We remove beams when rotten without injury to the terrace. The wall-hold is generally six inches less than the thickness of the wall. 8th. In the thickness of the wall, resistance to heat is not considered. Strength is alone considered. We cannot, as I remarked before, use any bond timber or ties of any kind on account of the destructive vermin. The bricks contain much sand, salt, alkali, and other fusible matter, and will vitrify before they are well burnt. You have seen many walls thicker than the dimensions I have given, but those are built with brick and mud, and having no cement require to be thicker. 9th. If you begin your work early, so that it will be completely dry, it will resist any frost, but if any moisture remain within, the frost will rend the work. 10th. Add a little strength to your timber, make the parapet low, take care before a thaw to throw off the snow, keep the spouts open, and it will sustain double (or more) the weight you mention. 11th. The floors the same as the roof, but a little lighter, and not so much cove. - 12th. Stick to the common and ancient rules of architecture in all cases; but doors and windows, make them much larger. 13th. Water cement is made of brick dust, lime, and the juice of the sugar cane. The Madrass plaister is as I before described ours. 14th. Stone lime is cheap and used for common purposes, shell lime is dear and only used with fine work; I believe it is no better than your own. 15th. No further than I have before described. 16th. These facts do come within my knowledge, and are true. It forms a crust impervious to water, and must protect any thing it covers. When dry, it will keep a ship afloat after her caulking is perished and loose. Much oil is saved in making this article by bestowing labour on the beating and mixing of it. 17th. Where manual labour is an objection I have stated it. E e No. LVI. Observations on the foregoing communications, by B. Henry Latrobe, Surveyor of the public buildings of the United States, and one of the Committee to whom it was referred by the Society. Copying the English standard, the bricks of the United States are very generally made 8 inches long, 44 inches broad, and 24 inches thick; so that in the wall with the joint, they shall take up nine inches in length, and half as much, viz. 41, in breadth; but the various degrees in which different sorts of clay shrink in drying and burning, occasion here, as well as every where else, variety in the size of the bricks; and I have scarcely ever known bricks, from two different kilns in the same city to work correctly together. The cupidity of the brick-makers contributes also to the diminution of the size of bricks in Philadelphia; a wall two bricks thick seldom measures, with the joint, more than 17 inches; a brick-and-half wall, barely 13 inches, and 5 courses in heighth, with the joints, measure one foot. This gradual diminution in the size of bricks is rather encouraged, than counteracted, by the interest of the bricklayers :-for, as it is the general practice for individuals, as well as public bodies, to find all their materials, and to pay the mechanic only for the labour, and as it is a very general practice to pay the bricklayer by the 1000 bricks, according to the brick-maker's account; or to count them on the outside of the wall, where they all pass for whole bricks and lie closest, it follows that in a given mass of wall, the small bricks, upon the whole, tell better than the large ones; and in both cases, especially the first, the bricklayer is not interested against admitting small bricks to be made. - On the other hand, the brick-maker in burning his bricks as well as in selling them by count, is benefitted; for small bricks can be burned at less expense of fuel than large bricks, and are less liable to warp and break. I am of opinion that great advantages would result from making our bricks larger than the usual standard; not only in the saving of labour, but of mortar, which here, as in India, is the most expensive part of the wall. The width of a brick should not be greater, than that a man can very easily and conveniently grasp it; and although Mr. William Jones has not given information as to the width of the Calcutta brick, (which is of more importance to the workman than its length) I am of opinion that the best possible size of a brick is the following, 11 inches long as in Calcutta,` 5 wide 2 thick. when burned. Such a brick would add 2 inches to our single brick walls, and in most cases permit them to take the place of walls now built of 1 bricks. A brick-and-half wall, in the fronts of our middling houses, would give room for the window-dressings and shutters; and, in a two-brick wall, there would be no necessity of making thicker the walls of our best houses for this purpose. This is not the place to enter into further details. Practical builders can easily investigate the results from such a change in the size of our bricks; but it will be difficult to be effected, while the astonishing increase of our buildings gives to the brickmakers such an influence over all our building operations. 1 Use of brick dust in mortar. In his answer to the 8th Query, Mr. William Jones has the following remark : The bricks contain much sand, salt, alkali, and other fusible matter, and will vitrify before they are well burned. We might then consider the brick dust, made by pounding the bricks of Calcutta, as so much sharp sand, and as having lost that contractility which clay unvitrified by the admixture of vitrescible substances, and unhardened by fire, universally possesses. In this state, it might be an unobjectionable ingredient in cement in America. But as it is evident from the process of laying on the chunam internally, and also from the beating required upon the materials of their terraces, that their brick dust is not generally in this hardened state, when mixed with the mortar, but that it continues to contract, and to require forcible compression by beating or rubbing until it is quite dry, it becomes, on more than one account, a very noxious as well as a very inconvenient ingredient in all cements, to be used where there is frost, and where labour is dear. I should be obliged to write a voluminous treatise on this subject, were I to submit to you all that my experience, as well as my reasonings suggest, to counteract the prejudice in favour of the use of brick dust in cements north and south of the tropics. Its utility beyond the reach of frost, I need not examine. It would be useless to establish or to refute it in our region of severe winter. I will therefore only endeavour to comprize, in as small a compass as possible, what may be useful to our own citizens. Belidor, Blondel, Sturm, Smeaton, Higgins, Adams, and many other French, German, English and Italian writers have all recommended brick dust in some cement or other. I have none of their works at hand, so as to refer to their receipts or their experiments, and no doubt their cements have possessed all the qualities ascribed to them, when the brick dust has been prepared of well burned bricks. I have also seen brick dust employed by engineers and architects whom I have personally known, and have employed it myself; but I do not recollect a single instance of the cement in which it has been used having resisted the effect of moisture and frost. Natural argillaceous stones are more apt to be forced to pieces by frost than any others. Bricks not sufficiently burned are always destroyed by frost. The effects of frost on the natural clay of the earth is well known,-it renders our roads almost impassable in spring. It seems therefore, to plain sense, a conclusive argument against the use of this material in cements, that wherever we see it present in any natural or artificial production, its dissolution by frost is certain. The freestone of Acquia, however, appears to be sand cemented by an alluminous (argillaceous) infusion: Some of it is dissolved by the frost, but the best stone resists it most perfectly. Water oozing through this stone covers the face of the rock with allum. I have not been able to detect in this sand stone any particle of calcareous matter. Its smell when moist is strongly earthy. See my memoir in the Philosphical Transactions, on this stone, page 283 of this Volume. If however the clay be hardened by being converted into a vitrified, or otherwise solid brick, then indeed it ceases to be under the dominion of frost, and is at all events, I should suppose, as good as so much sand. It remains to be enquired by chemical investigation, whether some affinity between clay, thus hardened, and lime does not exist, which expelling in their union their caloric, combines the two substances more intimately and in a smaller compass, than that of lime mixed with sand; and, of course, gives to the compound more hardness, and permanent continuity. If such affinity does exist, which I will not deny, such brick dust is so far superior in quality, as an ingredient of cements, to sand. But it is, I think, far counterbalanced by its other quality of infinite contractibility and expansion. Clay, in its purest state, is used in Wedgewood's pyrometer, on account of this very quality, which it appears never to lose; and from thence arises the perfection of this most useful instrument. When the cement, of which brick dust is an ingredient, is laid on in a moist state, it occupies in some cases (for I have last year had much unpleasant experience of the fact in the floors laid in Deniroth's cement at Washington) more space than when dry. On a wall exposed to heat, or upon a timber floor accessible, and at first pervious to the air, there appears to be a limit to the contraction of the cement. But in a heavy vaulted building, like the Capitol of the United States, at least, the moisture of which evaporates slowly, I would reject brick dust altogether as an ingredient of any kind of cement, either for mosaic floors, terraces, or facings. Full justice appeared to be done by the contractor and patentee, in beating his floors both as to time and labour, but after a year's drying they have cracked ́ into innumerable fissures. From what I have said, it will be evident, that I consider brick dust as an ingredient in cements, inapplicable to our climate and of course useless. Good clean washed sand, and stone lime, in the proportion of three of sand and one of lime, up to six to one, according to the size of the particles of sand, and the goodness of the lime, is a ccment that will never fail, if well mixed and worked, and laid on as soon as possible after being mixed. The lime in slack |