interest on the original investment, annuity to provide for the replacement of the structure when the time comes for its renewal, insurance when necessary, and charges for maintenance and repairs.

It is evident that, by increasing the original investment, we can usually provide a structure that will have a longer life and will require less annual expenditure for maintenance and repairs. However, we are considering here only the subject of maintenance and repairs, and shall take into account only such structures as are actually built, the economics of design and the types of structure best suited for various purposes and conditions having been covered in other chapters.

All structures should have periodical and careful inspection; and when anything is found requiring attention, it should be done promptly. The neglect of attending quickly to minor repairs needed will usually result in much more extensive and costly repairs later on; for, according to an old and well known proverb, "a stitch in time saves nine."

When an organization, such as a Railway Company or a City, has a large number of bridges to maintain, it is customary to have a department to look after the work, with the requisite force of men and the necessary equipment; and these should be continually engaged in making repairs to bridges and like structures.

Repairs to timber bridges are more frequent than those on permanent structures, on account of the liability of timber to decay. Wood preservation, especially by the process of creosoting, has taken wonderful strides in the last few years. With the growing scarcity of timber and the rise in its price, such preservation will increase until it will be the exception rather than the rule to have untreated timbers in exposed structures. The majority of wooden bridges now in service on railways are trestles; although, in some parts of the country, timber or combination-timber-and-steel truss-spans are still used. Timber structures can be kept up almost indefinitely by replacing members shortly before they become dangerously decayed; however, when a bridge is so old that a large portion of the timber shows more or less decay, it is more economical to renew the whole structure, salvaging such second-hand material as possible, after which the bridge will be serviceable without further repairs for a period of years.

In case of pile-driven trestles, it is cheaper under some conditions to replace the piles with frame bents than it would be to renew the structure as a pile-driven trestle. In this case the old piles are cut off under the ground until sound timber is reached, usually about two or three feet; and the new frame bents are supported on these old piles. The latter usually rot off close to the ground line; but in good soil they will remain sound at a depth of two or three feet for a long period of years. The objections to this procedure are that the sill, being underground, is difficult to inspect and that a frame bent structure is not as stiff as a pile structure; consequently, if a pile driver is available, it is preferable to drive pile bents rather than to construct frame ones.