As tar was known to be soluble in water, attempts had been made to defend cordage from the water by oils and fatty bodies, which do not mix with it; but the application of these unguents was found to inter fere with the twisting of the fibres. Tanning has also been used, but though it is found useful in the net manufacture, it is not employed in cordage.
Mr. Chapman, to whom the rope manufacture owes so much, directed his attention to the subject, and seems to have obtained very important results. He had found that rope yarn is considerably weakened by passing through the tar kettle; that tarred cordage loses its strength progressively in cold climates, and so rapidly in hot climates that it is scarcely fit for use in three years.
Mr. Chapman, therefore, set himself to discover a preserving substance with the following properties: 1. That it should not be soluble in water.
2. That it should not become rigid by length of time, as the rope which imbibed it would be weaken ed by sudden bendings.
3. That it should be free of any acid or essential oil capable of being disengaged by heat, for these in gredients occasion the dry rot in cordage.
Mr. Chapman found that tar could be rendered fit, by the following process, for becoming a preservative of cordage: 1. By boiling it in water, which will extract from it its superabundant acid, and its mucilage, which contains a disengaged acid.
2. By repeating and combining these processes till the tar has become more pitchy, by having thrown off a larger portion of its essential oil, and by restoring the plasticity which it has thus lost by the addition of suet, tallow, animal oils, or expressed oils that may have the same effect.
The following report drawn up by our celebrated countryman Mr. William Allen, on the advantages of using Mr. NV. Chapman's preparations of tar in cord age, is too valuable a document to be omitted here.
" Common tar, unprepared, contains a quantity of vegetable acid; and apprehending that this acid might injure the texture of cordage, the following experi ment was made:—A piece of twine, which, by previ ous trial, was found capable of supporting 61 lbs. without breaking, was immersed in vegetable acid, and after 46 hours it was so much injured that it broke with a weight of less than 16 lbs. A piece of the same twine was immersed for 46 hours in the es sential oil, which came over in distillation from the jar; and, although it had suffered no diminution of strength at the termination of its immersion, yet after being exposed three days to the air, it was only capa ble of bearing 31 pounds.
The Stockholm tar used in these experiments was found to contain about seven per cent. of vegetable mucilage, capable of being converted into acid in a hot climate, when the cordage is immersed in water; the tar also contained as much real acid as there is in an equal measure of common vinegar; but by repeated ly boiling the tar in water, according to the method prescribed, it is freed from its acid and mucilage, and may be employed in the manufacture of cordage with great advantage, in the place of common tar. Also,
if the prepared tar be boiled down so much further as to deprive it of that portion of its essential oil which it is found necessary to retain to prevent tarred cord age being too rigid, and the place of the essential oil be supplied with a due portion of fixed or expressed oil, it is probable that those injuries will be done away, which arise from the action of essential oil on the fibres of the hemp, and from the rigidity of cord age experienced in vessels returning to cold from hot climates where the essential oil is considerably thrown off." In 1802 the additional expense of using purified tar amounted to about one pound sterling per ton, and the experiments made on ropes tarred by this new method, will be found at the end of this article. A vessel was fitted out by Mr. Renwick of Newcastle, having the cordage partly tarred in the common way, and partly with purified tar, but she was unfortunate ly lost on her first voyage.
In 1802 another patent was taken out by Messrs. James Mitchell, Sen. and Junior, for an improved method of manufacturing cables, hawsers, and other cordage. We have already seen that, in his patent of 1799, Mr. Mitchell gave a slight twist to a small num ber of yarns, which were combined into the strand as if they were as many single yarns. The twist which he gave to these sets or parcels of yarn was only such as to shorten them between three and Jive fathoms in every two hundred fathoms. The object of the pre sent patent is to facilitate the progress of combining these parcels of yarn. After placing the sledge at a proper distance from the head of the ropery, they at tach as many parcels of yarns as are wanted to as many hooks on the tackle-board at the head of the ropery, and run them down over the stake heads in parallel lines. This is accomplished by what is called a bedder, which admits and compresses each division separately, and retires towards the sledge, the parcels of yarn receiving their twist from the hooks on the tackle-board. The strand is then made from those twisted parcels of yarn in the ordinary way. After giving an account of other analogous processes. the patentee states, that the parcels of yarn may be twist ed " without rotation on the axes, by the simple pro cess of thorough putting the parts or subdivisions when coiled above the boards," and drawing them on through the proper distances, after they had been passed through the holes in the tackle-board.