III. Generalisation of the Results of Experiments on the Tensile and Compressive Resistances of Glass.

Mean tenacity (T1) of glass in the form of bars =1(2286+2540+2890+2540)=2560 lbs. per sq. in. ; Mean tenacity (T',) of glass in the form of thin plates =1(4200+4800+6000)=5000 lbs. per square inch;

that is, the tenacity of glass in the form of thin plates is about twice that of glass in the form of bars. Mean resistance (T2) of glass to compression =1(27582+31876+31003)=30,150 lbs. per sq. in. ;

that is, the ultimate resistance of glass to a crushing force is about twelve times its resistance to extension.

IV. Resistance of Rectangular Glass Bars to a Transverse Strain.

Let 7=the length of the bar supported at the ends and loaded in the middle.

W=breaking weight in lbs.

K=area of the whole transverse section.
D=the whole depth of the section.

d, d=the respective distances of the top and bottom
edges from the neutral axis.

T1the tensile resistance of the material in lbs. per square inch.

T=the compressive resistance of the material in lbs. per square inch.

Then we have TATE'S "Strength of materials," equations (27) and (6)—

Substituting this value of the constant, equation (16)

hich expresses the transverse strength of a rectangular r of glass supported at the ends and loaded in the iddle.

III.

RESEARCHES ON THE TENSILE STRENGTH OF WROUGHT IRON AT VARIOUS TEMPERATURES.

(From the Report of the British Association for 1856.)

ON a previous occasion I had the honour of conducting, for the Association, a series of experiments to determine the effects of temperature on the strength of cast iron. In that inquiry I endeavoured to show to what extent the cohesion of that material was affected by change of temperature, and taking into account the rapidity with which iron imbibes caloric, and the facility with which it parts with it, it is equally interesting to know to what extent wrought iron is improved or deteriorated by similar changes. In the present inquiry, as in the former on cast iron, the expansion of the metal by heat is not the question for solution. Rondelet, Smeaton, and others, have already investigated that subject, and it now only remains for us to determine the effects produced on the strength of malleable iron by changes of temperature varying from -30° of Fahrenheit to a red heat, perceptible in daylight.

The immense number of purposes to which iron is applied, and the changes of temperature to which it is exposed, render the present inquiry not only interesting, but absolutely essential to a knowledge of its security under the varied influences of those changes; and when it is known that most of our iron constructions are exposed to a range of temperature varying from the extreme cold of winter to the intense heat of summer, it is assuredly de

sirable to ascertain the effects produced by these causes on a material from which we derive so many advantages, and on the security of which the safety of the public not unfrequently depends.

Independent of atmospheric influences, another consideration presents itself in reference to the durability and ultimate stability of iron under changes much greater than those alluded to above, and this is the strength of such vessels as pans and boilers subjected to the extreme temperatures of boiling liquids on one side, and the intense heat of a furnace on the other. But even these extremes, however great, do not seem seriously to affect the cohesive strength of wrought-iron plates, nor do they appear to cause any disruption of the laminated structure which results from the system of piling and rolling adopted in the manufacture, excepting only where small particles of scoria happen to intervene between the laminated surfaces. These not unfrequently prevent a perfect welding, as the plate is compressed by passing through the rolls, and the effects of temperature are strikingly exhibited in the production of large blisters upon the surface of the plate, as shown in the annexed sketch at a, a. Now the reason of

Fig. 23.

b

his is the want of solidity and homogeneity in the plate, nd the consequent expansion of the lower part exposed > the greatest heat. Let us suppose, for the sake of lustration, the plate to be 3ths of an inch thick, and the arface b to be the interior of a boiler-plate, and the surice a, a to be exposed to the action of the fire in the In this case it is evident that the temperature

irnace.

H

of the side a, a may be upwards of 1000°, while that of b is very little above 212°, or the temperature of boiling water; and supposing there be any imperfection or want of soundness in the plate, the result will be a greater expansion on the exterior surface, causing it to rise up in blisters in the manner we have described. These defects are invariably present when the plates are not sound; but in other respects, where the bars which form the pile are clear and free from rust or scoria, and are well welded in the rolling process, the wide difference between the temperature of one side and that of the other produces, apparently, no injurious effect on the strength of the plate. It is, however, widely different when the whole of the plates are exposed to the same degree of temperature, as in this position the strengths are increased or diminished according as the temperature approaches or recedes from the point where the strength is a maximum.

In order to show how the results were obtained, it will be necessary to describe the apparatus and mode of conducting the experiments.

The apparatus consisted of a powerful wrought-iron lever, Plate III. A, figs. 2 and 3, capable of imparting a force of more than 100,000 lbs., or 45 tons per square inch to the specimen to be broken. The lever is supported in a cast-iron standard or frame B, arranged for the reception of specimens of the material to be subjected to a crushing force or tensile strain. On the short arm of the lever the plates and bars (one of which is seen at a) were suspended by a shackle c, and held down to the bottom of the castiron standard by the rod and screw e; on this rod the box, b, was fixed, and prepared to hold a bath of oil or water, in which the iron to be broken was immersed. Below this box was a fire-grate, d, for heating the liquid in the bath to the required temperature, and this grate could be drawn backwards from the box, b, when the required tem