1843.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



121 



barrels and bags, therefore its first impulsive effort on being ignited 

 would be 1000 -r- 3 = 333i atmospheres, or about 5000 lb. the square 

 inch; and the force of the permanent generated elastic fluid, when 

 cooled, would be 244 -^ 3 = 81i atmospheres, or about 1200 lb. to 

 the square inch. Now, if we suppose the space within which the 

 gunpowder was confined to have been cubical, each of the six sides 

 would have exposed an internal surface to the action of the gunpowder 

 of 100 square feet, equal to 14,400 square inches. As the pressure, 

 according to the preceding calculation, would in the first instant be 

 equal to 5000 lb. the square inch, the impulsive effort on each side of 

 the cubical chamber would be 72,000,000 lb. or 32,143 tons. As the 

 point of least resistance must necessarily have been towards the face 

 of the cliff, the acting power maybe considered as having been exerted 

 'only in that direction ; therefore the cliff would be forced outwards 

 with an impulse upwards of 32,000 tons. As the rock yielded to this 

 immense power, the pent-up air would expand, and its force would 

 consequently be diminished. The cooling of the generated gas would 

 also greatly weaken its expansive force, and its gradual escape through 

 the fissures of the falling rock would prevent any sudden explosion. 



The sound which was heard was that of the rending of the solid 

 rock, and not the firing of the powder; for it is well known that the 

 explosive report of any detonating body is caused by the concussion 

 of the air. This fact is proved by the firing of explosive mixtures of 

 hydrogen and oxygen gases in a strong glass apparatus, for the pur- 

 pose of obtaining the product of the combustion of the two gases. 

 The apparatus usually employed contains about half a pint, the ex- 

 plosion of which quantity of the mixed gases, when in contact with 

 the atmosphere, is sufficient to produce a report as loud as the firing 

 of a pistol ; but when the gases are fixed in a closed vessel, no report 

 whatever is heard. This experiment proves also, on a small scale, 

 the posBibilitv of controlling explosive forces. The expansion of an 

 explosive mixture of hydrogen and oxygen gases at the moment of 

 combustion, amounts to 15 times its original volume, which gives a 

 pressure of 15 atmospheres, or about 225 lb. to the square inch; yet 

 the glass bottle in which the gases are fired is sufficient to control the 

 explosive effort, and to prevent even any sound from being heard. 



The quantity of gunpowder requisite in ordinary blasting operations 

 must depend altogether on the hardness of the rock, and the mass in- 

 tended to be moved. The proper adjustment of the quantity of pow- 

 der to the resistance to be overcome, forms, however, an important 

 consideration, for an excess of powder is not only a wasteful expendi- 

 ture of a valuable agent, but it renders the operation more dangerous 

 by the dispersion of fragments in all directions, and it not unfrequently 

 diminishes the effect of a blast by concentrating the direction of the 

 impulsive force. This is particularly the case in warlike operations, 

 where the object in springing a mine is to make the destructive effects 

 extend as far as possible. The aperture produced by the explosion, 

 when a mine is properly charged, is a cone, the diameter of the base 

 of which is double the height, taken from the centre of the mine. This 

 calculation is founded on the supposition that the materials to be re- 

 moved are either earth or soft clayey soil. The allowance of powder 

 recommended for such mines is about 10 lb. per cubic fathom when 

 the materials are loose earth, and for strong clayey soil about 16 lb. 

 It is found that when the charge of powder greatly exceeds those 

 quantities, the materials immediately above the powder are alone 

 blown up, that the aperture is nearly cylindrical instead of conical, and 

 consequently the sphere of its influence is diminished. 



The explosive effects of a charge of powder in blasting, depends 

 also materially on the mode of tamping. This is a point which, until 

 a comparatively late period, was altogether overlooked, and it is not 

 even now so generally attended to as its importance deserves. The 

 notion which formerly obtained was, that the impulsive effort of the 

 powder was greatly increased by ramming it tight. In gunnery 

 practice this is correct; for when the ball is rammed closely to the 

 powder, it is propelled with greater force than when it is not. 

 But the required effects in the operations of blasting are exactly the 

 opposite of those in gunnery. The ball and wadding of the gun may 

 be considered as the tamping of the mine. To blow out this tamping 



without bursting the gun, is the object in gunnery — to retain the 

 tamping and to burst the chamber holding the gunpowder is the object 

 in blasting. To attain these different ends, the methods adopted in 

 the two cases ought of course to be different. The well known fact 

 that when a ball is rammed only a short way down the barrel of a gun, 

 it will burst before the ball is forced out, affords a very instructive 

 lesson in the practice of blasting, and shows clearly that to produce 

 the most effect in rending the rock, and to run the least risk of blowing 

 out the tamping, ample space should be left between the tamping and 

 the powder. It is true that by leaving such a space for the expansion 

 of the generated elastic fluid, the intensity of its action is diminished, 

 but the same amount of power is distributed over a larger surface ; its 

 action accordingly approaches more to that of an ordinary mechanical 

 force, and the liability to split the rock into small fragments is thereby 

 decreased. 



One great advantage in mining operations, derived from the prac- 

 tice of leaving a space between the tamping and the powder, arises 

 from its admitting the use of loose dry sand for tamping, instead of 

 requiring the blast hole to be filled and rammed tight with hard sub- 

 stances. The labour, the trouble, and the danger of tamping by the 

 common process, renders the substitution of dry sand a great ad- 

 vantage, as the risk of igniting the powder by striking a spark, is 

 entirely removed ; and we suspect that in those cases where it has 

 been found to fail, and the tamping has blown out, that the cause of 

 failure may be attributed to the neglect to leave a sufficient space 

 between the tamping and the powder. 



The manner in which the space between the powder and the tamp- 

 ing operates in preventing the latter from being blown out, has been 

 explained in the following manner. The force of fired gunpowder 

 may be considered as proceeding from a point, and radiating in all 

 directions round it. This force must, therefore, participate in the 

 nature of all central forces, and diminish in intensity as the square of 

 the distance. When a ball is rammed close to the powder, it 

 approaches near to the point whence the force emanates, and sustains 

 consequently its full effect ; but when the ball is placed farther from 

 the point of radiation, the force acting on it may be diminished many 

 times within the space of half an inch. Suppose, for example, that a 

 musket ball which, when close to the powder, is within a quarter of 

 an inch of the centre of radiating power, were placed a quarter of an 

 inch from contact with the powder, the impulsive force it would re- 

 ceive would be diminished four times. If it were removed to a dis- 

 tance of one inch from the charge, the force acting on it would be 

 diminished sixteen times. If, therefore, we view the first impulsive 

 effort of fired gunpowder as a radiating force, we perceive at once 

 the cause of its diminished action on the tampirg of a mine, when a 

 space is left between it and the charge, and whether the tamping 

 materials be sand or hard fragments of rock, the vacant space must be 

 equally advantageous. Should the resistance be too great to yield to 

 the first explosive effort of the gunpowder, the direction of the action 

 of the pent up elastic fluid would cease to be radiating. It would 

 resemble the pressure of compressed fluids, and act uniformly in 

 all directions. The tamping would then be acted on by a power 

 equivalent to the compression of the generated gases, and in a direc- 

 tion tending most effectually to force it out. It is under such circum- 

 stances, we conceive, that the tamping, whether of sand or rock, is 

 most frequently blown. 



The mode of tamping with dry sand has been brought into notice 

 more particularly within the last two years, in connexion with Mr. 

 Martyn Roberts's plan of blasting by galvanism ; but it was known 

 and practised successfully, we believe, 30 years ago, at Lord Elgin's 

 mines at Charlestown, in Scotland. It is to be regretted that an im- 

 provement, attended evidently with so many advantages, and which 

 is calculated to prevent accidents in the dangerous occupation of 

 blasting, should have made such slow progress that more than 30 

 years have elapsed since its introduction without its general adoption. 



