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without much regard to their goodneis; andperhaps there is no manufacture in which thisis fo remarkably exemplified as the preparationof common mortar, efpecially in London andits neighbourhood.

One radical fault is the ufe of chalk inltead ofdone lime, for although chalk when perfectlyburnt is equally good as the hardeft lime, yetit poffeffes two capital difadvantages, firft, itwill fall into a coarfe powder on the applicationof water, when it is only partially calcined,which ftone lime will not; and fecondly, thecores or unburnt lumps may be broken down bya blow with the fpade, and are therefore veryfeldom rejeted as they ought to be. Thewhiteft chalk is the word of all, both as beingthe fofteft, and as being a fat lime, that is, con-taining no clay and iron, in confequence ofwhich a larger proportion of fand may be incor-porated with it, and the mortar becomes lefscompact. The yellowilh grey chalk on theother hand is the bed, fince it is fomewhatharder than the former, and being a meagre lime,containing a portion of clay, it will not admitof fo much fand. The Dorking lime which isthe bed of thofe ufed in London , is a chalklime of this defcription, and contains about T yof clay.®

Another effential fault in London mortar isthe bad quality of the fand with which it iscompofed. This is pit-fand, but very differentfrom the kind recommended by Pliny and Vi-truvius ; indead of being clean, large-grained,and {harp, it is of a dirty brownifh grey colour,being foiled by a mixture of clay, and is com-pofed of fmall round grains. Its finenefs andimoothnefs cannot be amended, but by wafhingit well in running water the clay might un-questionably be got rid of, and this would be notrifling improvement; for Smeaton has fhownby direct experiment that mortar of the bedquality when mixed with a fmall proportion ofunburnt clay never acquires that hardnefs anddrynefs which without this addition it wouldfpeedily have attained.

The method of preparing common mortar isalfo extremely imperfect. The lime being flackedby the addition of water, and the unburnt limebeing broken down and mixed with the reft, alarge quantity of dirty fand is added, and thewhole being incorporated by means of a fpadeis reckoned to be fit for ufe; thus the principalpoint in the making of mortar, namely, beatingthe ingredients together fo as to mix them tho-roughly, is flurred over in a bafty carelefs man-

ner, and the refult, as might he expected, is acrumbling mafs fcarcely fit for uf e . The Romanbuilders, on the other hand, after they hadmixed together the materials, employing for thispurpofe a fmaller proportion of water than iscuftomary at prefent, put the mafs into a largewooden mortar, and beat it till it ceafed to ad-here to the heavy wooden or iron peftle whichwas ufed on the occafion. A praftice this whichhas been long followed by the Dutch with com-pleat fuccefs, as will be Ihown in the nextfeftion, and the high utility of which is alfoproved by Mr. Smeaton, in his hiftory ofEddyftone light-houfe.

Frefti made mortar, if kept under ground inconfiderable maffes, may be preferved for manymonths without injury; and the older it is be-fore it is ufed the better, the builder taking theprecaution to beat it up afrefli previous to ufingit; for it not only fets fooner, but acquires agreater degree of hardnefs, and is lefs apt tocrack. Pliny informs us that the ancient Ro-man laws prohibited builders from ufing mortarthat was lefs than three years old, and to thiscircumftance he exprefsly attributes the remark-able firmnefs of the oldeft buildings in the city.A fimilar law prevailed, and, we believe, ftillprevails in Vienna , requiring the mortar to be ayear old before it is employed. But there isnothing which {hows in fo ftriking a point ofview the advantage and neceflity of beating mor-tar, and that the effect produced is owing tofomething more than a mere mechanical mixtureof the ingredients, as the preparation of grout orliquid mortar. This differs from common mortaronly in containing a larger quantity of water,fo as to be fufficiently fluid to penetrate thenarrow irregular cracks and interftices of roughftone walls, and is generally made by dilutingcommon mortar with water either cold orhot. It not unfrequently happens that thisgrout refufes to fet, and at all times it is a longwhile in acquiring the proper hardnefs: but ifinftead of common mortar, that which has beenlong and thoroughly beaten is employed, thegrout will fet in the fpace of a day, and foonafter acquire a degree of hardnefs much fupe-rior to what is made in the common manner.

§ 2. Water Cements.

Although a wed made mortar compofedmerely of fand and lime, if allowed to dry, be-comes impervious to water, fo as to ferve forthe lining of refervoirs and aqueducts, yet if thecircumftances of the building are fuch as torender it impracticable to keep out the water