In examining the strength of cordage, 3r inches of circumference and under, M. Duhamel found that the strength increased a little faster than the number of equal threads, thus: Ropes of 9 threads bore 1014 instead of 946 lbs.
12 1564 1262 18 2148 1893 According to the experiments of Mr. Huddart, no strength is lost in the common way when there are only three yarns in the strand. When there are three yarns, the loss is one-sixth, and with a hundred yarns it is about one-half. His registered cordage, according to theory, loses nothing, but by actual experiment it loses one-eighth.
The following rule is given by Dr. Robison for ob taining the strength of ropes.
Multiply the circumference of the rope in inches by itself, and the fifth part of the product will be the number of tons which the rope will carry.
For example, if the rope is 6 inches in circumfe rence, we have 6 times 6 36, the fifth of which is it tons.
There is no branch of the rope manufacture more important than that which relates to the of the cordage. The following experiments were therefore made by M. Duhamel on the relative strength of tar red and white or untarred cordage.
3. That untarred cordage resists the ordinary inju ries of the weather longer than when it is tarred.
These results of direct experiments have been con firmed by the observations of seamen; but they have invariably found, that untarred cordage is weaker than tarred cordage, when it is exposed to be alternately wet and dry; that tarred cordage is chiefly useful for cables and ground tackle, which must be constantly soaked in water; and that cordage, supel:fieially tarred, is always stronger than what is thoroughly tarred, and resists better the alternate conditions of dryness and wetness.
Several important experiments on the relative strength of tarred and untarred ropes were made by Mr. NV. Chapman, chiefly with the view of determin ing the effects of his method of preserving ropes with purified or washed tar. Three pieces of rope were made on the 10th August, 1808, of 12 threads in each strand. The first was an untarred rope, the second a rope made of washed tarred yarn, and the third a common tarred rope. A part of each of these
ropes had their strength tried on the breaking ma chine; and another part was steeped in water for about three months, and then taken to a foundry stove, which is supposed to have been at about 130° of Fah renheit. They remained in the stove about three months. After that they lay at Mr. Chapman's ropery till November 3d, 1803, when the following experi ments were made with them: The ropes with which the preceding experiments were made, were three French inches in circumference, and were made of the best Riga hemp.
M. Duhamel next examined the relative strength of a parcel of tarred and untarred cordage, which had been manufactured on the 12th of July, 1746. It had man laid up in the store-house, and the following re sults "ere obtained at the dates mentioned.
The tarred ropes were both brittle; but the latter was more so, and they both cracked on bending.
The following experiments were made, in 1807, by Mr. Chapman, for the purpose of showing the injury arising from the retention of that portion of the essen tial oil which cannot be dispensed with, and also the injury which arises from the progressive disengage ment of the acid of essential oil.
From these results M. Duhamel 1. That untarred cordage in constant service is durable than the same cordage when tarred.
2. That untarred cordage retains its strength lot much longer time when it is kept in store.
The first column shows the strength of the rope when made; and the second after having been expos ed to the heat of a stove from 85° to 100° of Fahr.
The following experiments also made by Mr. Chap man, confirm those of Duhamel, respecting the dimi nution in the strength of cordage produced by tarring. The ropes were registered on the improved principle, and were made with the same yarn, and with 17 threads in en' strand.
The following experiments were made with -I' opes made of the same yarns, and of nine in each strand.
The common tarred rope being only about two-thirds the strength of the other.