1845.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



217 



In the West, the Cross of equal parts was known and employed as 

 well as the other. The sarcophagi, cohimns, pillars, the stones for the 

 altar are still marked by this Cross ; the other, that of unequal dimen- 

 sions, belongs more ospecially to us. 



Although these two forms'werc originally common to both cjiurches, 

 the Cross of equal parts was eventually predominant iii the East, and 

 was called the Greek Cross. The Cross of unequal parts prevailed 

 amongst us, and was called the Latin Cross. 



In the Eastern Church the Greek Cross was predominant in tlie 

 principal and subordinate details of religious monuments, in architec- 

 ture as well as in decoration. The plan of many of the oriental 

 churches exhibited the form of the Greek Cross. The following 

 design gives the plan of a church in Greece. The shaft of this Cross 

 seems to be a little longer than the trans- 

 versa' nave, but this may possibly be an 

 error of the designer; even with this 

 error, it is a Greek rather than a Roman 

 Cross. 



The Greek Cross was that most used 

 in the capitals of the Byzantine Churches. 

 In the church of St. Demetrius at Salo- 

 nica, in those of St. Sophia at Constanti- 

 nople, St. Marc at Venice, and St. Vital 

 at Ravenna, edifices purely Byzantine, 

 the Cross of equal dimensions, whether 

 in a medallion or not, shines in the midst 

 of the volutes, the knots and leaves of 

 the acanthus. In painting, the robes of 

 St. Chrysostcm are adorned with small Greek Crosses in medallions ; 

 numerous Greek Crosses also, without medallions, ornament the cha- 

 stille of St. Gregory Nazianzen. It is a Cross of equal parts which 

 divides the nimbus of God. It was this same Cross which was borne 

 by the Knights of Malta (the descendants of the Knights Hospitallers) 

 as the distinctive decoration of their order. 

 { To be continued.) 



ARCHITECTURE IN IRELAND. 



Sir, — It is the most unlikely thing in the world that the Institute of Irish 

 Architects should take up a subject suggested by the inquiries of an indi\ I- 

 dual among the correspondents of a journal — althongh it he an architectural 

 one. The information I wished to obtain was not of a kind to require a 

 body of men to assemble in conclave in order to draw it up ; and I naturally 

 lookird to Dr. I'ulton, — he having been a frequent contributor to your Jour- 

 nal — as one both capable and ready to satisfy me off hand in regard to a 

 few matter-of-fact particulars. If my asliing for some information as to 

 what l)as been done of late years in the way of building, at Dublin, should 

 lead him to contemplate giving us an historical sketch or precis of the nio- 

 dern and recent architecture of Ireland, — so much the better, for it would 

 be quite a new chapter in general arcbiteetural history. Gwilt takes no no- 

 tice whatever of the architecture of our Sister Island. In his table of archi- 

 tects he mentions but one who belongs to Ireland by his works, though not 

 a native of it, viz. Gandon, and then he blunders by attributing to him the 

 Exchange at Dublin, erected before he settled there, and thereby robs Cooley 

 of the fame due to the author of so elegant a structure — one that deserves to 

 have been particularly noticed by Gvvilt where he speaks of that class of 

 buildings, as being a most tasteful example of the kind. I wish to ascertain 

 whether a ground plan or section of it has ever beeu published, and as tliat 

 is a simple query, easily answered, if it can be answered at all. I hope Dr. 

 Eulton will favour uie by doing so, should it be in his power. Perhaps, too, 

 he can inform me when Francis Johnstone, the architect of the Dublin Post- 

 office died, and if there exists any sort of memoir or notice of him. Ihe 

 laborious, painstaking, accurate and universal Navler, mentions neither him, 

 Cooley, nor Cassels, though his Dictionary is crammed with a host of names 

 of persons who are mere nonenties. Yet perhaps we ought not to reflect 

 upon the negligence of foreigners, when our own biographiial dictionaries 

 are exceedingly barren indeed in reeard to architectural names — I mean 

 those of our country — the United Kiiigdom. As to notices of the older 

 Italian architects and such well-known ones as Jones and Wren, they are 

 merely transferred from one work of the kind to another, by the aid of 

 scitiiirs and paste. 



I heg to sign myself as before, 



Philo-Hibermcus. 



ON THE CONSTRUCTION AND PROPER PROPORTIONS OF BOILERS 



FOR THE GENERATION OF STEAM. 



By Andrew Murray, .\.ssoc. Inst. C.E. 



(Paper read at the Institution of Civil Engineers.) 



This paper commences by investigating the quantity of air chemically re- 

 quired for the perfect combustion of a given quantity of coal, of the quality 

 commonly used for steam purposes. The amount of air to each pound of 

 coal is stated to he l.')0-3.'> cubic feet, of which -It.Ct cubic feet are required 

 for the various carburetted hydrogen gases given off, and 10j-71forthe 

 solid carbon. The practical utility, however, of this knowledge, is much 

 impaired by the circumstance that combustion ceases even in pure oxygen, 

 and much more in air, before the whole of the oxygen present has entered 

 into the new chemical combinations required. It is also known, that car- 

 bonic acid gas exerts a positive influence in checking combustion, as a candle 

 will not burn in a mixture composed of four measures of air and one mea- 

 sure of carbonic acid gas. Large quantities of this gas being generated by 

 the combustion of the solid carbon on the grate, and being necessarily me- 

 chaiMCally mixed with the inflammable gases as they rise, the quantity of air 

 required for their subsequent combustion must, on this account, be increased 

 to a very large extent. The whole of the air thus supplied in excess, must 

 he heated to a very high temperature, before any combustion can take place, 

 and the loss of the heat thus absorheil must be taken into account in calcu- 

 lating the ultimate economy of igniting these gases. 



The form of furnace now in general use, in which the fuel is spread over 

 a large surface of fire-bar, has not been subject, in so far as effects the sup- 

 ply of air through the bars, to much alteration, amongi-t the many patents 

 and proposals which have been made for the more complete combustion of 

 coal. The point most open to change in the common furnace, is the width 

 between the bars ; and as it is desirable to have the supply of air to the fur- 

 nace as abundant as possible, it should be made as large as can be done 

 without causing waste, by allowing the coal to fall through into the ash-pit. 

 A greater number of these bars is thus to he preferred to a smaller number 

 of broad or thick bars ; indeed to such an extent is this carried in France, 

 where coal is more valuable than in this country, and the chemistry of the 

 subject perhaps more generally understood, that the bars are made not more 

 than half an inch thick, the necessary strength being obtained by making 

 them 4 inches deep. AVith coke or wood, which cannot fall through the bars 

 and be wasted, in the same way as coal, the space is always made much 

 wider, and with great advantage ; so much so with coke, as to have led to 

 the opinion that a given quantity of coke would produce as much heat by its 

 combustion as the coal from which it was made. Any grounds for such an 

 opinion could only have arisen from the combustion of the coal having been 

 so imperfect, that not only had the whole of the gases passeil off uncon- 

 sumed, but even a large portion of the solid carbon must have been allowed 

 to escape in the form of carbonic oxide, without having generated its due 

 amount of heat, and been converted into carbonic acid gas. 



In the combustion of coke, or of the solid portion of coal, as left in an 

 incandescent state on the fire-bars of a common furnace, after the volatile 

 gases have passed off, the amount of heat generated by the whole of the car- 

 bon, uniting at once with its full amount of oxygen, will he the same as 

 what would be generated by its combination, first, with a smaller quantity 

 of oxygen, forming carbonic oxide ; and subsequently, by the ignition of this 

 gas, by its combination with the further quantity of oxygen, required to turn 

 it into carbonic acid gas. 



As some portion of the carbon is always cnnverted into carbonic acid gas 

 in the furnace, it folloivs, that the air for the ignition of any carbonic oxide 

 there formed, and allowed to pass into the flues, must be greatly in excess of 

 the quantity chemically required ; and the whole of this excess must be raised 

 to the temperature of the other gases, witli which it will he mingled. The 

 superior economy, therefore, of at once converting the whole of the carbon 

 into carbonic aciil, gas is apparent ; and there is no doubt hut that this very 

 desirable result may he obtained nearly to the full extent, by due care in the 

 formation, and subsequent management of the fiu'nace. 



The best mode of supplying air to the other inflammable gases residting 

 from the combustion of bituminous coal, whicli are composed of hydrogen 

 and carbon, and which will be treated ot under the common name of carbu- 

 retted hydrogen, has been a matter of much controversy, and been the sub- 

 ject of many patents. The mode proposed by the greater of the patentees 

 is, to admit the air immediately behind the furnace, at the hack of what is 

 termed the bridge. A bridge docs not exist in every case ; but where it does 

 exist, it is generally in the form of a wall or obstruction right across the 

 back of the furnace ; often placed there for no other purpose, than to pre- 

 vent the fire from being pushed back into the flue. The whole of the pro- 

 ducts of combustion, as formed in the furnace, necessarily pass over this 

 bridge, before entering the flue. The additional air is sometimes heated, 

 previously to its being admitted to the gases, after they have left the furnace, 

 and the manner in which it is supplicfl, varies exceedingly; one party advo- 

 cating its admission in a long thin film, another through a great number of 

 small orifices, and others again attach less importance to the manner of it» 

 admission, so that it is only admitted in stifiicient quantity. All these plans 

 proceed upon the supposition that large quantities of ir flammable gas pass 

 off from the furnace, and as none of them directly aftect the operations going 

 on within the furnace itself, the gases which are actually given off, would he 

 the same until they pass over the bridge, whichever plan might be adopted. 



