MOiiTAB: 



MORTARS. 



778 



it may suffice to state, that the chalk lime 13 -usually mixed with 3 

 portions of sand to 1 of lime ; the semi-hydraulic graystone lime, from 

 the chalk-marl, with 2 sand to 1 lime ; whilst the blue-lias lime is 

 mixed with proportions of 1 4 sand to 1 lime. 



The operation of ordinary mortar making depends for its success 

 upon the energy and the stability of the crystallisation of the lime ; for 

 the sand simply acts the part of furnishing a nucleus around which the 

 crystals may arrange themselves, and unless it should happen to contain 

 a notable proportion of silica in the soluble state it will remain chemi- 

 cally inert. Now the pure carbonates of lime yield a caustic lime, 

 which is very easily soluble in water, and which does not crystallise at 

 all when kept in large masses ; and the only lime compounds able to 

 solidify under water whether salt or fresh are those which contain 

 the various proportions of silicate of alumina, described in the article 

 LIME. It is on this account that in countries, such as Holland, or 

 Central Italy, where the natural limestones are remarkably pure car- 

 bonates, it is customary to introduce into the mortars to be employed 

 in hydraulic building the trass and puzzuolanos obtained in the same 

 districts, because those materials are composed of the dehydrised 

 silicate of alumina, which when ground fine will combine chemically with 

 the pure hydrated carbonate of lime. It was supposed that any 

 ilfliydriaed silicate of alumina would produce this result, and the 

 French and German engineers, following the occasional practice of the 

 ancient Romans, actually substituted underburnt-brick dust for the trass 

 or puzzuolano, in some marine walling, and some canal works. But it 

 would appear that peculiar conditions of the silicate of alumina are 

 requisite, at least in the case of mortars intended to resist the action of 

 sea-water ; because in almost every instance the mortars composed of 

 pounded bricks, lime, and sand, have disintegrated after a short ex- 

 pi >sure to the sea; those which were only exposed to the action of 

 fresh water have resisted more successfully, and it is notorious that the 

 Roman mortars possess a very extraordinary hardness and durability, 

 even at the present day. Without dwelling upon these still rather 

 obscure points in the theory of the action of limes, it may suffice here 

 to observe that the above considerations point out the nature of the 

 mortars required for particular positions, or for particular works. 

 Thus, the mortars required for hydraulic works in sea-water must be 

 composed of the most energetic cements, and the latter must even be 

 used without any mixture of sand, if the mortars are intended to be 

 at once exposed to the action of the sea ; if, however, the mortars can 

 be protected for a sufficient length of time to allow the crystallisation 

 to take place effectually, the cements may be mixed with sand, or even 

 the best hydraulic limes may be used. For river work, hi non-tidal 

 rivers especially, the best hydraulic limes should be used ; and for all 

 exposed walling, the common descriptions of hydraulic lime may be 

 resorted to ; whilst the purer limes should be reserved for internal 

 works, aud for surface renderings of small thickness. 



In the preparation of the various mortars the first operation to be 



fierformed is the slaking of the lime or cement. In the case of the 



latter class of materials, which are always ground before being used, 



the only precaution to be observed is to avoid bringing them into a 



fluid state ; or, in other words, it is advisable only to add enough 



water to bring them to the consistency of a tenacious modelling clay ; 



especial care being also taken to moisten the stones or bricks to be 



tised simultaneously with them, so as to prevent any absorption oi 



moisture from the cements. In the case of the hydraulic limes, when 



without being previously ground, the slaking is performed either 



in pit or by aspersion, or by immersion ; and the object to be 



nied is to cause the lime to fall into a fine dry powder, in the 



state of the hydrate ; which powder, however, would not possess the 



power of crystallising without the addition of a further quantity of 



water. This additional quantity of water is added at the same time 



as the sand, introduced for the purpose of affording nuclei arounc 



which the crystals of the lime may arrange themselves; and it is 



indispensable that the whole of the lime in a mortar should have beer 



thoroughly slacked before it is placed in the work. At the presenl 



day, in all large building operations the mortar is made by means o: 



i ILLS, or mortar-nulls, the latter being formed of large stone or 



iron wheels revolving in hollow basins ; and when those machines are 



rly used the mortar is more uniform in quality, and more free 



from core than in the ordinary process o manufacture. The core is 



in fiict, the under-burnt lime, which slakes, when in lump, less 



qmckly than the rest of the lime, and is therefore exposed to take on 



tion of expansion 'at a different period; this action would 



perhaps, disturb the crystallisation of the rest of the work, and grea 



pi" nations are required to prevent its occurrence ; it is known amongs 



" workmen by the phrase of " the lime blowing." The pure rich lime 



are more exposed to this class of action than even the hydrauli 



ones ; because in the process of calcination some portions of the limt 



stone become not only under, but also otiiers become mer burnt, am 



the periods of slaking of the core, produced by either of thos 



causes, differ materially from that of the normal lime. Great car 



is therefore required in slaking the rich limes, and the old Roman 



seem to have adopted the custom of slaking them at least three year 



rule; a far greater quantity of water mai 



('tl with these lime* than with the hydraulic varieties, and indeec 

 the London builders, when about to employ the ordinary chulk lim 

 for phwtcring purposes, actually run it, as they say ; or they rcduc 



to a fluid state and run off the liquid into a tank, where the excess 

 f water is allowed to evaporate. This operation of running would 

 ffectually destroy any of the hydraulic limes, and it is important 

 ith the latter only to present the quantity of water strictly required 

 or its hydration. Unfortunately working men are so careless that it 

 s practically impossible to ensure the observance of the precaution of 

 eeping the bricks or stones so wet as that they should not be able 

 o abstract moisture from the mortar ; and it thus almost always 

 appens that, even in the manipulation of hydraulic limes, a larger 

 uantity of water is used than would theoretically be required. 

 The mixture of the sand with the lime, or cement, is either effected 

 >y hand or by means of the pug-mill; and the proportion above 

 ndicated for the various descriptions of lime should be strictly adhered 

 /o, and ascertained by actual measurement; the lime itself being 

 laked, reduced to powder, and measured in that state. In ordinary 

 iperations it may not be necessary to observe such extreme accuracy, 

 >ut for hydraulic works it is essential that all precautions should be 

 akeii ; and therefore it is equally essential that the proportions of the 

 ime and of the sand should be ascertained by some safer test than 

 he eye of the labourer charged with making the mortar. Be this as 

 t may, the sand to be used for mortar should be a clean, sharp, 

 angular, silicious sand ; free from loam, clay, and large lumps of stone, 

 ir of oxide of iron : and it must be distributed as equally as possible 

 hrough the mass. 



The mortars obtained by the use of trass and puzzuolano do not 

 contain the same quantities of sand which they would ordinarily do, 

 >ecause the trass, &c., replace it to a very great extent. In the north 

 of Europe it is customary to mix S parts of lime in powder, 3 parts 

 of trass, and S of sand for hydraulic works; and in the south of 

 iurope, 33 parts of lime with 45 of puzzuolano, and 22 of sand ; but 

 ,he value of these mixtures depends, after all, on the care and attention 

 with which the various ingredients are brought in contact with one 

 another. Occasionally other substances, such as smith's ashes, pounded 

 tiles, the roasted clay nodules of the ironstones, &c., have been mixed 

 with lime and sand in the preparation of mortars ; and they commu- 

 nicate to them some unimportant qualities of colour, or even at times 

 lome additional powers of setting, which render their use in particular 

 laces desirable, as for instance, for pointing old brickwork, or for 

 jedding chimney-pots, or other analogous details of a building which 

 are much exposed to variations of atmospheric conditions. The 

 ordinary pointing mortar is made from the purer rich limes carefully 

 slaked and prepared for the purpose. 



In the neighbourhood of Paris the gypsum, or sulphate of lime, is 

 frequently used as a mortar, for walling purposes in positions which 

 are not exposed to much humidity ; and this description of mortar is 

 often used as a temporary protection to the slower setting hydraulic 

 lime mortars in sea-water. 



MORTAR-BOATS. Under the word GUN-BOATS the present mode 

 of arming small vessels has perhaps been sufficiently noticed. The 

 increased use of shells in naval warfare, to be fired from ordinary 

 cannon, converts, in fact, every ship-of-war into a bomb- vessel ; hence 

 the term mortar-boat will not long designate any particular build of 

 vessels, although a few small craft of light draft of water are still so 

 called in the navy, but the term will merge into that of gun-boat. 



MORTARS are pieces of ordnance which, compared with guns, are 

 very short, and which are employed to throw shells or carcasses at con- 

 siderable elevations (generally at 45, but sometimes as much as 70), 

 in order that the missile may range to a great distance, and, by falling 

 nearly vertically upon the object (a barrack, magazine, or casemate), 

 may crush it by the momentum acquired in descending. Mortars are 

 either of iron or brass ; they rest upon solid beds, and the trunnions 

 or cylinders upon which they turn, in giving the required elevation, 

 are placed at the lower extremity of the piece. A mortar platform, in 

 a battery, should be very strong, in order that it may bear the great 

 recoil or shock of the piece when fired ; and it should be carefully 

 laid in a horizontal position. 



The calibres of mortars in the British service are 4?, 5|, 8, 10, and 

 13 inches. All these different kinds of mortars are used on land, and 

 the two last are also employed in the navy ; but in this latter service 

 the pieces are about 16 inches longer than the land-service pieces of the 

 same calibre, and are much heavier, the sea-service 13 in. weighs 

 101 cwt. and the land-service 36 cwt. The two first-mentioned are 

 sometimes called royal mortars. 



By varying the charge of poWder in the same mortar, it has been 

 found that there is a particular elevation which, with each charge, gives 

 a longer range than is obtained from an equal charge at any other 

 elevation. It has also been found that the elevations which give the 

 longest ranges differ much hi two mortars of different calibres, but of 

 like proportions, even when charged with quantities of powder bearing 

 the same proportion to the weights of the shells, but for the sake of 

 simplicity they are all generally fired at 45, and the tables are all 

 arranged for that elevation. 



It has been supposed that mortars were employed in the year 1495, 

 at the siege of the castle of Naples, but on no other ground than a 

 statement that artillery of considerable magnitude was conveyed into 

 Italy with the army of Charles VIII. In 1588, however, the use of 

 mortars must have been well known, since, jn the Appendix to the 



Colloquies of Tartaglia,' which was published in that year, the method 



