1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



5S 



But we have also oliserved throughout the present inquiry that 

 the engineers have been already warned by experience of the 

 necessity for increasing the strength of bridges employed in rail- 

 ways; and of watching more narrowly their construction, so as to 

 render them as strong as possible. Accordingly we have found 

 that the original structure of all those bridges which had shown 

 the least signs of weakness, has been carefully altered and 

 strengtliened, so as to leave no ai>parent cause for apprehension; 

 while in new bridges, better and stronger combinations are 

 adopted. 



And in conclusion, considering that the attention of engineers 

 has been sufficiently awakened to the necessity of providing a 

 superabundant strengtli in railway structures, and also considering 

 the great importance of leaving the genius of scientific men un- 

 fettered for the development of a subject as yet so novel and so 

 rapidly progressi\'e as tlie construction of railways, we are of 

 opinion that any legislative enactments with respect to the forms 

 and proportions of the iron structures employed therein would be 

 highly inexpedient. 



^Ve would, however, direct attention to the general conclusions 

 we have arrived at from our oiin experiments and from the infor- 

 mation supplied to us, namely, — 



That it appears advisable for engineers in contracting for cast- 

 ings to stipulate for iron to bear a certain weight instead of en- 

 deavouring to procure a specified mixture. 



That to calculate the strength of a particular iron for large 

 castings the bars used as a unit should be eijual in thickness to the 

 thickest part of the proposed casting. 



Tliat, as it has been shown that to resist the effects of reiterated 

 flexure iron should scarcely be allowed to suffer a deflection equal 

 to one-third of its ultimate deflection, and since the deflection 

 produced by a given load is increased by the effects of peixussion, 

 it is adiisable that the greatest load in railway bridges should in 

 no case exceed one-sixth of the weight which would break the 

 beam when laid on at rest in tlie centre. 



That as it has appeared that the effect of velocity communi- 

 cated to a load is to increase the deflection tliat it would produce 

 if set at rest upon the bridge; also that the dynamical increase in 

 bridges of less than 40 feet in lengtli is of suflicient importance to 

 demand attention, and may even for lengths of 20 feet become 

 more than one-half of the statical deflection at high velocities, but 

 can be diminished by increasing the stiffness of the bridge; it is 

 advisable that, for sliort bridges especially, the increased deflec- 

 tion should be calculated from the greatest load and highest 

 velocit)' to which the bridge may be liable; and that a vveight 

 which would statically produce the same deflection should, in esti- 

 mating the strengtli of the structure, be considered as the greatest 

 load to which the bridge is suljject. 



Lastly, the power of a beam to resist impact varies with the 

 mass of the beam, tlie striking body being the same, and by 

 increasing the inertia of tlie beam without adding to its strength 

 the power to resist impact is within certain limits also increased. 

 Hence it follows tliat weight is au important consideration in 

 structures exposed to concussions. 



"Whilst, however, we lament that the limited means which have 

 been placed at our disposal, and the great time required for such 

 investigations, have compelled us to leave in an imperfect state, 

 or even to neglect altogether, many interesting and important 

 branches of experimental inquiry, we trust that the facts and 

 opinions which we have been enabled to collect will serve to 

 illustrate the action which takes place under varying circum- 

 stances in iron railway bridges, and enable the engineer and me- 

 chanic to apply the metal with more confidence than heretofore. 



Whitehall, 26th July, 1849. 



DotTGLAS GaLTON, 



Lieut. Hoyal Engineers, 



Secretary. 



Wrottesley. 

 Robert Willis. 

 Henry James. 

 George Rexxie. 



W. CUBITT. 



Eaton Hodgkinsox. 



Analysis of the Evidence received by the Commission. 



Chemical Const iluenls of Iron.— }.U.notT\es Stirling states, that iron in 

 its pure state is malleable, and that it is a combination of carbon with iron 

 which produces cast-iron. In addition to carbon, the cast-iron in this 

 country contains silica, lime, magnesia, alumina, occasionally some of the 

 phosphates and other admixtures ; but iron made from magnetic ores is much 



purer. The strength of cast-iron depends upon its freedom from impurities 

 and upon the proportion of carbon it contains. The strongest cast-iron con- 

 tains about three per cent, of carbon, or, according to Mr. Charles May, 

 when the carbon is in the smallest proportion that produces fluidity, a 

 larger proportion tends to make the iron soft and weak, and a smaller hard 

 and brittle. Mr. Glynn states, that the strongest iron generally shows a 

 clear grey, or slightly mottled fracture, and he considers that that colour 

 indicates the combination of carbon with iron which produces the greatest 

 strength. Mr. Morries Stirling states, that while colour is admissible as a 

 test of strength it is not bo of chemical constitution, for though dark-coloured 

 iron is usually weak, grey iron usually strong, and white iron usually brittle, 

 yet black iron when chilled becomes white, although it must be supposed to 

 contain the same quantity of carbon j hence, as a general rule, he concludes 

 that colour indicates the treatment to which iron has been subjected, and in 

 some cases only the quantity of carbon. Mr. Charles May coincides in con- 

 sidering the question of strength to be very much reducible to the quantity 

 of carbon contained in the iron, as some of the tenderest iron skilfully 

 treated will produce some of the strongest castings. Mr. Stephenson and 

 Mr. Morries Stirling mention that the fluidity of the Berlin iron is due to the 

 presence of arsenic, and the latter has observed that manganese mixed artifi- 

 cially with cast-iron closes the grain and is an improvement boih to cast-iron 

 and steel. On wrought-iron the effect of manganese is stated to be to give 

 it the hot short property, whilst the cold short is produced by the presence of 

 a small quantity of phosphorus ; and the admixture of arsenic renders 

 wrought-iron hard and brittlc- 



Qualities and Mixtures of Iron. — The use of the hot-blast in the manufac- 

 ture of iron, it is stated by Mr. Glynn, does not of itself make iron worse or 

 better ; but by its means, materials, otherwise intractable, yielding alloys of 

 iron may be smelted, instead of ores yielding purer metal. Mr. .Morries 

 Stirling has not found any distinct difference between the chemical consti- 

 tuents of hot-blast and cold-blast iron, but apparently there is more carbon 

 in the hot-blast iron, and graphite is more commonly to be seen on the sur- 

 face of No. 1 hot-blast than on cold-blast iron. Mr. Charles May considers, 

 that by the use of the hot-blast the quantity of carbon which can be com- 

 bined with the iron is increased. Mr. Hawkshaw and Mr. Fairbairn consider 

 hot-blast iron weaker than cold-blast ; the latter gentleman and Mr. Stephen- 

 son state that the use of the hot-blast renders the metal very fluid ; and Mr. 

 Glynn says that its use is to produce in large quantities and at a cheap rate 

 a soft fluid metal to be employed in light castings, and that in that respect 

 he considers the invention to be of great pubhc benefit, as enabling Scotch 

 iron-masters to use a new kind of ore, which though of a weaker character, 

 further experience may enable them to purify and improve. 



At the same time the hot-blast is essential for smelting the iron-stone 

 from South Wales with anthracite coal, and the metal yielded is of the 

 strongest character. Mr. Glynn and Mr. Stephenson mention that generally 

 hot-blast iron is dark grey in colour and very fine in the crystal ; but it 

 appears to be universally agreed that there is no certain method of distin- 

 guishing hot-blast from cold-blast iron. Mr. Uastrick states that the 

 temperature of the hot-blast at the Gartsherrie furnaces was 680^ Fahrenheit. 



Mr. Stephenson does not attach much importance to the variation in 

 strength of diftereut sorts of iron, he considers that taking the average of 

 irons generally throughout the country there is a proximity to an uniform 

 standard. He concludes, from a series of experiments made by him for the 

 High Level Bridge at Newcastle, that hot-blast iron is less certain in its 

 results than cold-blast; that mixtures of cold-blast are more uniform than 

 those of hot-blast ; that mixtures of hot and cold-blast give the best results; 

 that simple samples do not run so solid as mixtures ; that simple samples run 

 too hard and sometimes too soft for practical purposes. Mr. Kastrick would 

 prefer making girders of forge iron. Mr. Hawkshaw would use the Low- 

 moor iron. It is, however, generally admitted that mixing irons from dif- 

 ferent parts of the country produces the best castings, and since the object 

 in mixing them is to obtain the proportion of carbon to iron which gives the 

 greatest strength combined with the required degree of fluidity, the exact 

 proportion will be regulated by the appearance of the fracture of the several 

 irons. Mr. Morries Stirling states that No. 1, hot-blast iron, mixed with 

 No. 3, cold-blast, will give the right proportion of carbon ; but that if iron 

 containing that proportion could be obtained at once from the blast-furnace, 

 it would be very superior. Mr. Charles May, however, observes, that the 

 strength of cast-iron depends upon the bulk into which it is to be run as 

 well as upon its constituent parts, and that the art of theironfounder consists 

 in his ability to produce the required amount of strength without any very 

 definite knowledge upon the subject, either chemical or mechanical. Jlr. 

 Fox considers a very good mixture for girders to be cold-blast Blaenavon, 

 two-thirds, and of hot-blast Scotch two sorts, from the black band and the 

 red hccmatite ores, one-third. Mr. Grissell considers the use of old scrap 

 iron to be of immense value, and would use Scotch iron, cold-blast Welch, 

 and old scrap. Mr. Fairbairn names as the best mixture independently of 

 price — 



Lowraoor, No. 3 . . . . 30 per cent. 

 Blaina, or Yorkshire, No. 2. . . 25 „ 

 Shropshire, or Derbyshire, No. 3 . 25 „ 

 Good Old Scrap .... 20 „ 



100 



