Results of experiments, by the same, on the transverse strength of cold blast iron; length of bars, and distance between the points of support the same as in the preceding Table.

312. The following remarks are extracted from the same Report: "I had remarked, in some of the experiments, that the elasticity of the bars was injured much earlier than is generally conceived; and that instead of its remaining perfect till one third, or upwards, of the breaking weight was laid on, as is generally admitted by writers, it was evident that th, or less, produced in some cases a considerable set or defect of elasticity; and judging from its slow increase afterwards, I was persuaded that it had not come on by a sudden change, but had existed, though in a less degree, from a very early period."

"From what has been stated above, deduced from experiments made with great care, it is evident that the maxim of loading bodies within the elastic limit, has no foundation in nature; but it will be considered as a compensating fact, that materials will bear for an indefinite period a much greater load than has hitherto been conceived."

66

313. We may admit," from the mean results, "that the strength of rectangular bars is as the square of the depth."

314. Effects of time upon the deflections caused by a permanent load on the middle of horizontal bars.

The following Table exhibits the results of Mr. Fairbairn's experiments on this point. The experiments were made on bars 5 feet long, 1.05 inch deep; the one of cold blast iron, 1.03 inch broad; the other of hot blast, 1.01 broad; distance between the points of support 4 feet 6 inches. The constant weight suspended at the centre of the bars was 280 lbs. This weight remained on from March 11th, 1837, to June 23d, 1838.

315. Mr. Fairbairn in his Report remarks on the above and like results: "The hot blast bar in these experiments being more deflected than the cold blast, indicates that the particles are more extended and compressed in the former iron, with the same weight, than in the latter. This excess of deflection may in some degree account for the rapidity of increase, which it will be observed is considerably greater in the hot than in the cold blast bar."

"It appears from the present state of the bars, (which indicate a slow but progressive increase in the deflections,) that we must at some period arrive at a point beyond their bearing powers; or otherwise to that position which indicates a correct adjustment of the particles in equilibrium with the load. Which of the two points we have in this instance attained is difficult to determine : sufficient data, however, are adduced to show that the weights are considerably beyond the elastic limit, and that cast iron will support loads to an extent beyond what has usually been considered safe, or beyond that point where a permanent set takes place." 316. Effects of Temperature. Mr. Fairbairn remarks: "The infusion of heat into a metallic substance may render it more ductile, and probably less rigid in its nature; and I apprehend it will be found weaker, and less secure under the effects of heavy strain. This is observable to a considerable extent in the experiments" on transverse strength "ranging from 26° up to 190° Fahr." "The cold blast at 26° and 190°, is in strength as 874: 743,

The hot blast at 26° and 190°, is in strength as 811: 731, being a diminution in strength as 100: 85 for the cold blast, and 100 to 90 for the hot blast, or 15 per cent. loss of strength in the cold blast, and 10 per cent. in the hot blast."

"A number of the experiments made on No. 3 iron have given

extraordinary and not unfrequently unexpected results. Generally speaking, it is an iron of an irregular character, and presents less uniformity in its texture than either the first or second qualities; in other respects it is more retentive, and is often used for giving strength and tenacity to the finer metals."

66

At 2120 we have in the No. 3 a much greater weight sustained than what is indicated by the No. 2 at 190°; and at 600° there appears in both hot and cold blast the anomaly of increased strength as the temperature is advanced from boiling water to melted lead, arising from the greater strength of the No. 3 iron." 317. Influence of Form in Cast Iron upon the Transverse Strength of Beams. Upon no point, respecting the strength of cast iron, have the experiments of Mr. Hodgkinson led to more valuable results to the engineer and architect, than upon the one under this head. The following Tables give the results of experiments on bars of a uniform cross section, (thus T,) cast from hot and cold blast iron. The bars were 7 feet long, and placed, for breaking, on supports 6 feet 6 inches asunder.

Table exhibiting the results of experiments on bars of hot blast iron of the form of cross section as above.

Note. The annexed diagram shows the A form of the uniform cross section of the bars. The linear dimensions of the cross section in the two experiments were as follows:

Length of parallelogram AB

Depth
Total depth of bar CD
Breadth of rib

66

AB

DE

0.36 "

1.56
0.365"

66

Expt. 5.

Table exhibiting results of experiments on bars of cold blast iron 5 feet long, of the same form of cross section as in preceding Table.

Note. The linear dimensions of the cross section of the bars, in the above Table, were nearly the same as those in the prece

ding Table, with the exception of the total depth CD, which, in these last two experiments, was 2.27 inches, or a little more.

318. The object had in view by Mr. Hodgkinson, in the experiments recorded in the two preceding Tables, was twofold; the one to ascertain the circumstances under which a permanent set, or injury to elasticity takes place; the other to ascertain the effect of the form of cross section on the transverse strength of cast iron. The following extracts from the Report, give the principal deductions of Mr. Hodgkinson on these points.

"In experiments 4 and 5," (on hot blast iron,) "which were on longer bars than the others, cast for this purpose, and for another mentioned further on, the elasticity (in Expt. 4) was sensibly injured with 7 lbs., and in the latter (Expt. 5) with 14 lbs., the breaking weights being 364 lbs., and 1120 lbs. In the former of these cases a set was visible with, and in the other with of the breaking weight, showing that there is no weight, however small, that will not injure the elasticity."

"When a body is subjected to a transverse strain, some of its particles are extended and others compressed; I was desirous to ascertain whether the above defect in elasticity arose from tension or compression, or both. Experiments 4 and 5 show this; in these a section of the casting, which was uniform throughout, had

C

the form L. During the experiments the broad part ab was laid

a b

horizontally upon supports; the vertical rib c in the latter experiment being upward, in the former downward. When it was downward the rib was extended, when upward the rib was compressed. In both cases the part ab was the fulcrum; it was thin, and therefore easily flexible; but its breadth was such that it was nearly inextensible and incompressible, comparatively, with the vertical rib. We may therefore assume, that nearly the whole flexure which takes place in a bar of this form, arises from the extension or compression of the rib, according as it is downward or upward. In Expt. 4 we have extension nearly without compression, and in Expt. 5 compression almost without extension. These experiments were made with great care. They show that there is but little difference in the quantity of set, whether it arises from tension or compression."

"The set from compression, however, is usually less than that from extension, as is seen in the commencement of the two experiments, and near the time of fracture in that submitted to tension. The deflections from equal weights are nearly the same, whether the rib be extended or compressed, but the ultimate strengths, as appears from above, are widely different."

319. Form of Cast Iron Beam best adapted to resist a Trans