THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



252 



^. considered more as dependent on situation. The proportion of 

 ?ar s 1 s Serally been estimated in parts of the span or chord, and 

 r'^ up'toi, has been stated as the proper depth for the vousso.rs 

 I^'tK'c own ; apex-the thickness of abutment i of the span, and of 

 the piers -. The rise or versed sine should not .n any case be less 

 han of U e chord. When the height of the piers exceeds the nse 

 of th; arc an addition of i (if their size be estimated by the previous 

 ind.rd) i made to their thickness, and .-hen the span of the arch 

 excetds 50 feet, it is usual to increase the voussoir as the abu ment 

 or ke vback i approached. The success that has attended Mr. 

 Brune • bold experiment of the Maidenhead Bridge, carried into suc- 

 Sssfu execution amid the sneers and jibes of his -mpeers, >n the 

 ppl cation of brick to a span for which, a few years ago - ^t-e^av e 

 granite was considered competent, augurs favourably or the use ot 

 f ct?of brick to an extent never before contempU^ted, e^pec^l y 

 ^vhere hieh banks offer the additional inducement to try the ettect, 

 he being then no approaches to make. The Maidenhead Bridg 

 s turned ^in half-brick rings in cement, 5 ft. 3 in. in depth at the 

 IS vmnea m^ 6 sorineine; the two main arches are 



S:;".i'l£S ^a^ wUhTri^ % of 24ft.3in.; the land 

 arch re 21 ft. and Is ft. span, there being six of the latter on one 

 side and two of the former on the other. The immense mass of - ft. 

 iHepth of brick in the arch is, in my opinion, the true source of 



''The-proper position of the joint in complex figures, such as the 

 ellipse, parabola, and cycloid, is a good exercise in geometry; the 

 posUion of the joint should be perpendicular to a tangent at that point, 

 n confident that arches of stone might 1^^ ^^^'-^^'f^^'^^^f 

 their present extent as regards span; there would be no danger o 

 compression of the materials, as the cohesive power per square i h 

 of granite, sandstone, and Bath stone, are respectively 2bUU. 8Jb, and 

 755 lbs, the weight per cubic foot of each material being 1/3, 144, and 



92 lbs. Q ^_ 



Newcastk'upon- Tyne. 

 The following works have been consulted :-The C. E. & A. Journal; 

 Elmes on Architecture; Woodward on the Wisdom of the Egyptians; 

 Sir James Hall's Origin of Gothic Architecture; !^tuart ^ f'^^^^ 

 Athens, and Stuart's Dictionary ; Papers by Stukely, Gunn, and b.r H. 

 Wotton, in the London Archsologia. 



NOTES ON STEAM NAVIGATION. 



[AususT, 



Holding down bolls.— Those which go through the bottom of the 

 ship, if of iron, are verv quickly eaten away. It is a usual and bene- 

 ficial practice to tin them before they are inserted, but even in this 

 case thev do not last very long; the tinning is probably in most cases 

 iniured in driving the bolt : zincing would be better. When the vessel 

 has much rise of floor, and the keelsons are consequently very deep, 

 and the bolts therefore very long, it is often very inconvenient to get 

 the bolts introduced at all, especially if the vessel draw much water ; 

 for the blocks upon which the vessel has to be placed in the dock 

 require to be very high, and when high there are few docks in which 

 there is a sufficient rise of water to enable the vessel to surmount 

 them It has, therefore, been found expedient in some cases to make 

 the holding-down bolts with a nut at each end, and to reeve them from 

 above ■ but in such cases it is almost indispensable that the bolts 

 should be of copper or Muntz' metal. If of iron the threads of the 

 screw would be soon worn away, or the nut would get so fixed by 

 corrosion that it would be very difficult to get the bolts out when they 

 required to be renewed. Holding-down bolts of the common descrip- 

 tion last from three to five vears, but if made of copper or Muntz' metal, 

 they will last as long as the ship. Those holding-down bolts which 

 go through the keelson merely with a notch cut upon the under side 

 of the keelson for the reception of the nut, waste away more quickly 

 than those which go through the ship's bottom. As a general practice 



the best plan is to make twelve bolts go through the bottom in each 

 engine, four at the crank framing, four at the cylinder, and four at the 

 main centre ; and these bolts should be of copper or Muntz' metal, with 

 screws at both ends. The rest of the holding-down bolts should be 

 of iron, and should not go through the keelsons, but be screwed into 

 them, as wood screws, by means of a square neck, and be furnished 

 with a screw and nut above. It is of course necessary that the threads 

 of the bolts screwed into the wood should be wood screw threads, and 

 the bolts themselves should have considerable taper. 



Expansion Faim.— Slide expansion valves are, in our opinion, the 

 best, yet they are very little used; probably from the difficulty of 

 obtaining a good combination for altering the degree of expansion ; 

 yet such a combination is, we conceive, attainable enough, and there 

 are already several exquisite contrivances of Mr. James Whitelaw's 

 which go far towards the removal of the difficulty. Of the double- 

 beat description of expansion valves, that description is extremely 

 objectionable which has two discs of brass upon one spindle, fitting 

 into the upper and lower valve seats, situated in an upright pipe. 

 Such valves can never be tight except by accident:— if tight when 

 cold, they cannot be tight when hot, for the spindle being surrounded 

 with steam, will expand more than the pipe in which the seats are 

 situated, which has steam only on one side, and which is, moreover, 

 generally of cast iron, the spindle being of brass. The double-beat 

 Cornish valve is the only species of double-beat valve that should 

 ever be employed. Messrs. Fawcett, of Liverpool, have, in some 

 cases, applied a single-beat valve for expansion, with perfect success. 

 It would, of course, be extremely difficult to open this kind of valve, 

 but for the combination of levers, or rather the principle of the com- 

 bination, used in Mr. Watt's original valve gearing; but Messrs. 

 Fawcett have greatly simplified that arrangement, the proper action 

 being produced by means of a single crank. 



Heat.— It cannot be doubted that combustion would be rendered 

 much more perfect in steam engine furnaces if the incandescent fuel, 

 instead of being surrounded with water, which, by a rapid absorption 

 of heat, depresses the temperature of the fire, were surrounded by 

 some substance which would prevent the heat from being abstracted 

 by conduction, therebv rendering the heat of the fire the greatest 

 possible under the existing conditions. But it becomes a question 

 •.vhether if by lining the furnaces with fire brick, and thereby pre- 

 venting the heat from being carried immediately from the furnace 

 into the water, the same amount of beneficial effect would be obtained 

 as by the present practice ; and this question resolves itself into the 

 stiU'more abstruse one whether heat is a material substance or a mere 

 property of matter. If heat be a material substance, whatever heat 

 is not given out in the furnace will be given out in the flues, supposing 

 the temperature of the chimney to be uniform ; but if it consist m 

 mere vibration, either in the particles of bodies or any etherial fluid 

 pervading them, we may suppose the possibility of those vibrations 

 'o counteracting one another as to subside or cease without the pro- 

 duction of any adequate effect upon material substances. Sound, it is 

 well known, consists in the vibration of the air; and it is further 

 known that two sounds may conspire to counteract each other's effects, 

 and actually produce silence. Two rays of light, in like manner, may 

 be so managed as to produce darkness; and mny we not, in like man- 

 ner, imagine the possibility of two rays of heat producing absolute 

 cold ' It is not impossible that in all furnaces such neutralization may 

 take place, in a greater or less degree ; and it is not at all consequen- 

 tial that bv making the combustion more perfect in the manner we 

 have indicated, a greater available heat will be produced. The subject 

 is indeed involved in much uncertainty : it is a very important subject, 

 and we beg leave to recommend its investigation to those who have 

 time and opportunity for experimental researches. 



Engine Frames.— ^Ve think the adoption of the Grecian or Gothic 

 orders of architecture in the cast iron work of steam engines, is an 

 evidence of bad taste ; but the rejection of every species of embellish- 

 ment is worse still, being indicative of the want of taste altogether. 

 The beauty of a piece of machinery is, in our eyes, a point of no tritling 

 moment; a handsome engine will always be better taken care of and 



