538 



Royal Society . 



The formula which expresses the relation of the bursting pressure 

 to the thickness aud diameter of the globe, is — 



«T. 

 A ' 



P = 



where a = the longitudinal sectional area of the material in square 

 inches, that is in the line of rupture or line of minimum strength ; 

 A = the longitudinal sectional area of the globe in square inches ; 

 and T = the tenacity of the glass in pounds per square inch. 

 Hence from the above experiments we deduce — 

 Pounds. 

 T=4200 for flint-glass, 

 =4800 for green glass, 

 ^6000 for crown-glass. 



5000 = mean tenacity of glass. 

 Here the mean tenacity is nearly twice that obtained in the experi- 

 ments upon thick bars ; a result, which perhaps corresponds with the 

 difference between the crushing strength of cylinders and cubes, and 

 is largely attributable to the condition of annealing. 



On the Resistance of Glass Globes and Cylinders to an external 

 pressure. — The manner of conducting these experiments did not 

 differ in any essential detail from that pursued in the experiments 

 upon wrought iron. The globes and cylinders, after having been 

 hermetically sealed in the blowpipe flame, were fixed in a wrought- 

 iron boiler communicating with a hydraulic pump. In this position 

 an increasing pressure was applied until the globes broke, the 

 amount of pressure at the time being noted by means of a Schiiffer 

 pressure-gauge. During the collapse the globes were reduced to 

 the smallest fragments, so that no indication of the direction of the 

 primary lines of fracture could be discovered. 



The following Table contains a summary of the results on glass 

 globes subjected 'to an external pressure : — 



* These globes remained unbroken. 



