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, etc. 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 Southampton 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, etc.„ 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. R R, reading-room for the accomodation 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 Fig. 1 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 to, 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. S S, two circular spiral staircases, which proceed from the basement to the gallery. l t is the lecturers' table, behind which, at P, is a large frame for the exhibition of plans, diagrams, charts, drawings, etc.; 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 folding-doors 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.)

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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 reading-room 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, etc. 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. H S 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 ad. C R is the committee-room, 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 Ris 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. M R 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, etc, 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 G G, show the seats in the gallerv 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.

N S is the top of the circular stone staircase leading from the entrance in Northumberland Court; and M S, 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. L L 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 extremities, and another in the middle.

Fig. 2.

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