degree, and then remains stationary: this may be rectified by elevating the scale of the instrument the required quantity. The other takes place at every change of temperature; it is small and scarcely perceptible, with small changes of temperature, but by considerable changes becomes very evident and important. Singular consequences sometimes result from the influence of these changes. If two liquids be taken of different temperatures, a greater difference will be found between them, by trying the hot fluid, and then the cold fluid by the same thermometer, than what will appear to exist by trying the cold fluid first. Again, if a new thermometer be graduated by an old one preserved as a standard, although it may be made to agree with it, yet, after a while, the two will not accord; and if two old thermometers be taken that do agree, and the one be heated whilst the other remains unused, they will no longer indicate the same temperatures. The reason now becomes evident, why alcohol thermometers are so much less affected in this manner than those filled with mercury. Alcohol expands several times more than mercury, so that an instrument constructed with it having a tube of the same diameter, and degrees of the same size, will require a bulb several times less than if mercury had been used. Hence, as the elevation is in proportion to the capacity of the bulb, independent of the liquid it contains, the alcohol thermometer will exhibit a much smaller effect than the mercurial instrument. MM. A. de la Rive and F. Marcet have also investigated the elevation of the mercury in thermometers, which is due to the cause pointed out by Mr. Flaugergues, namely, the continued pressure of the air on its external surface: and by opening the top of the thermometer, by submitting the instrument to condensed or rare atmospheres, and by comparison with thermometers otherwise constructed, have abundantly proved the effect due to this power. These philosophers had occasion also to remark some curious effects due to the absorption and evolution of heat, by the expansion and condensation of gases, which however we cannot at this time further attend to, than by copying the conclusions at the end of the memoir. 1. That atmospheric pressure exerts an influence on the bulk of thermometer bulbs. 2. That in experiments, where this effect may influence the results, it is better to use thermometers open at the top. 3. That certainly cold is produced in making a vacuum by the air-pump, but in smaller quantity than was supposed. 4. That when gases enter an exhausted vessel, there is at first a production of cold, and then of heat. 5. That various modifications may render the cold produced at the moment of the entrance of air into a vacuum more intense. Sig. Bellani has undertaken a series of experiments, to determine whether the air or vapor, the last portions of which are found to remain so obstinately in barometers and thermometers, is introduced with the mercury, or is a portion of that which originally occupied the tube before the introduction of the metal. The conclusion he comes to is, that it is always a portion of that which previously adhered to the glass, and that mercury is utterly incapable of absorbing either air or moisture. The extraordinary way in which air and water is held as it were in a film over glass is insisted upon, and reference made to many authors in proof of it. The following, however, are more interesting, as being some of the facts he advances to prove that the mercury never contains either of these substances. Fill a barometer tube and boil it very carefully; then prepare a kind of funnel made of a small capillary tube, which will reach through the mercury in the barometer tube to the closed end, and is enlarged at top; let it be recently made, so as to be dry, and introduce it into the barometer tube; prepare some mercury by agitating it in a bottle with water and air, then drying its surface with bibulous paper, and afterwards passing it through paper cones three or four times into dry vessels; pour a little of this mercury into the funnel tube, and with a horse-hair or fine wire remove the air, so that the column may be continuous; then pour in so much of this prepared mercury as will fully displace the mercury that was boiled in the tube; afterwards remove the funnel tube, and put the barometer to its proper use. It will be found to stand exactly at the same height as before in the same circumstances; and if the mercury be now boiled in the tube, none of those bubbles will appear which arose on the first boiling; care being taken throughout that the inner surface of the tube has not been exposed to the air. Perhaps an easier mode of making the same experiment is to make the barometer terminate at top in a bulb, which will hold more mercury than is required to fill the tube: then when it is boiled it need only be placed upright in a basin of common mercury, and, when inclined, the mercury will enter and replace that which was boiled in the instrument; the results will be as above. An experiment proving the same thing may be made still more easily thus: fill a mercurial thermometer and boil it well; then heat it till nearly all the mercury is expelled, but preserve its open extremity under common mercury: the latter metal will enter as the instrument cools, and behave in every respect as the well-boiled mercury did. If a bulb of a thermometer be suddenly squeezed between the finger and the thumb, the mercury will rise in the stem several degrees, and will again sink as quickly after the pressure is removed. To prevent any derangement from communication of heat, the hand may be covered with a thick glove. This is a very important fact, and it may be shown in a less exceptionable way :-let a mercurial thermometer, with a large bulb and a long stem, be first held upright, and then immediately inverted; between these two positions the column of mercury will descend through a visible space: thus proving that a variable pressure in the atmosphere, or mercury, will produce anomalies in the thermometer. Mr. Breguet's thermometer consists of slips of two metals, unequally expanded by heat, twisted into a spiral: to the extremity of the spiral is fixed an index, which moves round a graduated circle, pointing out the temperature. It is obvious that, when the spiral is heated, the index will move in one direction, and in another when the spiral is cooled, because it will twist or untwist itself according to the changes of temperature to which it is subjected. The two metals employed are silver and platinum; and in order to render the extreme points more fixed, and to prevent sudden starts, a slip of gold, the expansibility of which is intermediate between that of silver and platinum, is soldered between these two metals. This thermometer is more delicate than any mercurial thermometer whatever. It is even more delicate than an air thermometer. This spiral thermometer, and a mercurial one, were placed together under the receiver of an air pump. The temperature at the time of the experiment was 66-2°. The mercurial thermometer, when the air was pumped out, sunk 3.6°; but the spiral thermometer fell 41.4°, and descended to 24-8° Fahrenheit. The differential thermometer, invented by professor Leslie, consists of two tubes, each terminating in a small bulb of similar dimensions; a small portion of dark-colored fluid, formed of sulphuric acid tinged with carmine, having previously been introduced into one of the balls. The instrument is then fixed on a stand, and furnished with a graduated scale. When the column is equally pressed in opposite directions, the fluid will point at zero, and whatever heat may be applied to the whole instrument, provided both bulbs receive it in an equal degree, the fluid must remain at rest. But, if the one ball receives the slightest excess of temperature, the air which it contains will be proportionally expanded, and the column will be depressed with a force equal to the difference between the temperature of the two balls. A self-registering thermometer is a most important instrument, and, as such, must not be passed unnoticed. It is employed to indicate the extreme changes that occur in the temperature of the air. Dr. Rutherford employed two thermometers. The one which marks the minimum is filled with alcohol; and the other, which indicates the maximum, is filled with quicksilver; and they are both attached to the same frame, or, what is still better, affixed to separate frames, placed nearly horizontal, or rather elevated about five degrees, to prevent the separation of the thread of liquid. The tubes have bores from the twenty-fifth to the fifteenth part of an inch wide, and include a minute tapered or conical piece of ivory, or of white or blue enamel, about half an inch long. This mark, having in either thermometer its base turned towards the bulbs, is drawn to the lowest point by the alcohol, which again freely passes it; but it is always pushed forward to the highest limit by the inercury, which afterwards leaves it. Mr. Crichton has contrived a self-registering thermometer, somewhat similar to that of M. Breguet; consisting of two oblong slips of steel and zinc, firmly fixed together by their faces; so that the greater expansion or contraction of the zinc over those of the steel, by the same variations of temperature, causes a flexure of the compound bar. As this is secured to a board at one end, the whole flexure is exercised at the other, on the short arm of a lever index, the free extremity of which moves along a graduated arc. The instrument is originally adjusted on a good mercurial thermometer; and the movements of the arm are registered by two fine wires, which are pushed before it, and left at the maximum deviation to the right or left of the last observed position or temperature. M. Fourier has invented a new instrument which he calls a thermometer of contact. It consists of a conical vessel constructed of very thin iron, with the exception of the bottom, which is made of thin pliable skin; it is filled with mercury and has a thermometer, the bulb of which is immersed entirely in the mercury, and the scale has degrees of such magnitude that they may be divided into tenths. The skin must be preserved perfectly clean and never be overheated; it is better than any other similarly flexible substance, because of its superior conducting power. This instrument is to be accompanied by a support consisting of a block of marble; and any substance operated upon is to be in sheets or reduced to thin plates. When an experiment is to be made, the sheet, cloth, or thin plate, is to be placed upon the marble, both being at the temperature of the room; the conical vessel, with its contents, is to be heated on a stove or other hot body, until about 46° or 47° C.; and then being removed, at the moment it has fallen to 45°, it is to be placed on the substance to be tried; the time when it arrives at 40° is to be exactly noted by a watch, and then the temperature noted minute by minute for five minutes. If the experiment be repeated with the same substance on another part of the marble, exactly the same results will be obtained, provided the temperature of the place has not changed. If the experiments are to be made on rigid plates, then these are not to be placed directly upon the marble, but upon a mercurial cushion, made by confining mercury under a surface of skin. If the substance first tried be replaced by another, and then the fall of temperature in a given time be noted, the variation will be found very sensible, however slight the difference between the substances; the addition of a single sheet of the finest paper makes a great difference in the effect. The slightest difference in the nature of the stuff is immediately indicated. If a piece of linen cloth be replaced by flannel, or by woollen cloth, or a thin piece of woollen cloth by a thick piece, not only are the differences produced very evident, but they can be obtained over and over again with the utmost constancy, care being taken that the pressure of the mercury upon the skin, and therefore upon the substance, be the same in all cases. The same instrument also indicates the heat of contact of bodies. In such cases, after being heated as before-mentioned, it is to be placed on a thick mass of the substance to be tried, and the fall of temperature in a given time noted as before: striking effects were thus obtained. Being first applied to iron at the temperature of 8° C., and then upon a mass of stone, the difference at the end of the second minute was 5°. The differences are much greater when iron is compared with brick or wood. Although the conducting powers thus obtained for different substances are only approximations, yet there are many bodies, as bricks, stones, wood, clothing, &c., for which these are quite sufficient. Another still more delicate method of ascertaining the conducting power of bodies is then described, but it is also more difficult. Two vessels are used; the lower one is maintained at a constant temperature, as 100° C.; upon that is placed the substance to be tried, and upon that again the upper vessel. The lower part of the upper vessel is enclosed, and constitutes the bulb of an air thermometer; the upper part is retained at the temperature of ice; the air therefore in the thermometer is cooled by the ice and warmed by the lower heated vessel; the latter producing an effect greater or smaller according to the nature of the substance between it and the air-vessel; the temperature of the air and the indication upon the scale connected with it soon becomes permanent, and, as it is higher or lower, indicates the greater or less conducting power of the interposed substance. When the experiments are carefully made they accord with those of the former instrument, but are more delicate. By means of these instruments M. Fourier was able to ascertain that many substances when put together, conducted heat differently, according to the order in which they were placed. Two pieces of cloth being put between the instrument and the marble, the order of substances traversed by the heat was skin, cloth-cloth, marble. After observing the effect a thin plate of copper was placed between the cloth and the marble; the fall of temperature was then slower than before; the copper was then placed between the pieces of cloth, and the cooling was as if no copper were present; then placing the copper beneath the skin of the instrument and above the cloth, so that the order was skin, copper, cloth, cloth, marble, the temperature diminished more rapidly than if no copper had been there: thus the interposition of the metal facilitated the transmission of heat from the skin to the cloth, but diminished the transmission from the cloth to the marble. The chevalier Landriani has described in the Giornale di Fisica, &c., a method contrived and adopted by himself in the construction of very delicate thermometers; and, from his experience, he is induced to consider instruments made in his way much superior to the common mercurial thermometer. The form of the instrument is nearly that of the common thermometer; but the tube is of extreme fineness, a quarter of a grain of mercury occupying in it a length of three, four, and even five inches. In order to blow a ball at the end of such a tube, it is found necessary to attach a condensing syringe to it, the elastic gum bottle not being sufficient for the purpose; and in forcing in the air when the end of the glass has been heated to produce the ball, great care must be taken that no. moisture or oil enter the tube, as the smallest particle completely closes up its minute passage. The ball and tube are then filled with alcohol in the usual manner; and, after this is done, the bore of the tube is to be expanded into two small bulbs near to each other, and to what is to be the top of the instrument, or the instrument may be reversed; the ball may be considered the top, and the other extremity being turned round, may have the two bulbs blown on it so as to resemble a common form of the barometer; this being done, alcohol is to be introduced, until not only the ball and tube, but the lower bulb, and part of the upper are filled with it. In these thermometers, one object was to avoid the injurious effect occasioned by the adhesion of the surface of the fluid in the tube with the glass; the surface of the fluid is therefore not regarded as any indication of the state of the instrument; it is always in the upper bulb, and is very little altered by any alteration of temperature; but a point is taken in the column of alcohol in the tube, by which to make observations, and this point is marked by a small cylinder of mercury; and in addition to the advantage thus obtained, of perfect freedom of motion, the column which, before from its minuteness was with difficulty visible, becomes readily distinguishable at the necessary point. The mercury is readily introduced into the tube of the instrument by warming it, and then introducing its extremity into the metal on cooling; it passes first into the bulbs, and may then be placed in any required part of what is to be the scale, and this being done the instrument is to be closed and graduated. In this way thermometers have been made so delicately that with a ball of three lines and a half in diameter each degree, (of Reaumur) has been ten and twelve inches in length, which extension allows of a division to the four hundredth and even the six hundredth part of a degree, without affecting the accuracy of the instrument. In graduating it the principal points may be taken from a mercurial thermometer, and the division into equal parts adopted for the others. Landriani enumerates some of the advantages this instrument has over common mercurial thermometers. It is more readily constructed, the adhesion of the mercury to the glass being obviated, and even the adhesion of the surface of the alcohol being of no consequence. Its material, the alcohol, has more fluidity and expansibility than mercury. In mercurial instruments the weight of the metal endangers the bulb, which. being necessarily thiu, is liable to accidents in a much greater degree than when filled with alcohol. Another important defect to which mercurial thermometers are liable, and from which these are very nearly free, is the expansion of the ball at the extremity by the weight of the column of mercury in the tube; and this column varying with the temperature, and its pressure by position, errors of a very changeable nature are introduced. Thus, with a mercurial thermometer having a ball of four or five lines in diameter, and degrees of four or five lines in length, the temperature indicated is not the same in a vertical and in a horizontal position. Landriani proposes also the use of his instrument in determining fractions of degrees which cannot be observed by the common thermometer. This is done by graduating the instrument into degrees according to common thermometers, but not affixing numbers to them; and then by dis placing the mercury from part to part, the scale may be made to commence at any given degree. If the mercury be made to descend into the ball of the instrument, or to rise into the bulb, and the instrument be placed horizontally, the temperature of the whole may then be brought to any required point; and that done, by placing the thermometer vertically with the ball upwards or downwards as required, the mercury is made to enter the tube, and passes over degrees graduated upwards or downwards from the temperature to which the whole instrument was brought. M. Landriani, in a succeeding number of the Giornale di Fisica, has proposed these thermometers to be used in meteorological observations as self-registering thermometers, and they appear very applicable to this purpose. They are to be constructed as before described, except that besides the cylinder of mercury, which is the indicator of temperature, there is to be another portion of mercury within, either the ball or the first bulb, as the instrument is to measure the extreme point of heat or of cold. The use is as follows:-Supposing it put by, the indicating cylinder of mercury will, of course, be somewhere in the stem, and the other portion of metal should be in the ball; if it be required to mark the lowest degree of cold during the night, it is to be placed upright with the ball upwards, and the point where the indicator stands noted; the mercury in the ball will rest just over the orifice of the tube, and will enter it on any descent of the column beneath; if the temperature diminishes, however, that column will ascend, the spirit in the ball contracting; but, whenever it begins to expand again, the mercury in the ball will descend, dividing the alcohol above and below it. When the instrument is next observed, therefore, nothing more is required to ascertain the extreme cold of the night than to mark the numbers of degrees between the two cylinders of mercury, and these, subtracted from the numbers of degrees between the indicator, and the ball or the mercury at the first observation, give the degrees of cold. In ascertaining the extreme heat, M. Landriani proposed to use another thermometer with the ball downward, when the first bulb will become the receptacle for the registering portion of mercury, and the difference between the two columns of alcohol included between the indicator and the bulb at the first observation, and the indicator and registering mercury at the second, will give the extreme heat of the instrument between the two observations. It would be easy, however, to make one instrument answer both purposes, and one which M. Landriani depicts is very fit for them; the ball is above, and the tube is bent just above the bulbs, so that they shall also stand perpendicularly and rising upwards from the tube. If then a small portion of mercury be appropriated to the ball, and another to the first bulb, the former will indicate the lowest temperature in the absence of the observer, and the latter the highest, the indicator of course always being present. The thermometers above described are very limited in their extent; they indeed point out to the lowest degrees of heat which are commonly observed even in cold climates, but they by no means reach to those degrees of heat which are very familiar to us. The mercurial thermometer extends no farther than to 600 of Fahrenheit's scale, the heat of boiling mercury; but we are sure that the heat of solid bodies, when heated to ignition, or till they emit light, far exceeds the heat of boiling mercury. To remedy this defect, Sir Isaac Newton, whose genius overcame those obstacles which ordinary minds could not approach, attempted by an ingenious experiment to extend the scale to any degree required. Having heated a mass of iron red hot, and exposed it to the cold air, he observed the time which elapsed till it became cold, or of the same temperature with the air; and, when the heat so far decreased that he could apply some known measure (as a thermometer) to it, he observed the degrees of heat lost in given times, and thence drew the general conclusion, that the quantities of heat lost in given small spaces are always proportional to the heat remaining in the body, reckoning the heat to be the excess by which it is warmer than the ambient air. So that taking the number of minutes which it took to cool after it came to a determined point in an arithmetical progression, the decrements of the heat of iron would be continually proportional. Having by this proportion found out the decrements of heat in a given time, after it came to a known point, it was easy, by carrying upwards the same proportion to the beginning of its cooling, to determine the greatest heat which the body had acquired. This proportion of Sir Isaac's was found by Dr. Martine to be somewhat inaccurate. The heat of a cooling body does not decrease exactly in proportion to that which the body retains. As the result of many observations, he found that two kinds of proportion took place, an arithmetical as well as the geometrical proportion which Sir Isaac Newton had adopted; namely, that the decrements of heat were partly proportional to the times (that is, that quantities of heat are lost in equal times), as well as partly in proportion to the remaining heat; and that if these two are added together the rule will be sufficiently accurate. By the geometrical proportion which Sir Isaac Newton adopted, he discovered the heat of metals red hot or in fusion. The method above-mentioned, so successfully pursued by Sir Isaac, was sufficient to form a scale of high degrees of heat, but was not convenient for practical purposes. Accordingly the ingenious Mr. Josiah Wedgwood, who is well known for his great improvement in the art of pottery, applied himself in order to discover a thermometer which might be easily managed. After many experiments recorded in the Philosophical Transactions, but which it is unnecessary to detail here, he invented a thermometer which marks with much precision the different degrees of ignition, from a dull red heat visible in the dark to the heat of an air furnace. This thermometer is extremely simple. It consists of two rulers fixed upon a smooth flat plate, a little farther asunder at the one end than the other, leaving an open longitudinal space between them, Small pieces of alum and clay mixed together are made of such a size as just to enter at the wide end; they are then heated in the fire along with the body whose heat we wish to determine. The fire, according to the degree of heat it contains, diminishes or contracts the earthy body, so that, when applied to the wide end of the gage, it will slide on towards the narrow end, less or more according to the degree of heat to which it has been exposed. Mr. Wedgwood found that ten cwt. of the porcelain clay of Cornwall required all the earth that was afforded by five cwt. of alum. But, as the clay or alum differs in quality, the proportion will also differ. There can now, however, be no difficulty in making thermometers of this kind, as common clay answers the purpose very well, and alum earth can easily be procured. Those who wish to see a more particular account of this subject may peruse Mr. Wedgwood's papers in the Philosophical Transactions for 1782, 1784, and 1786. As Mr. Wedgwood's thermometer begins at the lowest degree of ignition, and Fahrenheit's goes no higher than the boiling point of mercury, Mr. Wedgwood continued to fill up the interval of the scale by using a piece of silver instead of his common thermometer pieces; and in this way he has found that 130° of Fahrenheit are equal to one of his. He has accordingly, by observing this proportion, continued We Fahrenheit's scale to the top of his own. are now, therefore, enabled to give a scale of heat from the highest degree of heat produced by an air furnace to the greatest degree of cold hitherto known, which was produced at Hudson's Bay in December 1784, by a mixture of vitriolic acid and snow. Of the remarkable degrees between these extreme points we shall lay before our readers Brandy boils Alcohol boils Fahrenheit's Serum of blood and white of eggs Sulphuric acid of the specific gra- Common vinegar freezes A mixture of one part of alcohol and three parts of water freezes A mixture of snow and salt freezes Brandy, or a mixture of equal parts of alcohol and water freezes Spirit of wine in Reaumur's thermometer froze at Torneo. MERCURY FREEZES Cold produced by Mr. Macnab at Hudson's Bay by a mixture of sulphuric acid and snow. THERMOPYLE, in ancient geography, a narrow pass or defile, between the Sinus Maliacus on the east, and steep mountains, reaching to Oeta, made dreadful by impassable_woods, on the west; leading from Thessaly to Locris and Baotia. These mountains divide Greece in the middle, in the same manner as the Appennines do Italy; forming one continued ridge from Leucate on the west to the sea on the east, with thickets and rocks interspersed; so that persons even prepared for travelling, much less an army encumbered with baggage, cannot easily find a commodious passage. In the valley verging towards the Sinus Maliacus the road is only sixty paces broad; the only military way for an army to pass if not obstructed by an enemy; and therefore the place is called Pylæ, and by others, on account of its hot water, Thermopyla. It is famous for the brave stand made by Leonidas and 300 Spartans against the whole army of Persia; and also for the Amphyctiones, the common council or states-general of Greece, assembling there twice a year, in spring and au tumn. THER MOSCOPE, n. s. Fr. thermoscope; Gr. θερμος and σκοπεω. An instrument by which the degrees of heat are discovered; a thermo meter. By the trial of the thermoscope, fishes have more heat than the element which they swim in. Arbuthnot on Aliments. |