Marvels of Scientific Invention - Thomas W. Corbin

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the nitro-cotton is another kind of explosive and the saltpetre, one of the ingredients in the old gunpowder, provides the necessary oxygen for burning up the wood-meal. Nitro-cotton is made in much the same way as nitro-glycerine, except that cotton takes the place of the glycerine. Cotton is almost pure cellulose, another organic substance, like glycerine insomuch as it is composed of carbon and hydrogen, but, unlike it, containing also oxygen. Treated with nitric acid it also forms a combination of carbon, hydrogen, oxygen and nitrogen, which is called nitro-cotton, nitro-cellulose, or gun-cotton.

      It may be asked, why, if these two substances are thus similar, need they be mixed? The answer is that although alike to a certain degree they are not exactly the same, and the modern manufacturer of explosives in his strife after perfection finds that for certain purposes one is the best, and for others another, while for others again a combination may excel any single one.

      For some work another kind of explosive altogether is to be preferred. This is based upon chlorate of potash, a compound very rich in oxygen, which it is prepared to give up readily to burn any other suitable element which may be at hand. A well-known explosive of this class is that known as cheddite, since it was first made at a factory at Chedde, in Savoy.

      For the sake of simplicity, however, I propose in the following descriptions to refer to all these explosives under the common term "dynamite," since that will probably convey to the general public an idea of their nature better than any other term or terms which I could choose.

      So now we come to the great question, how can the modern farmer benefit by the use of high explosives such as these? The answer is, in many ways. Let us take the most obvious one first.

      A farmer has been ploughing his land and growing his crops upon it for years. Perchance his forefathers have been doing the same for generations. Every year, for centuries possibly, a hard steel ploughshare has gone over that ground, turning over and over the top soil to a depth of six to eight inches. Each season the plants, whatever they may be, grow mainly in that top layer. They take the goodness or nourishment out of it and it eventually becomes more or less sterile. By properly rotating his crops he mitigates this to a certain extent, in addition to which he restores to the land some of its old nitrogenous constituents by the addition of manure. Yet, do what he will, this thin top layer is bound to become exhausted. And all the while a few inches lower down there is almost virgin soil which has scarcely been disturbed since the creation of the world.

      Nay, more, that virgin soil, with all its plant food still in it, is not only doing little for its owner, it is positively doing him harm. For every time his plough goes over it it tends to ram it down flat; every time a man walks over it the result is the same; every horse that passes, everything that happens or has happened for centuries in that field, tends to make that soil just below the reach of the ploughshare a hard, impervious mass, through which only the roots of the most strongly growing plants can find a way, and which tends to make the soil above it wet in wet weather and dry in dry weather. Thus roots have to spread sideways instead of downwards; or, growing downwards with difficulty, each plant has to expend vital energy in forcing its roots through the hard ground which it might better employ in producing flowers or fruits. And there is no natural storage of water. A shower drenches the ground. In time it dries, through evaporation into the air, and then when the drought comes all is arid as the Sahara.

      That hard subsoil is known by the term "hard-pan," and, as we have seen, it is produced more or less by all that goes on in the field. Even worse is the case—a very frequent one too—wherein there is a natural stratum of clay or equally dense waterproof material lying a few feet down.

      Beyond the reach of any plough, this hard stratum can be broken up by the use of dynamite. The usual method is to drive holes in the ground about fifteen to twenty feet apart and about three or four feet deep, right into the heart of the hard layer. At the bottom of each hole is placed a cartridge of dynamite with a fuse and a detonator. This latter is a small tube containing a small quantity of explosive which, unlike the dynamite, can be easily fired, and initiates the detonation of the cartridge.

      When these miniature earthquakes have taken place all over a field a very different state of things prevails. The "hard-pan" has been broken. The explosive used for such a purpose has a sudden shattering power, whereby it pulverises the ground in its vicinity rather than making a great upheaval at the surface. The sudden shock makes cracks and fissures in all directions, through which roots can easily make their way. Moreover, it permits air to find an entrance, thereby aerating the soil in such a way as to increase its fertility. The heat, or else the chemical products of the explosion, seem to destroy the fungus germs in the ground. Finally a natural storage of water is set up. Heavy rain, instead of drenching the upper soil, simply moistens it nicely, while the surplus water descends into the newly disturbed layers, there to remain until the roots pump it up in time of drought.

      It is stated that an acre of hay pumps up out of the soil 500 tons of water per annum, so it is easy to see what an important feature this natural water-storage is.

      Farmers say that their crops have doubled in value after thus dynamiting the subsoil.

      This operation has been spoken of as a substitute for ploughing, but that may be put down to "journalistic licence," for while it truly conveys the general idea, it is hardly correct. The ordinary plough turns over about eight inches, the special subsoil plough reaches down to about eighteen inches, but the dynamite method loosens the ground to a depth of six or seven feet. Corn roots if given a chance will go downwards from four to eight feet. Potatoes go down three feet, hops eight to eighteen feet and vines twenty feet, so it is easy to see how restricted the plants are when their natural rooting instincts are restrained by a hard layer at a depth of eighteen inches or so.

      The holes are made by means of a bar or drill. A great deal depends, of course, upon the hardness of the soil. Sometimes a steel bar has to be driven in by a sledge-hammer. At others a pointed bar can be pushed down by hand. In some cases it will be found that the best tool to employ is a "dirt-auger," a tool like a carpenter's auger, which on being turned round and round bores its way into the earth. However it may be done, one or more cartridges of dynamite are lowered into the finished hole, one of them being fitted with the necessary detonator and fuse. Then a little loose earth or sand is dropped into the hole until it is filled to a depth of six inches or so above the uppermost cartridge. Above that it is quite safe to fill the hole with earth, ramming it in with a wooden rammer. This is called "tamping," and it is necessary in order to prevent the force of the explosion being wasted in simply blowing up the hole. What is wanted is that the explosion shall take place within an enclosed chamber so that its effect may be felt equally in all directions. The holes are generally about an inch and a half or an inch and three-quarters in diameter.

      There 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.

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