Report on the Progress and State of Applied Mechanics. 213 



called "sea-worms," whose burrowing would otherwise reduce most 

 immersed pieces of timber to the condition of a honeycomb. 



27. The discovery of new fibeous tegetable substances, in addi- 

 tion to those ah-eady known, occupies the attention of many naturalists. 



28. Few materials have contributed more, by their discovery, to the 

 advancement of practical mechanics than iNDiA-EtrBBER and Gutta- 

 PEECHA ; and especially the compound of the former substance with 

 sulphur, called Vulcanized India-eubbee, which, by the perfection of 

 its elasticity, the great variations of form that it is capable of undergo- 

 ing, and its preservation of those properties throughout a great range 

 of temperature, is equally well adapted to act as an extensible spring, 

 and as a compressible cushion, to prevent shocks between hard surfaces. 

 GuTTA-PEECHA, tliough softencd by a moderate degree of heat, possesses 

 a strength and an elasticity, at ordinary temperatures, which enable it 

 to be employed as a substitute for leather belts in machinery. Its 

 recent application to the coating and insulating of telegraph-wires, is 

 well known. 



29. A new process of dressing leather was introduced a few years 

 ago, by which its strength is rendered much greater than it formerly 

 was ; and it is better fitted for supporting heavy loads, and transmitting 

 intense forces in machinery. 



30. As an artificial substitute for natural fibrous material, may be 

 noted the Wiee-eopes of Smith and of Newall; and the wire-cables, 

 applied to suspension-bridges, by Boebling and other engineers, in 

 which the flexibility of a fibrous material is combined with strength 

 greater than is possessed by any animal or vegetable fibre. In most 

 suspension-bridge cables, the wires are parallel ; but in wire-ropes, they 

 are sjiun into a spiral form, hy an apparatus which makes them revolve 

 round each other without turning about their own axes, — so that each 

 wire, although it is hent into a screw, is not tvmted, — a condition 

 essential to the preservation of the strength of the wires unimpaired. 



31. The AET OF PUTTING TOGETHEE the materials of structures, 

 requires for its accomplishment the observance of two kinds of princi- 

 ples, — those of STABILITY and those of steenqth. Stabihty insures 

 that the pieces, of which the structure is made up, shall preserve their 

 proper positions, without being upset or dislocated ; — strength, that each 

 piece shall preserve its figure, and remain whole under the utmost load 

 that is to be laid on it. The theory of stability forms a brancli of the 

 science of statics, depending for its advancement on the application of 

 mathematics to questions whose experimental data are few and simple, 

 such aa the weights of bodies, and the friction lietweeu their surfaces ; 

 the theory of strength, depending on mathematical investigation also. 



