NATURAL PHILOSOPHY. 139 



moderate cost. It was also found desirable to test the separate 

 strands of each cable, as well as the wires themselves. For this pur- 

 pose a number of strands similar to those employed in the manufac- 

 ture of the different cables were produced, and the tensile breaking 

 strain and elongations carefully observed and recorded. In oi-de'r to 

 ascertain whether the length of the lay of the hemp and Manilla 

 round the strand was of that spiral form which produced a maximum 

 of strength, the yarn separated from the strand was als;) tested, and 

 comparing the sum of the breaking strains of the wire and yarn sep- 

 arately, with that of the two in combination in. the strand, the object 

 by these means was approximately obtained. Another very impor- 

 tant question arises in the construction of this cable, and that is, the 

 strength of the core and its conducting wire, and how it is to be pro- 

 tected under a pressure of 7,UUU lb. to 8,000 per square inch, when 

 lodged at the bottom of the ocean. This appeared a question well 

 entitled to consideration, and provided a properly insulated wire, of 

 one or more strands, can, without any exterior covering, be deposited 

 with safety at these great depths, it is obvious that the simpler the 

 cable the better. Assuming, therefore, that gutta-percha is the most 

 desirable material that can be employed as an insulator, it then re- 

 solves itself into the question, what additional covering and what 

 additional strength is necessary to enable the engineer so to pay out 

 of the ship a length of 2,000 miles, into the deepest water, as to 

 deposit it, without strain, at the bottom of the ocean? This is the 

 question the Committee had to solve, and for this very important 

 object experiments were instituted. Regarding the circumstances 

 bearing directly upon the ultimate strength of the cable, the Com- 

 mittee have arrived at the conclusion that the cable No. 46, composed 

 of homogeneous wire, calculated to bear not less than from 850 lb. to 

 1,000 lb. per wire, with a stretch of -j 5 - of an inch in 50 inches, is the 

 most suitable for the Atlantic cable. The following is the specifica- 

 tion of No. 46 cable : The conductor consists of a copper strand of 

 seven wires (six laid round one), each wire gauging *048 (or No. 18 

 of the Birmingham wire-gauge), the entire strand gauging '144 inch, 

 (or No. 10 Birmingham gauge), and weighing 300 lb. per nautical 

 mile, imbedded for solidity in the composition known as "Chatterton's 

 Compound." The insulator consists of gutta-percha, four layers of 

 which are laid on alternately, with four thin layers of Chattel-ton's 

 compound, making a diameter of the core of '464 inch, and a circum- 

 ference of 1'392 inches. The weight of the entire insulator is 400 

 lb. per nautical mile. The external protection is in two parts. First, 

 the core is surrounded with a padding of soft jute yarn, saturated with 

 a preservative mixture. Next to this padding is the protective cov- 

 ering, which consists of ten solid wires of the gauge 'Oi)5 inch, drawn 

 from homogeneous iron, each wire surrounded separately with five 

 strands of Manilla yarn, saturated with a preservative compound ; 

 the whole of the ten strands thus formed of the hemp and iron being 

 laid sptrally round the padded core. The weight of this cable in air 

 is 34 cwt. per nautical mile, the weight in water is 14 cwt. per nau- 

 tical mile. The breaking strain is 7 tons 15 cwt., or equal to 11 times 

 its weight per nautical mile in water, that is to say, if suspended 

 perpendicularly, it would bear its own weight in 11 miles depth of 



