24 EFFECTS OF TARRING CORDAGE. 
exactly equal to the twist in the laying, the strands lie less obliquely to the 
axis than in other ropes, and therefore bear a greater load. 
# * * * * 
In examining the strength of cordage, 3} inches in 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 Ib. 
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 more than three yarns, the loss is one sixth, and with a hundred yarns it 
is about one half. His registered cordage, according tv theory, loses nothing, 
but by actual experiment it loses one eighth. 
The following rule is given by Dr. Robison for obtaining 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 circumference, we have 6 times 6 = 
36, the fifth of which is 7} tans. 
Tarring Ropes.—There is no branch of the rope manufacture more im- 
portant than that which relates to the tarring of the cordage. The fol- 
lowing experiments were therefore made by M. Duhamel on the relative 
strength of tarred and white or untarred cordage. 
August 8th, 1741, 
Untarred Rope. Tarred Rope. Difference. 
Broke with 4500 pounds. 3400 pounds, 1100 
4900 3300 1600 
4800 3250 1550 
April 25th, 1743. 
Broke with 4600 pounds. 3500 pounds. 1100 
5000 3400 1600 
5000 3100 1900 
Sept. 3d, 1746. 
Broke with 3800 pounds. 3000 pounds. 800 
4000 2700 1300 
4200 2800 1400 
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 been laid up in the storehouse, and the following results were obtained 
at the dates mentioned. ‘ 
Difference 
« Untarred . 
ae. s.| Rope. ae Difference. 
1746. April 14th . 2645, | 3312" P33 
1747. May 18th . 11 2762 2155 607 
1747, Oct.21st . 6 2710 2050 660 
1748. June 19th 9 2575 1752 823 
1748. Oct.2d . . 4 2425 1837 588 
1749. Sept. 25th . 12 2917 1865 1052 
