14

again saturated and copiously painted. No further precaution has been consid-ered necessary for the preservation of those cables which have been constructedin Europe . But in order to insure their durability still further, and to improvetheir appearance, I have invented a machine by which a continuous wrappingof wire can be laid around, perfectly close and tight. Neither air nor moisturecan penetrate a good wrapping, and if it is kept painted, no further precautionis necessary for the preservation of the cable. This improvement has beenapplied on the cables of the Monongahela bridge, as well as on those of theaqueduct.

The advantage of a well manufactured wire cable is, that while it lastsequally as long as solid bars, it possesses a far superior degree of strength, whichcan be depended on at all degrees of temperature. But in order to cause eachstrand to bear an equal share of the whole strain, the tension of the differentwires must be exactly alike, and this is not so easily effected as might at firstsight appear. To insure a perfect cable, much experience and judgment isrequired, and a constant vigilance and caution is to be exercised. Green handscannot be trusted with any part of this delicate operation without being previ-ously well instructed, and constantly watched duriug the progress of the work.It is not considered necessary here to enter into the details of construction, norcould the operation of a somewhat complicated machinery be well understood,without being illustrated by numerous drawings.

Before closing my remarks on the cables, I will explain the mode by whichthey, as well as the anchor chains, will be preserved under ground. Themethod usually adopted by European engineers in the construction of the anchor-age of suspension bridges, for the passage of the chains or cables, is to form ahollow channel in the anchor-masonry or in the rock, as the case may be, andto leave this passage open for the free admittance of those persons who havecharge of the structure, and whose duty it is to examine the fastenings fromtime to time. When I devised the plan for the Pittsburgh aqueduct, I hadnecessarily to depart from the old and long established mode of building. Icould not, with any hope of success, resort to open passages and expect to keepthe water out, while it was in the aqueduct. I therefore concluded to wall thechains in solid masonry, and to preserve them against oxidation, first by twocoats of pure red lead, and secondly, by surrounding them completely, while thewalls were carried up, with hydraulic cement. Pure calcined lead, mixed withboiled linseed oil, furnishes, as is well known, an indestructible paint. The oilmay, in course of time, disappear, but the lead never will; it will preserve theiron against oxidation as long as it remains in contact with it. The hydrauliccement forms a compact body around the iron bars and the cables, impervious toair and water. And as it is left below ground undisturbed, its settling will takeplace slowly, but surely, and without shrinking. As a solid incrustation, thecement will afford an adequate protection to the chains, as well as to the cables.In addition to this, it is well known that a coat of lime or calcareous cement isa most effective preservative against rust. The greater affinity of oxygen forlime will prevent its combination with iron until the former is completelycrystalized. Should moisture find its way to the iron, it will not reach it with-out passing through a large body of lime mortar and cement; it would, there-fore, be rendered calcareous, and add another coat of lime to the iron. By theabove method, it is proposed to insure the chains and cables undergroundagainst oxidation, and thereby to render that constant vigilance unnecessary,which otherwise would be indispensable for the preservation and safety of thestructure.