j ;<-...-. 



F.xperi- 

 nicnts on 

 the strength 

 of Cordage. 

 Dr.Hooke'i 

 experi- 

 ments. 



Salvages. 



Reaumur'* 

 experi- 

 ments. 



of the ropo al'u-r being stretched or used ; and an 

 h c p,, .new the proportion oftwivt in tl: 



out, and in hanienm-, to vary those acrording to the 

 cilwrva'ioiif. wlik'h IK- makes. In hardening the strands 

 laid rope, the earn ie* in fully 



more than one-tenth of the length of the strand. Inlay- 

 in;,' tin Hands together, it only requires to be attend- 

 ihat, \\hile a.-: imieh alter turn is given as is ne- 

 cessary for the ri^ht formation of the rope, just so much 

 twist should be communicated to the btrands by the 

 forehooks, as will counteract the opposite or after turn. 

 This is easily seen by observing whether the strands be- 

 fore the top, are shortening, or otherwise, while the 

 rope is cloMng. A chalk mark upon the strand, 

 close by a stake-head, will show this. If this is 

 not attended to, and too much turn given, some of 

 tie outside yarns will most probably snap, even in the 

 closing. 



The above observations will be in some measure il- 

 lustrated by the following comparative lengths of yarn, 

 t,t rands, 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, - - 1C6 



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, - - 1 83 



Each of the 9 single strands, after 

 hardening, - - lC3 



Each of the 3 great strands, when 

 closed, - - - MTl 



Each of the 3 grand strands, after 

 hardening, - - IS 5* 



Length of the cable or hawser, 120 fathoms. 

 Experiments on Rones. 



It was long ago shewn by Dr. Hooke, from several 

 experiments on the strength of cordage in 1669, that 

 the strength of the component parts of the rope was 

 diminished by twisting. This tact, 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 slinging 

 great guns, rolling tackles, and for every kind of work 

 where great strength and great pliancy are required. 



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 interesting 

 results. 



1 . A thread of silk, composed of 832 fibres, broke with 

 from five to five and a half pounds. Each fibre sus- 

 tained one drachm 18 grains. Sum of the absolute 

 strengths of the fibres, 1040 drachms, or bibs. 2oz. 

 Real strength . . 58 



Loss of strength by twisting 2lbs.lOoz. 



VOL. XVII. PART II. 



ROPEMAK.ING 465 



The yarn of a ckein of white thread bore each, at 



rage 



Two yarns twisted black into a cord broke ^^> 



with .... 16 



. e the absolute strength of 

 two yarns . 19$ 



Real strength 



Loss of strength by twisting 3$ Ibt. 



3. The average strength of tome thread was buch, 

 each broke with 8lbs. whereas when three we re 



twisted, they bore only . 1? Ibs. 



Hence we have absolute strength 



Heal strength . . . . 17$ 



Loss of strength by twisting 6' Ibs. 



4. The average strength of some thread was eucb, 

 that each broke with 7 Jibs. ; whereas vthenfour were 

 twisted, they broke with 2llbs. 



Hence we have absolute strength . 30 Ibs. 



Kcal strength . . . 21 



Loss of strength by twisting 8| lb*. 



5. The average strength of other four threads was 

 such that each broke with plbs. whereas when twisted, 

 they broke with 22lbs. 



Hence we have absolute strength . 3(> Ibs. 



Real strength . . . S3 



Loss of strength by twisting 1 4 Ibf. 

 G. A well made and small hempen cord broke in 

 different places with 58, 63, 67, and 72lbs., so that its 



58 + 63 + 67 + 72 

 average strength was = 051bs. 



4 



The cord consisted of three strands, and another part 

 of it was untwisted, and its three strands separated. 

 One of them bore 29, another 33^, and the third 35. 

 Hence the absolute strength of the three strands, when 

 separate, is .... 9* Ibs. 



Real strength when twisted . 65 



Loss by twisting 33 lb. 



7. Another part of the same cord, which broke with 

 78lbs. was separated into its strands, when they bore 

 20, 28, and 30 pounds. 



Hence we have absolute strength . 84 Ibs. 



Real strength .... 72 



Loss by twisting 12 Ibs. 



Dr. Robison has given an account of a very interest- Experi- 

 ing experiment by Sir Charles Knowles, upon a piece menu of 

 of white or untarred rope, 3^ inches in circumference. Sir C. 

 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 90lbs. 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 Ibs. 



Hence we have absolute strength of yarns 6480 

 Real strength 



Loss of strength by twisting 1928 Ibs. 

 As the diminution of strength in the yarns, demonstra- 

 ted by the preceding exam pie, obviously arises from their 

 position when twisted, in consequence of which they do 

 not all bear the load at the same time ; and not from any 

 permanent weakness produced by the twisting it 

 became reasonable to believe, that the twist given to 

 3 N 



