quickly driven. A variation in the length of the bolt makes no difference, provided it is long enough. It is not necessary to point the ends. Lag screws may be used, if available.

Through bolts, with nuts and washers, are the most secure form of fastenings; and, if procurable, they should be used for all important joints, except sills and caps which are usually "riders" (butted on the trestle legs), and for which drift bolts, dogs, or scabbing, are employed. For round timbers, through bolts have the disadvantage that they are generally either too long or too short, requiring the use of wooden washers in one case, or dapping out the timber in the other—in either instance involving a loss of time. Turned bolts are invariably used in lieu of field rivets in the assembly of sectional steel trusses.

A full supply of nails and spikes of assorted sizes should, of course, be provided.

Sizes of Individual Members

For bridges at the front, the sizes of individual members must generally be such that they can be readily handled and placed by unskilled labor, with the aid of animals and ordinary tackle, gin-poles, or shears. In the cases when plant is available, heavier pieces may be used to advantage. The principal considerations limiting the size of individual pieces are the requirements of transportation, by motor truck as well as by rail.

Plant and Tools

The military bridge builder in the past has made comparatively little use of heavy construction plant in his operations, but this resulted from necessity rather than choice. Construction plant is utilized whenever practicable; and in view of the great capacity now required of military bridges, portable plant is necessary even in the combat zone. However, in the tactical operations of the combat troops the opportunities for the employment of plant will be relatively few. The work of the military bridge builder is spread over a considerable area; he moves rapidly from place to place; and he cannot be unduly hampered in his movements by the necessity of transporting heavy construction plant. Usually the plant is not at hand when needed, and it can seldom be depended upon. Moreover, one of the principal uses of plant is to save manual labor, and this consideration has less weight in military than in civil constructions.

There are, however, a number of pieces of light, portable construction-plant that may often be advantageously employed, even at localities quite close to the front. These include small hoists, pile-drivers, compressors; pneumatic tools, concrete mixers, and rock crushers. Gasoline is the best motive power, because of the compactness of both machine and fuel. For works on the lines of communication, heavy, standard construction-plant is frequently adopted. For the future, portable hoists and pile-driving outfits will be provided for corps and army engineer-troops.