are two ways of firing the charges. One is by means of fuses. The detonator is fastened to one cartridge and a length of fuse is attached to the detonator, which passing up the hole terminates above the ground. The fuse is a tube of cotton filled with gunpowder, and it burns at the rate of about two feet a minute. Thus if three feet of fuse be used the man who lights it has a minute and a half in which to find a place of safety from falling stones.
The other way is by electricity. In this case an electric fuse is attached to the cartridge and two wires are led up the hole. These are connected to an electrical machine, which causes a current to pass down into the fuse, where, by heating a fine platinum wire, it fires the detonating material with which it is packed. This detonating material in turn fires the dynamite.
The advantage of the electrical method is that twenty or thirty holes being simultaneously connected to the same machine can all be fired at once.
And now let us think of another kind of farming, in which fruit trees are concerned. With a large tree the need of plenty of underground space for its roots would seem to be more important even than in the case of annual plants like wheat. Yet we know very well that the usual procedure is to dig a small hole just about big enough to accommodate the roots of the sapling when it is planted, while the ground all round is left undisturbed. The assumption is that the tree will, in time, be able to push its roots through anything which is not actually solid rock. So much is this the case that one authority has thought fit to warn tree-growers in this picturesque fashion. "When planting a tree," he says, "forget what it is you are doing, and think that you are about to bury the biggest horse you know." How many people when planting any tree dig a hole big enough to bury a horse? It is fairly safe to reply, only those who do it by dynamite.
The method of working is to bore a hole nearly as deep as the hole you want to blast. At the bottom place a powerful charge, far stronger than you would use for "subsoiling," as just described. That will not only blow a hole big enough for you to put your tree in, but it will loosen the ground all around the hole for yards. The main debris from the hole will fall back into it, but that will not matter much, since, being all loose, it is an easy matter to remove as much as is necessary to plant the young tree. The advantages are the same as those enumerated in the previous case—namely, the loosened ground gives more scope for the roots—apple-tree roots want twenty feet or so—the ground holds moisture better, and the explosion kills the fungus germs. In addition to these there is the advantage that to blast a hole like this is cheaper than digging it.
And that the advantages are not merely theoretical is shown by the fact that trees so planted actually do grow stronger, bigger and quicker than precisely similar ones under the same conditions, but set in the ordinary way with a spade.
And not only do new trees thus benefit; old trees can be helped by dynamite. Many an existing orchard has been improved by exploding dynamite at intervals between the rows of trees. Care has to be taken to see that the disturbance is not so violent or so close as to damage the trees, but that can be easily arranged, and then the result is that the soil all around the trees is loosened, the roots are given more freedom and the water-storing properties of the ground are greatly improved.
Again, how often a farmer is troubled with a pond or a patch of marshy ground right in the midst of his fields. It is of no use, and simply serves to make the field in which it occurs more difficult to plough and to cultivate—besides being so much good land wasted. Now the reason for the existence of that pond or marsh is that underneath the surface there is an impervious layer in which, as in a basin, the water can collect. Make a hole in that and it will no more hold water than a cracked jug will. And to make that hole with dynamite is the easiest thing in the world.
If the pond be merely a collection of water which occurs in wet weather, but which dries up quickly, there simply needs to be drilled a deep hole and a fairly strong explosion caused at the bottom of it. How deep the hole must be depends upon the formation of the earth at that point, and how low down is the stratum which, being waterproof, causes the water to remain. It is that, of course, which must be broken through, and so the explosion must be caused at a point near the under side of that layer. With a little experience the operator can judge the position by the feel of the tool with which he makes the hole. If the pond is permanent but shallow, men can wade to about the centre, there to drill a hole and fire a shot. If it be permanent and deep, then the work must be done from a raft, which, however, can be easily constructed for the purpose. Once broken through, the water will quickly pass away below the impervious stratum and useless land will become valuable.
The same may be done on a larger scale by blasting ditches with dynamite. This is in many cases much cheaper than digging them. A row of holes is put down, or even two or three rows, according to the width of the proposed ditch. In depth they are made a little less than the depth of the ditch that is to be. And for a reason which will be apparent they are put very close together, say three feet or so apart. Preparations may thus be made for blasting a ditch hundreds of feet long and then all are fired together. The earth is thrown up by a mighty upheaval, a ditch being produced of remarkable regularity considering the means by which it is made. The sides, of course, take a nice slope, the debris is thrown away on both sides and spread to a considerable distance, so that, given favourable conditions and a well-arranged explosion, there is constructed a finished ditch which hardly needs touching with spade or other tool.
It not being feasible to fire a lot of holes electrically, the limit being about thirty, the simultaneous explosion of perhaps hundreds has to be brought about in some other manner, and usually it is accomplished by the simple device of putting the holes fairly near together and firing one with a fuse. The commotion set up by this one causes the nearest ones to "go off," they in turn detonating those farther on, with the result that explosion follows explosion all along the line so rapidly as to be almost instantaneous.
A farmer who is troubled by a winding stream passing through his land, cutting it up into awkward shapes, can straighten it by blasting a ditch across a loop in the manner just described. In the case of low-lying land, however, ditches are obviously no use, since water would not flow away along them. In that case the principle suggested just now for dealing with an inconvenient pond can sometimes be used, for if the subsoil be blasted through at several points it is very likely that water will find a way downwards by some means or other.
And the list of possible uses is by no means exhausted yet. A man opening up virgin land often finds old tree stumps his greatest bother. He can dig round them and then pull them out with a team of horses, but by far the simpler way is with a few well-placed dynamite cartridges, for they not only throw up the stump for him, but they break it up, shake the earth from it, and leave it ready for him to cart away or to burn.
Boulders, too, can be blown to pieces far more easily than one would think. The charges may be put underneath them as with the tree stumps, but in many cases that is not necessary, all that is needed being some dynamite laid upon the top of the rock and covered with a heap of clay. So sudden is the action of the explosive that its shock will break up the stone underneath it. Yet another way, perhaps the most effective of all, is to drill a hole into the stone and fire a charge inside it. It behoves the onlooker then to keep away, for the fragments may be thrown three or four hundred feet, a fair proof that the stone will be very thoroughly demolished.
Even in the digging of wells explosives may be useful. In that case the holes are made in a circle, and they slant downwards and inwards, so that
