lbs.

A wire of one-tenth inch diameter of Silver breaks with

The relative cohesive strength of the metals are, according to Sickengen, as follow:

But their hardness, according to Cavallo, follows this order, viz., Iron, Platinum, Copper, Silver, Gold, Tin, Lead.

Banks observes that iron is about four times as strong as oak, and six times as strong as deal. Wood is from seven to twenty times weaker transversely than longitudinally. It becomes stronger both ways when dry.

TENON. The end of a bar of metal or piece of wood reduced in its dimensions, so as to fit a hole in another piece, called a mortise, and thus joining the two together.

TENSION. Is the state of a thing stretched; this term is much used by engineers to express the tenacity of metals and other substances, when pulled in the direction of their length; thus a wire of one-tenth of an inch in diameter, is said to be capable of resisting a tension of 450 pounds.

TENTER, trier, or prover, a machine or frame, used in the cloth manufac tory, to stretch out the pieces of cloth, and make them set even and square. It is usually about 4! feet high, and for length exceeds that of the longest piece of cloth. It consists of several long square pieces of wood, placed like those which form the barriers of a menage; so, however, as that the lower cross pieces of wood may be raised or lowered, as is found requisite, to be fixed at any height, by means of pins. Along the cross pieces, both the upper and the lower one, are numerous sharp hooked nails, called tenter-hooks, on which the selvages of the cloth are hooked.

TESSELLATED PAVEMENTS. Pavements of different coloured stones, tiles, or brick, laid chequer-wise, or like dice (tesselæ.) The term tessellated is, however, extended to all kinds of mosaic patterns or designs.

TEST. Any solid or fluid body, which, added to a substance, teaches us to distinguish its chemical nature or composition.

THEATRE. An edifice or great room for the public exhibition of scenic representations, the performance of the drama, of concerts, the delivery of scientific lectures and demonstrations, &c. Considering that the description of a theatre for the latter purpose will not be out of place in this work, and be acceptable to our readers, we shall here annex an account of the lecture theatre of the London Mechanics' Institution; which may serve the purpose of a model whereon similar undertakings may be constructed and arranged, making such alterations and modifications as will better adapt them to other circumstances. The front of this institution is a large dwelling-house, situated in Southamp ton Buildings, at the corner of Staple's Inn, in Holborn; the lower rooms of which are employed for the library, reading-rooms, apparatus rooms, laboratory, &c., and the upper as the private apartments of the Secretary. The theatre is an entirely new structure, built at the back of, and in connexion with, the house. The first stone of the theatre was laid on the 2d December, 1824, by Dr. Birkbeck, the munificent patron and enlightened president of the institution, and it was opened for use on the 8th of July, 1825.

The annexed Fig. 1 exhibits a plan of the ground floor of the whole building,

on a scale of 1 inch to 20 feet.

The doors in front of the house in Southampton Row are represented at D. O, office. H, hall and principal staircase. RR, reading-room for the accomo

dation of the members, supplied with all the periodical journals and reviews, and where all the books in the library may be perused. L, the library, containing upwards of 5000 volumes; including almost every work of reputation on

science and literature, which may either be consulted in the reading-room, or taken home by the members. A R, ante-room to theatre. E, principal entrance from the house into theatre. e e, side-entrances into theatre. T, the theatre, bounded by a wall w w w, of a horse-shoe form. G G G are the seats appropriated to the members in general. C are those allotted exclusively to members of the committee; and V, those for the accommodation of honorary members, and visitors. N is the entrance into the theatre from Northumberland Court; and M is that leading from Middle Row, Holborn. SS, two circular spiral staircases, which proceed from the basement to the gallery. It is the lecturers' table, behind which, at P, is a large frame for the exhibition of plans, diagrams, charts, drawings, &c.; and when these are made into transparencies, they are illuminated by a series of gas jets arranged behind the frame. F is a furnace, employed in the chemical lectures. This furnace, when not in use, is closed by two folding-doors, which are elegantly painted to correspond with the foldingdoors of the entrance E. The six black spots arranged in a semi-circle, show the site of the iron pillars that support the principal gallery, which is also of the horse-shoe form, as shown by the curved dotted line of that figure, (also exhibited in Fig. 2.)

The foregoing plan, although only descriptive of the ground-floor of the building, will enable us to explain the appropriation of the rooms and offices of the basement underneath it.

Underneath the hall H, is a kitchen and store-room; underneath the readingroom RR, are the porter's rooms; and underneath the library L, is the laboratory of the same area, containing furnaces, and other requisites for chemical investigations. In this room a class of the members meet weekly for mutual instruction in chemistry, minerology, &c. Adjoining to the laboratory is a small workshop, furnished with an excellent turning-lathe, work-bench, and various tools for the construction and repair of apparatus.

Under the theatre is an extensive class-room, lighted by gass, where practical geometry, perspective, architectural, mechanical, and ornamental drawing, are regularly taught.

The annexed figure (2.) exhibits a plan of the first-floor of the house, together with a plan of the gallery of the theatre. HS is the ascending staircase from the hall to the first-floor; H G, the gallery leading therefrom to the several apartments, all the doors or entrances to which are marked with a d. CR is the committeeroom, furnished with a large table, and other requisites, to accommodate the meetings of the committee of managers, who conduct the affairs of the institution. This room is 19 feet by 21 feet, and one of the side-walls is covered with a glass case, furnished with a splendid collection of minerals. A R is the apparatus-room, or museum, furnished with glass cases around it, containing an extensive assortment of mechanical, pneumatical, hydrostatical, optical, and electrical apparatus; besides a great variety of very large diagrams, for the illustration of those subjects; and an assortment of mineral and geological specimens. This room is open for the accommodation of the members every Tuesday evening, from eight till ten o'clock, to afford them opportunities of inspecting the apparatus, conversing together, and explaining to each other the results of their experience and observations. This mutual interchange of information is calculated to be productive of important advantages to the members. A class for mutual instruction in experimental philosophy, also holds a weekly meeting in this room. MR is another room, similarly appropriated to the last mentioned; it contains various models, and large pieces of apparatus, inconvenient for exhibition in the museum, (A R,) and an extensive collection of transparent illustrations of various sciences. B S, the staircase, leading to the upper floors of the house. The room over the committee-room is a class-room, in which writing, drawing, the English and Latin languages, &c., and occasionally stenography, are taught in the different evenings of the week; and the room over the museum is also a class-room, where mathematics and arithmetic are taught. The other rooms in the upper part of the house are the private apartments of the secretary, who resides on the premises. G GG, show the seats in the gallery of the theatre, rising up an inclined plane; the front, or lowest

row, being upon a level with the first-floor of the house, and the highest, or back row, being about 17 feet above the lowest. NS is the top of the circular stone staircase leading from the entrance in Northumberland Court; and MS, that appertaining to the entrance from Middle Row, Holborn. P is the pit, or

rather ground-floor, of the theatre, the plan of which is given in Fig. 1. LL are two jointed branches for gas-lights, each containing three burners, which can be moved in various positions, to suit the objects to be illuminated. The

dotted lines ooo, show the plan of a lofty rectangular gallery, even with the top of the semi-circular gallery G, from which there are two entrances at the extre mities, and another in the middle.

THEODOLITE. A mathematical instrument used by land-surveyors, for taking angles, distances, altitudes, &c. This instrument is variously made, and provided with subordinate apparatus, according to the price, or the requirements of the purchaser. We shall describe one of the most generally useful. This consists of two concentric horizontal circles, the inner of which has, at the ends of one of its diameters, two perpendicular columns, on which rests the horizontal axis of a small meridian telescope. The vernier of the inner circle is made fast to an arbitrary division line of the outer one, and both circles are moved, together with the telescope, until the object sought for appears in its field. The outer circle is now fixed, and the inner one is turned round, until the telescope strikes the second object, whose angular distance from the first is to be measured. The inner circle is now fastened to the outer, and by means of a micrometer screw, the thread of the telescope is brought exactly upon the object. The arc which the vernier of the inner circle has described on the outer one measures the angle which the two objects make at the common centre of the two circles.

THEOREM. A proposition which terminates in theory, and which con siders the properties of things already made or done. Or, theorem is a speculative proposition, deduced from several definitions compared together.

THEORY. A doctrine which terminates in speculations, without any view to the practice or application of it.

THERMOMETER. An instrument for measuring the temperature of bodies; founded upon the principle of augmentation in volume of fluids, in proportion to their absorption of caloric; and as regards aeriform fluids, the principle is probably very correct: but solids, and still more liquids, expand unequally, by equal increments of heat. Thermometers were invented about the beginning of the seventeenth century; but a knowledge of their author is involved in some obscurity. For the first half century, after their introduction, they were made in a very rude and imperfect manner; but they were at length considerably im proved by the Florentine academicians, and received subsequent ameliorations from Mr. Boyle, Dr. Halley, and Sir Isaac Newton, as well as from contemporaneous philosophers on the continent. The changes which the instrument underwent in their hands, (described in the Oxford Encyclopædia,) we shall not here insert, as all that had at that time been proposed, were liable to many conveniences, and could not be considered as exact standards for pointing out the various degrees of temperature.

The thermometers which at present are in most general use, are Fahrenheit's, De l'Isle's, Reaumur's, and Celsius's. Fahrenheit's are used in Britain, De l'Isle's in Russia, Reaumur's, and the thermometer centrigade, in France, and Celsius's, the same as the last named, in Sweden. They are all mercurial thermometers.

Fahrenheit's thermometer consists of a slender cylindrical tube, and a small longitudinal bulb. To the side of the tube a, is annexed a scale b, which Fahrenheit divided into 600 parts, beginning with that of the severe cold which he had observed in Iceland in 1709, or that produced by surrounding the bulb c of the thermometer with a mixture of snow or beaten ice, and sal ammoniac, or sea salt. This he apprehended to be the greatest degree of cold; and accordingly he marked it, as the beginning of his scale, with 0; the point at which mercury begins to boil, he conceived to show the greatest degree of heat, and this he made the limit of his scale. The distance between these two points, he divided into 600 equal parts or degrees; and by trials, he found that the mercury stood at thirty-two of these divisions, when water just begins to freeze, or snow or ice just begins to thaw; it was, therefore, called the degree of the freezing point. When the tube was immersed in boiling water, the mercury rose to 212, which, therefore, is the boiling point, and is just 180 degrees above the former, or freezing point. But the present method of making the scale of these thermometers, which is the sort in most common use,