scales extends only to 700°, but that of the larger includes the melting-point of iron. There is also a spirit-level for placing the instrument in a vertical position. The graduation of the scale is independent of any imperfection in the bore of the tube, and is not affected by the expansion of the bore from heat, the volume for measuring which, being permanent and not expanding, affords greater accuracy in the readings at high temperatures. The pyrometer comes into action when the thermometer ceases to be effective; the air or nitrogen in the bulb of the former allowing it to remain unchanged under extreme variations of temperature, whilst the latter explodes on being thrust into an ordinary flue or vessel of over-heated lead. The Jury Report from which this description is abstracted, offers no opinion as to the value of the instrument, nor does it appear that the jury tested its value by experiment.

(C.T.)