give the best evidence of the value of the invention in that respect, and from the vast facilities of executing the most ponderous and acquired forms, the saving of time and finish which can be attained under such a hammer, will also prove that a great step has been made in the mechanical arts.

In conclusion, it may perhaps be as well to remark on the valuable and important influence which such a hammer will have upon the quality of iron, as in the case of boiler plates and such like, the quality of which, as regards soundness, entirely depends on the efficient manner in which they have been hammered and consolidated in the primary process of faggoting, or shingling, namely, the forming into one perfectly solid mass, the block of iron from which such boiler plates, &c., are rolled. Nine-tenths of the defects which are met with in boiler plates, and which have caused such disastrous results, namely, defects from blisters, have arisen from, or may be traced to, imperfect consolidation, resulting from inadequate means of hammering the original mass into a truly solid block, by our not having the power to Fig. 3.

force out all the scoria, which, otherwise, lodging between the pile of pieces of which the faggot is composed, gives rise to the most serious desects, which every practical man has had to deplore. It will, in like manner, be scarcely requisite that I state any of the advantages that will arise in our having, by means of the energetic action of the steam hammer, a perfect security against unsound anchors, the importance of which requires no words to set forth. In short, we have now at command an almost new power,

inasmuch as, by means of this steam hammer, we have an accession to our means of dealing with power in the form and state of percussion, such as has never been attained before, and that in the most simple, straightforward, and effective manner.

Fig. 3 shows the application of the hammer A for forging an iron shaft laid over the anvil, or block, B, and is made self-acting, as will be seen by a reference to the cut, that when the tappets D D come in contact with the pin, or spring, on the block E, the steam valve C is opened, or closed.

1

B

pans, &c.

Fig. 4 shows the application of the steam hammer for coppers,

The hammer M works in the guides P P, suspended by the rods R to the beam above, like an inverted truss; the action of the man pulling down the lever N opens the valve, so as to admit the steam for raising the piston, and, with it, the hammer.

I may remark, that one boiler can be made to work any number of steam hammers, as the steam has only to be conducted to each by pipes, and the power let on and shut off in the same manner as gas; and in most iron forges, the waste heat of the furnace will more than furnish the requisite steam. There are many other applications and details connected with this important invention; but reluctance to further trespass on your readers' attention, and the space of your columns, causes me to defer to a future opportunity.

But I trust the high importance of the subject will plead my excuse for the length to which I have allowed my remarks to extend. With most sincere respect, I am, very truly, yours,

JAMES NASMYTH. Bridgewater Foundry, Patercroft, Jan. 17.

Civ. Eng. and Arch. Jour.

The Practice of Fresco Painting.

(Continued from page 134.) Methods of Fresco Painting, described by writers on Art. The observations on the practice of fresco-painting by early writers on art coincide generally with the statements above given; the only point on which those writers do not appear to insist is the necessity of keeping the lime for a very long period. In other respects, Cennini and Leon Battista Alberti, in the fifteenth century; Vasari, Armenini, and Borghini, in the sixteenth century; Andrea Pozzo in the seventeenth; and Palomino in the beginning of the eighteenth, describe, more or less fully, the same process. But before referring to these writers, it may be desirable to take a glance at the ancient authorities who have described the modes of preparing walls with stucco on which fresco-paintings were executed.

Vitruvius suggests that where there is danger of damp affecting the coats of plaster, a thin (brick) wall should be carried up within and in some measure detached from the main wall.* When timber

partitions were to be covered with stucco, two layers of split reeds were nailed with broad-headed nails on the upright and cross pieces, the one vertically, the other horizontally; “ the double row of reeds thus crossed and firmly fixed prevents all cracks and fissures." The coats of plaster, from the rough-cast to the finished surface, were numerous, namely, after the rough-cast, three of sand and lime, and three of marble-dust and lime. The last coat was often highly polished. “ When," Vitruvius afterwards observes, “ only one coat of sand and lime and one of marble-dust and lime are used, the plaster is easily broken and cannot receive a brilliant polish.”

When frescos were added, the surface was necessarily somewhat less smooth.

The passage that follows, relating to paintings on walls, has been • De Architect, 1 7, c. 4. This is the mode in which the stuccoed and painted walls of Pompeii are constructed; the bricks, or rather tiles, are placed edgewise, and are connected by leaden cramps to the brick or tufo wall, without being in immediate contact with it.

often the subject of controversy, but when compared with the practical details of fresco, already described, it can hardly fail to be understood as referring to that method. The ancient writer's mode of accounting for certain effects is, of course, unimportant. “Colors," Vitruvius observes, “when carefully applied on moist stucco, do not therefore fade, but (on the contrary) last forever; because the same having been deprived of moisture in the kiln, and having become porous and absorbent, readily imbibes whatever (moisture) comes in contact with it; and the whole, when dry, seems composed of one and the same substance and quality. Hence stuccoed walls, when well executed, do not easily become dirty, nor do they lose their colors when they require to be washed, unless the painting was carelessly done, or executed after the surface was dry.” The general evenness of the wall is here explained to be essential to the due effect of the paintings: the opposite evil, that of an undulating surface, on which dust lodges irregularly, is seen in some of the frescos of the Vatican.

This general evenness of the plaster does not suppose unpleasant smoothness of surface in the fresco; in many Italian, and indeed many antique, mural paintings, the traces of the brush often indicate a considerable body of color; but care seems to have been taken not to load the surface unequally. In a London atmosphere this comparative evenness of the surface might, on the Vitruvian principle, protect the painting longer from smoke and dust, while it would assist the operation of cleaning. But the work might be protected by other means; the plaster might be applied so that the face of the wall-at least in the portions intended to receive frescos-should not be quite perpendicular, but incline a little inwards (with reference to the room) towards the upper part. In connexion with the question of surface, it may be remarked that the hardening of the lime takes place sooner in proportion to the roughness of the surface. In plate 2 of Smith's translation of Vicat (“Résumé sur les Mortiers et Ciments Calcaires') will be found representations of sections of lime a year old, exhibiting the progress of the carbonic acid and the comparative redintegration of the original carbonate of lime. Captain Smith remarks (p. 173) “ It would be difficult to credit, did we not see it, how great an obstacle a smoothness of surface presents to the penetration of the carbonic acid.”

Leon Battista Alberti copies Vitruvius in many points: he observes generally that the more coats a wall receives the better the surface may be polished, and the longer it will last, and speaks of ancient examples in which there were nine successive coats. He alludes more directly to the practice of his own time when he says that no stucco should be composed of less than three coats :* these he afterwards describes. “ The first rough coat," he observes, “ should

• He is still so far true to the Vitruvian rules, that he speaks of each layer in the plural, as if the number of coats was indefinite. His Italian translator (Cosimo Bartoli, 1550,) reduces these half classical directions to the practices of the day, and gives the Florentine technical terms for the general expressions of Alberti; the rinzaffato rough-coat, the arriciuto sand coat, and the intonaco (tunica) fine plaster.

be composed of pit sand and pounded bricks; the pieces of brick should not be broken too small. For the second coat river sand is best adapted, and is less apt to crack; this second coat also should be somewhat rough, because nothing that is applied to a smooth surface will adhere to it. The last coat should be as white as marble, in fact pounded white marble should be used instead of sand. This coat need not be thicker than half a finger's breadth, some make it no thicker than the sole of a shoe. In many places,” he proceeds, 6 we find nails fastened in the wall to keep on the coats of plaster, and time has shown that they had better be of bronze than of iron. Instead of nails, I much approve the practice of inserting thin pieces of flint, projecting edgewise from the joints of the stone ; these should be driven in with a wooden mallet." Various directions fol. low, partly derived from Vitruvius, partly from his own experience. Speaking of colors that are fit and unfit for fresco, his expressions are at once in accordance with an ancient authority,* and with modern practice; in this as in other instances Leon Battista Alberti appears as the connecting link between ancient and revived art. He speaks of the “newiy-invented art of painting with linseed oil,” as calculated to last for ever on walls, provided they are perfectly free from damp; on this subject he could of course have no experience. He concludes by observing that he had seen even fresh lime painted with colors prepared from vitrified substances. Cennini, who has recorded the old Florentine methods, states that “both the lime and the sand should be well sifted. If the lime is what is called rich lime, and has been recently slaked, there should be two parts of sand to one of lime.t On being slaked it should be well mixed and stirred, and a quantity should be made, sufficient to last for 15 or 20 days. It should then be suffered to remain for some days, in order to render it less caustic, for if too caustic, the intonacos will blister.” The mortar composed as above serves for the first coat, the surface,

• Pliny (l. 35, c. 7) observes that certain colors, which he enumerates, are unfit for fresco (udo), but may be employed on a dry ground of gypsum (cretulam). So elsewhere (l. 33. c. 13) speaking of an artificial blue, he states that it would not stand on lime,“ usus in cretâ, calcis impatiens.” Andrea Pozzo observes that all colors may be used on a ground of gypsum ; the word creta or its diminutive is probably to be understood here to mean gypsum; the similar Italian word is often employed in this sense. Sir Humphrey Davy observes, “the ancients were not acquainted with the distinction between aluminous and cal. careous earths, and creta' was a term applied to every white fine earthy powder.” (Philosophical Transactions for 1815, p. 112, note.) The precise meaning of creta is, however, here less important; the above passages of Pliny, together with that before quoted from Vitruvius, are sufficient to establish the fact that the ancients painted on moist lime. The analy. sis of some antique paintings by Sir H. Davy, confirms this.

† This is the general proportion mentioned by the ancient writers, (Cato, Vitruvius, Pliny, and Palladius,) and appears to be now commonly in use. According to some modern au. thorities, the proportion of sand (for general purposes) may be very much increased with advantage; see Higgins, Experiments and observations made with the view of improving the art of composing calcareous cements, &c., London, 1780,” p. 51. But Vicat, by a series of accurate experiments, ascertained that “the resistance of mortars made from very rich limes slaked by the ordinary process, increases from 50 to 240 parts of sand to 100 of lime in stiff paste, and beyond that decreases indefinitely.” (Résumé sur les Mortiers et Ciments Calcaires, p. 51.) Thus two parts and half of sand to one of rich lime are already beyond the due proportion.

Cennini mentions two coats only, and applies the term intonaco to both.