TEMPLATE-TERRA COTTA-TESTS-THEODOLITE.

TERRA DE SIENA, a brown ferruginous ochre, used in painting. See OCHRE.

TESTS are those substances in solution, or in the gaseous form, which are used in chemistry to detect the presence of other bodies also in solution; which they do by forming insoluble precipitates, by changing colour, or by certain other characteristics well known to the practical chemist. Thus a few drops of a solution of carbonate or acetate of barytes added to a solution containing sulphuric acid produces a dense insoluble precipitate of sulphate of barytes. Now, as the carbonate or acetate of barytes does not produce this effect with any other acid, the formation of such a precipitate is a certain test of the presence of sulphuric acid. So also the presence of silver in solution

See STRENGTH OF MATERIALS is made evident on the addition of muriatic acid, by

There is a masterly article on the "Theodolite" in the Penny beneath the telescope to indicate the horizontality of

the line of collimation. The adjustments for parallax | the compass box, the zero of this vernier may be set and collimation are the same as those noticed under to the zero of the limb, and the vertical limb will be LEVELLING for the Y level. The adjustment of the in perfect adjustment. In large theodolites a second horizontal limb is made by first setting the instrument telescope is placed beneath the horizontal limb, for the as accurately as possible by eye, by moving the legs purpose of detecting any accidental derangement of of the stand until the bubbles in B B are nearly cen- the instrument during an observation, by noting tral, and the plummet which is suspended from a whether it is directed to the same point of a distant hook under the body of the instrument hangs freely object at the end of an observation as it was at the above the centre of the station. One leg should be beginning. The vertical limb also, in the larger theomoved, and each leg is capable of a double motion. dolites, admits of an adjustment to make it move The collar D is then tightened by the clamping accurately in a vertical plane when the horizontal screw c; the vernier plate is unclamped, and turned limb has been set in perfect adjustment. And also in round until the telescope is over two of the parallel small theodolites, when the vertical limb is permaplate-screws. The bubble b of the level s s is to be nently fixed to the horizontal, Mr. Heather remarks, brought beneath the telescope to the centre of its that an instrument which will not bear the following run, by turning the tangent screw i. The vernier test should be returned to the maker for better adjustplate is turned half round, and the telescope again ment. The azimuthal axis having been set truly brought over the same pair of parallel plate-screws; vertical, direct the telescope to some well-defined if the bubble of the centre be not in the centre of its angle of a building, and making the intersection of the run, it must be brought back to the centre, half way wires exactly coincide with this angle near the ground, by turning the parallel plate screws over which it is elevate the telescope by giving motion to the vertical placed, and half way by turning the tangent screw i. | limb, and if the adjustment be perfect, the intersecThe operation is to be repeated until the bubble re- tion of the cross-wires will move accurately along the mains truly in the centre of its run in both positions angle of the building, continuing in coincidence with of the telescope, and the vernier plate being turned it. A plumb line suspended from the cornice would round until the telescope is over the other pair of however be more trustworthy than the verticality parallel plate screws, the bubble is to be again of the angle itself. brought to the centre of its run by turning these screws. The bubble will now retain its position while the vernier plate is turned completely round, showing that the interior azimuthal axis about which it turns is truly vertical. The bubbles of the levels B B being brought to the centre of their tubes will therefore be adjusted to show the verticality of the internal azimuthal axis. The vernier-plate having been clamped, the collar D is to be loosened by turning back the screw c, and the whole instrument moved slowly round upon the external azimuthal axis; and if the bubble of s s beneath the telescope maintain its position during a whole revolution, the external azimuthal axis is truly parallel with the internal, and both are vertical at the same time; but if the bubble does not maintain its position, it shows that the two parts of the axis have not been accurately ground, and that the instrument is imperfect.

In adjusting the vertical limb the bubble of the level s s being in the centre of its run, the telescope (with the level attached and already adjusted parallel to its line of collimation) is to be reversed, end for eud, in the y's, and if the bubble do not remain in the same position, correction for 1-half the error is to be made by the capstan-headed screw at one end, and for the other half by the vertical tangent screw i. The operation is to be repeated until a satisfactory result is attained. On turning the telescope a little to the right and to the left, if the bubble do not remain in the centre of its run, the level ss must be adjusted laterally by means of the screw at the other end. This will probably disturb the first adjustment, and the whole must then be carefully repeated. | By means of the small screw which fastens the vernier of the vertical limb to the vernier-plate over

In an extensive survey the principal points should be determined by a system of triangles proceeding from an accurately measured base of considerable length. The angles of these triangles should be observed with a large and perfect theodolite, the corrections for the curvature of the earth, refraction, &c. being applied. The boundaries of the space to be surveyed being accurately determined, and a series of stations laid down throughout, the spaces included between these stations may be subdivided into spaces of 3 or 4 miles, the boundaries of which may be surveyed by a traverse, i. e. with a chain and a portable theodolite, and the details of the country within these spaces may be sketched by means of the prismatic compass. See SURVEYING.

THERMOMETER. Having, in the article HEAT, stated at some length the laws which regulate the action of instruments constructed for the measurement of temperature, and having also, under STEAM and STEAM-ENGINE, given a further exposition of those most important laws, our attention will be chiefly confined, in the present article, to a description of the modes of preparing thermometrical instruments, the methods of using them, and the reliance which is to be placed upon their indications.

Thermometrical instruments may be divided into three classes:-1st. Those which are intended merely to compare the temperatures of bodies, and to which the term thermometer is usually restricted. 2dly. Those which are intended to compare the quantities of sensible heat evolved by different actions or contained by different bodies: such instruments have been usually called calorimeters. 3dly. Those which are intended to compare the effects of different radiations or heating rays. As their indications depend on the difference

of temperature between their different parts, they have been named differential thermometers.

The first thermometer was constructed about the beginning of the 17th century. It consisted of a tube t, Fig. 2157, open at one end, and a glass bulb B at the other: the air in the bulb being first expanded by heat, the open end of the tube was inserted into the coloured liquid in the cup c. As the bulb became cool, the enclosed air contracted, and a portion of the liquid ascended the tube, by atmospheric pressure on the surface of the liquid in the cup, until the liquid in the tube equalled the bulk of air expelled by the heat. Any process tending to heat the bulb above the temperature of the surrounding air would cause the enclosed air to expand; thereby depressing the liquid in the tube; but if the temperature of the bulb were lowered beyond that of the surrounding air, the enclosed air would contract, and more liquid be forced into the tube. If a scale of equal parts were applied to the tube, a rough idea would thus be afforded of the differences in temperature of bodies affecting the bulb. Instruments of this kind are called air-thermometers; and formerly weather glasses, since they served to indicate cold and warm weather.

Fig. 2157.

the greatest summer-heat of Florence for the upper limit. As these points are variable, they were of no value for the purpose intended.

On the introduction of this thermometer into England, the first improvement of Boyle was to substitute coloured spirit for colourless. He proposed to fix one point of the scale by observing the height of the liquid in the stem when the bulb was placed in thawing oil of aniseed; a temperature which he preferred to that of melting ice, because it could be procured at any time of the year; for this oil melts or freezes at about 50°, and is therefore in this country seen about as often in the solid state as in the liquid. Hooke is thought to have suggested the temperature of freezing water as one of the fixed points; Halley proposed the boiling point of spirits as another. The differences of opinion respecting the standard minimum of temperature arose from the supposition that the freezing point of water was higher in one country than in another.

As spirit boils at a comparatively low temperature, the tubes filled with it were liable to burst on being exposed to temperatures beyond their boiling points. Newton proposed to remedy this defect by using oil instead of spirit; but this was found to be very sluggish in its motions within the tube, to adhere to it strongly, and to vary in fluidity at different tempe

ratures.

This instrument was improved by Boyle, who ex- The present mode of rendering thermometers comchanged the cup e for a bottle, Fig. 2158, into the parable with each other is commonly ascribed to neck of which a tube was cemented, so as to confine Newton, who was the first to take advantage of the a portion of air in the bottle instead of in the bulb, fact, that whenever a thermometer is placed in melting which was dispensed with, the tube being open at snow or ice, the liquid always stands at the same point both ends. The bottle could thus be dipped into of the tube, and that the boiling point of water is liquids, and their relative temperatures compared; but almt equally constant in all ordinary states of the the open tube, and the evaporation or weather. Now, if we divide the space between these contamination of the liquor contained two points into any arbitrary number of degrees, and in it, were objections which prevented continue degrees of the same magnitude both upwards this form, and other attempts at im- and downwards, all thermometers thus made will inprovement, from having any value indicate the same degree when exposed to the same scientific observations.

temperature. Before this capital invention of Newton, the instrument could scarcely be said to be more than a toy; but it now became almost an artificial organ of sense; the observations made with it being comparable, however widely they might be separated by time or place.

The Florentine Academicians saw the necessity of removing the registering fluid from the pressure of the air, and also of obtaining a scale with fixed points, such as should be applicable to all thermometers. Their instrument Another decided improvement was the adoption of Fig. 2158. depended on the expansion of spirits of mercury enclosed in a bulb and tube purged of air. wine instead of air, and the mode of construction Halley conceived the idea of employing mercury; but was as follows:-A tube, with a bulb at one end and he rejected it on account of the small amount of its open at the other, was heated, and when a portion of expansibility, which he estimated at ✈ from freezing the air was expelled, the open end was plunged into to boiling water; but it has since been found to be spirits of wine, which, as the bulb cooled, ascended; that is, 55 measures at 32°, expand into 56 into the stem: the bulb was then held downwards measures by being heated to 212°. Römer is said to over a flame, so as to boil the spirits and expel the have first adopted this important improvement and remaining portion of air. While the vapour was also to have constructed Fahrenheit's scale. Fahrenissuing from the end of the tube the flame of a blow-heit was an instrument maker, a native of Dantzic, pipe was applied to it, thus melting the glass and sealing it hermetically. The application of a scale was less happy than the structure of the instrument. The points selected for the graduation were, 1st, the cold of ice and snow for the lower limit, and, 2dly,

residing at Amsterdam : his thermometers were known over Europe early in the eighteenth century.

The advantages attending the use of mercury as the thermometric fluid are-1. It enlarges in bulk more equably for equal increments of heat than most other

liquids. 2. From its great density it is more easily | from one end to the other, and kept stationary at freed from air than either alcohol or oil; a quality of different parts by holding the tube horizontally: the much importance in the construction of these instru- space which it occupies in the tube at different ments. 3. It has a very convenient range; for while places is to be carefully measured; if this be the oil becomes viscid and very tenacious at low temper- same at every part, the bore is evidently uniform atures, and water1 and alcohol boil before they attain throughout; but if the mercury occupy a less extent a high temperature, mercury will retain its liquidity at one part than at another, the bore must vary in unimpaired for a range of more than 700°. 4. It accom- diameter, and hence cannot afford accurate results modates itself more speedily to the temperature of sur- when used as a thermometer tube. The application. rounding bodies than many other liquids. It was stated of this test will enable the careful maker to select under HEAT, that a less quantity of heat is required to such tubes or portions of tubes as most nearly apraise a given quantity of mercury to a certain temper-proach to perfect accuracy of bore. Thermometer ature, than is necessary to raise an equal quantity of water to the same temperature; .and this is in effect the same as saying that mercury becomes heated and cooled more speedily than water. This property is due to the metallic nature of mercury; metals are the best conductors of heat, and a fluid metal would conduct heat from one particle to another more rapidly than water or any other liquid.

As liquids do not follow the same law of expansion, that is, do not equally increase their expansibility with increase of temperature, it follows that two thermometers made with different liquids, although both be graduated in the manner above described so as to coincide in their indications of two fixed points, would nevertheless not coincide exactly when the temperature was at any other point. Thus, as mercury is more equable in its rate of expansion than any other liquid, a thermometer of any other liquid, although made to coincide with the mercurial one at the boiling and freezing points of water, would yet give a rather lower estimate of all temperatures between these two points, and a rather higher estimate of all temperatures above boiling and below freezing. Now in the comparison of scientific results at different times and in different places, it is evidently necessary to adopt some standard of universal reference; and in the case of the thermometer pure mercury is the standard. The methods of obtaining it pure are given

under MERCURY.

The glass capillary tubes which are prepared for the construction of thermometers should be precisely equal in diameter throughout, if we would have them measure equal increments of bulk. But in fact the tubes as obtained from the glass-houses are generally frusta of very elongated hollow cones, which by extension become more or less perfectly cylindrical. We may see how this happens by drawing out a thread of sealing-wax, which will always be larger at the ends than towards the middle. If one part of the bore be larger than another, a division at that part in a scale of equal parts would belong to a greater change in the volume of the mercury than a division at the other part, where the diameter is less. In order to determine the value of one of these tubes a drop of mercury may be drawn into it so as not to occupy a space in the bore of more than inch. The mercury is to be gradually moved through the tube

(1) Water is also inapplicable as a thermometric fluid from the

unparalleled singularities of its changes of bulk at low tempera

tures. See HEAT.

VOL. II.

tubes are sometimes made with an elliptical bore, to allow a small column of mercury to be more visible when expanded at right angles to the line of vision.

When the tube is selected a bulb is blown upon it at one extremity by softening the end in the flame of a table blow-pipe, and tying the other end to a bottle of India-rubber containing dry air; on compressing the bottle the air will expand the softened glass into a bulb. Air from the mouth would be injurious in this operation on account of the moisture which would be introduced into the tube. The size of the bulb must depend on the purposes to which the thermometer is to be applied; but, of course, the larger the bulb, in proportion to the stem, the more sensitive will the thermometer be to changes of temperature, for the greater will be the rise or fall produced by a change of bulk bearing any given ratio to the whole bulk of the liquid. But although there is no limit to the size or length allowable in a thermometer used only for indicating atmospheric temperature, it is otherwise with an instrument intended for discovering the temperatures of small bodies, for unless its bulb be much smaller than the bodies to which it is applied, it will sensibly alter their temperature, and not acquire the exact temperature which they had maintained in its absence.

As the pressure of the air acts more equally upon a spherical than on a cylindrical or pyriform body, the spherical is preferred; but when the bulb is very large in proportion to the stem, one of the two latter is adopted. With bulbs of large size containing mercury, a slight pressure of the fingers, apart from their natural heat, causes the mercury to ascend in the stem: and with such bulbs the varying pressure of the atmosphere has an influence on the capacity of the bulb. It may seem strange that the atmospheric pressure should have more effect in compressing the bulb than the weight of the mercury in distending it; but such is always the case in thermometers of ordinary length.

The bulb is filled with mercury much in the same way as the first thermometer, Fig. 2157, was filled with liquid, viz. by applying heat to the bulb so as to rarefy the air, and in allowing it to cool with the open end under mercury, a portion thereof will enter the stem and bulb. The tube should be placed in a nearly horizontal position, with its open end below the surface of the mercury. Heat is again applied to the bulb, and the mercury in it is boiled: the vapour of mercury expels the remaining air, and when the

3 II.