BLASTING.

Grey River Argus, Volume XII, Issue 1243, 24 July 1872, Page 3

BLASTING.

The rabjeot of improvements in the process of blasting is still occupying attention in Sandhurst if we may judge by the letters in, the correspondence column of the Advertiser. A snort time ago we reprinted a letter which referred to a certain method of blasting known as the "cone system, 11 but which did not explain the operation. We find the explanation in the latest Advertiser to hand. A Mr W. H. Reynolds, writing to that journal, ■ays:— "Not knowing what Mr S. W.

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* Viney calls the '^^^^^^^^^^^H in a position to j^ T^P>i^^^^^^^^| superiority, but 3 it can^^-^HH| practicable, aud less dangerous thlnTnW of tamping, let it by all means be adopted. About forty yean ago the late Mr TreKMkie, a well-known manager and power of Cornish mines, introduced as a substitute for the usual tamping, a plug of wood about six inches long made to the size of the hole, with a cat in the end intended to be uppermost, (similar to that of a clothes peg), in wnioh a wedge of wood was placed before inserting it in the hoJe^ and when down on the charge, the wedge was driven into the wood with a bar, which tightened the wood in the hole. That gentleman obtained a prize for this from the Falmouth Polytechnic Society, but the plan was not found to answer, being in many cases "blown out" instead of the intended burden. Cone-shaned pieces of wood were also tried, being placed in a hole with the base end on the charge, and sand dropped round the sides to tighten it. This plan has been found to answer with deep holes, but such a course cannot be adopted with Bhallow ores, or in those having an horizontal direction, much less in what are commonly known as " uppers." The; effect of the explosive power would be to tighten the plug, but as veiy few holes are perfectly round, owing to the varying hardness of the rock, and oth.er reasons, this power operates on the sand around the conical wedge, which is blown out if the hole is at all heavily burdened. If I understand Mr S. W. Viney, he attributes the explosions to the effect of continuous, percussion on compressed air. causing 'it to 'become sufficiently heated for the powder to ignite. But, if this were the correct theory, then, where the greater amount of air is admitted into a Hole with the charge, the greater would be the danger of an explosion, whereas we i find that accidents more frequently occur where there is but little room for air. The master-gunner 61 the Falmouth Garrison always places about two inches of loose, wool or hemp on the top of the powder in the cartridges He makes for the mines, in order to confine a portion of air in the holes, which, by the expansion with heat, he considers to give greater effect to the charge. Now, thousands of these have been used in mines under my management, and I never knew an explosion from their use, which is certainly singular, if Mr S. W. Viney's theory be correct. In repjy to this Mr Viney says :— : Allow me to briefly explain that the •'.cone . theory" is simple. It is a cone placed base downwards on the charge, held steadily upright by a wire screwed into its apex, and withdrawn when sufficient packing ia round the cone to keep it steady. ; I do not propose to drop sand in for packing, but small cubical pieces of quarts, settling them down by mild pressure, with a tool, and then finer stuff shattered, down amongst it, and so filled up with layers of lighter stuff. As to horizontal holes, the cone being held in position as described, the cubes of quartz . for packing may be mixed with a sufficient quantity of clay to make them cohesive, and so can be settled into their places as in a perpendicular hole. As to upper?, by which I presume Mr Reynolds means holes going directly or horizontally upwards, they are, no doubt, exceptional cases, but could be dealt with in like manner with a little additional care. I take ; the liberty of suggesting to Mr Reynolds the propriety of testing the cone principle, packed not with sand, but denying it, because some one tried it forty years since, and it partly answered. As to shallow holes, I know that it will answer in. any hole of two inches in diameter over a foot deep. As to the compression of the air in the hole, I have nothing to do with that, it is to the air in> the interstices of the powder I allude ; and all the' argument in the world will not destroy the philosophical or chemical fact that air compressed by repeated percussion becomes changed in its character— becomes fcreinfaofc.