DESCRIPTION OF A MODE OF ERECTING LIGHT VAULTS OVER CHURCHES AND SIMILAR SPACES.

BY M. DE LASSAUX.

(Communicated by Professor WHEWELL, of Cambridge.)

M. DE LASSAUX, of Coblentz, architect to the King of

Prussia, is the discoverer and restorer of this process,

and gives the following account of his investigations.

He had arrived in various ways at the conviction that what are called the gothic and ante-gothic styles of architecture (the pointed arch and round arch styles), are not only the most appropriate for churches, but also the cheapest. He had attempted to discover some easy means of erecting stone vaults in such cases, thinking them highly desirable, whenever the funds at the builder's disposal will permit them. Vaults which are at the same time wide and light, belong incontestably to the boldest and most ingenious of human inventions: they are peculiarly suitable in religious edifices; they are secure from the devastations of fire; and, when introduced in public buildings, they correspond to the spirit of the celebrated decree of the republic of Florence, enacted in the year 1294, that all which is executed for the commonwealth should bear the lofty impress of the common will.

M. de Lassaux was also aware, that at Vienna, at the present time, very wide and flat domes are erected almost entirely free-handed (i. e. without centering), and that in the neighbourhood of that city, very flat ovens and wide mantlepieces * are constructed almost in the same manner, and with the help only of a few slight posts or poles. He endeavoured, therefore, to discover some mode of facilitating, by similar means, the execution of wide vaults in churches.

His attempts for some time led him to nothing bearing on

*Brunelleschi constructed the cupola of the church of Santa Maria del Fiore at Florence, without a centre.-J. S. 'At Bassora, where they have no timber but wood of the date tree, which is like a cabbage-stalk, they make arches without any frame. The mason, with a nail and a bit of string, describes a semicircle on the ground, lays his bricks, fastened together with a gypsum cement, on the lines thus traced, and having thus formed his arch, except the crown brick, it is carefully raised, and in two parts placed on the walls. They proceed thus till the whole arch is finished; this part is only half a brick thick, but it serves to turn a stronger arch over it.'-Eton's Survey of the Turkish Empire.-J. S.

the point in question, except the usual methods of laying down the vaulting lines, and some historical notices, which will be mentioned subsequently. In the old church vaults which are extant, there was little to be seen, as they are in almost all cases covered with a coat of mortar or plaster.

About six years ago, however, happening to go into the space above the vault of the fine church at Ahrweiler, he observed in the extrados of the vaults so remarkable a dissimilarity in their height and curvature, that the thought in an instant struck him, that it was impossible these could have been built upon a regular centering. On a closer examination, it appeared impossible to entertain any further doubt on this subject; and in various places, where the rubble work had been laid bare, the whole mode and manner was exhibited of the process which had been employed, and the opinion thus formed was more and more confirmed by subsequent examination of a number of other vaults.

The whole mystery resides in this, that these pointed-arch cross-vaultings consist of separate, generally horizontal, courses; of which courses each has a small concavity, and consequently forms a small vault by itself, as soon as its terminating points have their due counterpoise. Now, as the bed-faces of the individual courses of a regular pointed arch, that is, of one which is described about an equilateral triangle, recede very slowly from the horizontal line, and even at the summit make with it an angle of only 60°, the adhesion of each individual vaulting-stone of moderate dimensions, such as brick and similar stones generally have, to the layer of mortar, is sufficient to prevent the sliding of the stone before the termination of the course; and hence there is no difficulty in executing each individual course free-handed and independently, and in locking it against its counterpoise. Against each course already locked, and consequently fixed and immoveable, we may begin a new one, and so continue to the final termination of the whole vault. All that is required, therefore, is a solid resistance for the terminating points of each course. Now, such a resistance may be supplied not only by solid obstacles, as the external walls, but equally well by the reaction of a contiguous course. Hence, if the groining-ribs or diagonal lines of the

separate compartments are properly supported beneath, the courses which rest on the same point perpetually keep each other in equilibrium, and consequently no further contrivance is needed than to execute the whole courses in the individual horizontal planes at the same time, or nearly at the same time; that is, to carry the courses all the way round; consequently the process in such cross-vaulting is the same fundamentally as in domes, when each course is locked by itself as a ring, one ring is gradually laid upon another, and thus finally the dome itself is locked, except that in these domes the upper courses have steeper bed surfaces, and consequently the stones will no longer remain in their places without the application of other auxiliary means, but would slip down as soon as they were laid, if not prevented in some other way. This is now done in Vienna in a very simple manner, by means of some strong ends of rope, which are fastened above, and somewhat backwards, from the course to be vaulted, and hang down like plummets, being loaded below by some stones tied to the rope. As soon as a stone is laid, and by a moderate blow with the hammer pressed against the preceding stone, one of these ropes is brought over the stone, and the pressure produced by the weight of the appended stone, combined with the adhesion of the mortar, is sufficient to hold the stone till it is sufficiently supported by the contact of the next stone; and this in its turn is prevented from slipping down by the pressure of the cord upon it.

We very often find, however, over ancient churches, crossvaults, where the diagonal lines consist of semicircles, and consequently the transverse and longitudinal lines, and also the lines of subdivision in compound vaults, form somewhat depressed pointed arches, of which the radii are usually threefourths, but sometimes only two-thirds of the diameter ;—here the same difficulty occurs in the upper courses, and probably has been met by the same or similar methods. We sometimes observe also a back-vaulting of those courses, of which we shall have to speak again in describing the locking of the vault.

The only difference between these old cross vaults and the usual ones, consists in this; that the latter are formed by the

motion of two horizontal straight lines over four arches set opposite two and two, and consequently all the horizontal lines in the vault are straight lines; and each course must be constructed as a vault in an upright position, so as to support itself freely, which requires a very careful shaping of all the individual stones, and a considerable thickness; whereas in the ancient vaults no horizontal line whatever occurs, and each course is laid in a line somewhat curved outwards; and consequently incomparably less thickness and less labour is requisite, and yet a far stronger arch is produced. A single cross vault of the first kind, according to the profile Fig. 1, cut

horizontally, in the line a b, forms a rectangular composition of straight lines, as Fig. 2, while the other, according to Fig. 3, consists of curves, which push with their extremities e e against the outer walls; and, resting with their other extremities. at d d in the diagonal lines upon two centerings which cross each other, keep each other mutually in equilibrium, till by closing up, or locking the vault, complete diagonal arches are obtained, which now support themselves, and enable us to remove both the centerings.

The way in which the separate divisions are closed up in the old vaults is represented in Fig. 4, and the last courses are generally back-vaulted; that is, the joints are flatter and not so steep as those which the regular joint-section requires, and as the above derivation of the forms of the stones would produce. At the same time, the two sides of the pointed arch

unite in a point, as in a simple-pointed arch, at the ribs or groins only, and not in the intermediate spaces. In the latter portions they form rather a very acute ellipse; and the wallscutcheon also has this form not unfrequently, indeed almost universally, where there is not introduced in this part some ornamented band or moulding. Hence the vertex of these intermediate compartments is higher than the vertex of the diagonal ribs, and forms a flat arch from their point to the wall, as is manifest from the section in Fig. 6.

The portion of the wall, bounded by the arch, resembling an inverted escutcheon.