The above observations will be in some measure il lustrated by the following comparative lengths of yarn, strands, and rope, in two descriptions of rope of most common use.
Thus, to form a 'three-inch strap-laid rope (that is a rope three inches in circumference, and composed of three single strands,) 120 fathoms in length.
An outside yarn of the strand, when drawn out by the carriage, and twisted as above will be 174 fathoms long. The length of each strand, or of the centre yarn before being hardened, 166 The length of the same, after har dening, and before the top is put in, 150 The length of the rope, 120 Again, to form a hawser or cable, 120 fathoms long, and 6 inches in circumference.
An outside yarn, as above, will be 190 fathoms.
Each of the 9 single strands, be fore hardening, 183 Each of the 9 single strands, after hardening, 163 Each of the 3 great strands, when closed, 1471 Each of the 3 grand hardening, 1351 Length of the cable or hawser, 120 fathoms.
Experiments on Ropes.
It was long ago shewn by Dr. Hooke, from several ex periments on the strength of cordage in 1669, that the strength of the component parts of the rope was di minished by twisting. This fact, indeed, has been long practically known to sailors who are familiar with the superior strength of rope yarns when made up in to a salvage, which is nothing more than a skein with out twisting. Salvages are invariably used for sling ing great guns, rolling tackles, and for every kind of work where great strength and great pliancy are re quired.
In the memoirs of the Academy of Sciences for 1711, M. Reaumur has given an account of his experiments on the strength of ropes, compared with that of their parts. The following are some of the most interest ing results: 1. A thread of silk, composed of 832 fibres, broke with from five to five and a half pounds. Each fibre sustained one drachm 18 grains. Sum of the absolute strengths of the fibres, 1040 drachms, or Sibs. 2oz.
Real strength, - - - 5 8 Loss of strength by twisting 2lbs. 10oz.
2. The yarn of a skein of white thread bore each, at an average - - - - - - 9.? lbs.
Two yarns twisted slack into a cord broke with - - - - - - 16 Hence we have the absolute strength of two yarns - - - - - - 191 Real strength - - - - - 16 Loss of strength by twisting 31 lbs.
3. The average strength of some thread was such, that each broke with Sibs. whereas when three were twisted, they bore only 17i lbs.
Hence we have absolute strength 24 Real strength - 17i Loss of strength by twisting 61 lbs.
4. The average strength of some thread was such, that each broke with 73lbs.; whereas when four were twisted, they broke with 214Ibs.
Hence we have absolute strength 30 lbs.
Real strength - 2,11 Loss of strength by twisting 81 lbs.
5. The average strength of other four threads was such that each broke with 9lbs. whereas when twisted, they broke with 221bs.
Hence we have absolute strength 36 lbs.
Real strength 22 Loss of strength by twisting 14 lbs.
6. A well made and small hempen cord broke in different places with 58, 63, 67, and 721bs., so that its 58 ± 63 + 67 + 72 average strength was — 65Ibs.
4 The cord consisted of three strands, and another part of it was untwisted, and its three strands separated. One of them bore 291, another 331., and the third 35. Hence the absolute strength of the three strands, when separate, is 98 lbs.
Real strength when twisted 65 Loss by twisting 33 lbs.
7. Another part of the same cord, which broke with 7Slbs. was separated into its strands, when they bore 26, 28, and 30 pounds.
Hence we have absolute strength, 84 lbs.
Real strength 72 Loss by twisting 12 lbs.
Dr. Robison has given an account of a very interest ing experiment by Sir Charles Knowles, upon a piece of white or untarred rope, 3 inches in circumference. It was cut into many portions, and from each of those portions a fathom was taken off, and carefully opened out. It consisted of 72 yarns, each of which was ex amined separately, and found to bear 901bs. at an aver age for the whole. Each piece of rope corresponding to these was examined separately, and the mean strength of the same pieces was 4552 lbs.