TABLE IV.-Summary of the Results of Experiments on the Resistance of Cut Glass Cubes to Compression.

Hence the mean resistance to crushing of cubes of glass

is equivalent to a weight of—

lbs.

Comparing these with the preceding results on glass cylinders, we have the mean resistance of the former experiments to the mean resistance of the above as 30,153: 18,401, or as 1.6: 1.

General Observations relative to the Results of the Experiments on the Resistance of Glass Cylinders and Cubes to Crushing.

With iron and some other materials, when a short column undergoes a pressure in the direction of its length, rupture takes place in a plane having a determinate angle to th f the column, this plane being the section of

Neglecting the friction of the surfaces,

Coulomb found this angle to be 45°, and allowing on an average 10° for the limiting angle of friction, the angle of the plane of rupture may be taken at 55°. To fulfil this condition, the length of the column to be crushed should be at least three times its radius: when the length greatly exceeds this limit, the rupture will be effected by the tendency of the column to bend; and when the length is within this limit, the force requisite to produce rupture will be increased in consequence of the irregular form of the line of fracture. These theoretical deductions have been confirmed by experiments made upon columns of iron, wood, bone, stone, and other materials. The results of the experiments here recorded, however, show that when the length of the cylinder does not greatly differ from three times its radius, the resistance to a crushing force is pretty nearly a constant, viz. on an Average 12.313 tons per square inch in the case of flintglass, 14.227 tons in the case of green glass, and 13.84 cons in the case of crown-glass. But, according to Couomb's law, the cubes of flint-glass (their lengths being considerably less than three times their semi-diameters) hould have presented higher powers of resistance than he cylinders; this discrepancy is probably owing to the njury which the glass had sustained in the process of utting, and to the imperfect annealing of glass when ast in the form of cubes and cylinders.

SECTION III.

RESISTANCE OF GLASS GLOBES AND CYLINDERS TO INTERNAL PRESSURE.

In the following experiments it has been sought no ly to determine the law of resistance to internal pr are, which is already well known from theoretical c

siderations, but to ascertain the direct tensile strength of the glass (of which the bursting pressure is a function) by a method free from many of the objections to that described in Section I. The bursting pressure of cylindrical and spherical vessels is well known to be in the ratio of the tenacity of the material, other things being the same, and the determination of the tensile strength upon this principle presents in the case of glass peculiar advantages. As glass can be obtained in tolerably perfect spheres, and as the fracture of these may be effected by a uniform water pressure, increasing slowly and regularly without vibration, there is a better chance of ascertaining the ultimate resistance of the material, from the absence of those shocks and irregularities which are inseparable from any process depending upon the piling up of weights, however carefully conducted.

Fig. 9.

In making these experiments, a number of glass globes were procured of varying size and thickness. The stems were then flanched out by the blowpipe (fig. 9), and the diameter having been carefully measured, they were ready for experiment. To effect their rupture, each globe, k (fig. 10), was attached by means of a stuffing-box (a) to the cover of a strong wrought-iron boiler B, and was enclosed by the iron cylindrical vessel d, to prevent the dispersion of the frag

ments when rupture took place. In the stuffing-box the flanch of the stem of the globe was bedded upon vulcanised india-rubber in such a manner as to secure a water-tight attachment without impeding the access of the water to the interior of the globe. The boiler was connected with a hydraulic pump by means of the pipe b, and an accurate gauge of the Schaeffer construction was fixed to the boiler to register the pressure. With this ar

ll be seen that as the pumping was con

tinued the water would rise in the globe, compressing the air in its interior, progressively, up to the point at which the resistance of the glass was overcome by the expansive force of the fluid; at that point explosion would take place, the pressure in pounds per square inch being noted both by the eye of the observer and by the maximum finger of the gauge.

round the globe as meridians of longitude, and splitup into thin bands, varying from 1th to 1th inch h. In the case of some elongated ellipsoids, it apthat the fractures occurred horizontally, or perhaps ly, from the condition of the fragments attached to m. In most cases, however, it was not clear from gments which had been the direction of the fracIthough the mode of rupture was the same in every

ascertain the thickness, several specimens were sefrom the thinnest fragments, and each being measeparately by a micrometer screw of fifty threads to ch and reading on a graduated head to th of an the minimum thickness was assumed as that of the hich ruptured, and has been employed in reducing sults.

must also be observed that the globes were usually y elliptical, in some cases seriously so; the vertical er, b d (using the same form of expression as before) generally less than the horizontal, a c. In the folTables the two diameters are given in each case :

mcknesse

Bur

Gl

Thicknes

Bu

Thickn

B