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DRAINING.

The covering in of drains, or leaving them open, is a question, at times, of neces-sity (as generally in B); at others, of economy,—the decision resting on the balanceof profit and loss between the value of the unoccupied surface and the expenses ofcovering; and in other cases of mere convenience—as in the more expeditious casesof military draining, Aa. A b., where nothing else could be gained by such an operation.

All that concerns this part of the work will be A a. A b. Ac. A/., the remainderbelonging to Permanent Works, except as relative to ‘ Sanitary Precautions,’which see.

A a. A b. —These are most likely to occur in facilitating siege operations, either bydepriving the enemy of the means of flooding the approaches, or to lay open anotherwise inaccessible front. From the probable nature of the case, the measureto be adopted will most likely be of the simplest description, though in deter-mining the volume and course of the stream, care should be taken, if possible, toreserve some control as’to continuance; and to calculate effects not only as tothe purpose in hand, but on communications in general, by such a body of waterentering a new channel. If possible, also, the excavating parties should com-mence at the end farthest from the point where the waters are to be let out.

The ‘ measure ’ above mentioned is the determining a new channel approaching,on the whole, to the straight line ab (figs. 1, 2), by which the contents of thelake, &c., a (which has hitherto received all the waters on the right of the water-shed c c), are to cross that line c c, and join the new course b, which may be a gully,or streamlet, leading to quite a different part of the country.

Now it by no means follows that the straight line a b will be the best, though,geometrically, the shortest. Greater facility in excavation, less amount of cutting,greater suitability of ground for the channel, &c., &c., may warrant even such adeviation as a d b ; which line need not be constantly in the same plane with a b(as a g h i j b, fig. 3) ; nor always on a winding course inclined to the horizon atone unvaried angle (as a n' m' if b, fig. 4),—but so regulated, if possible, that wherethe ground is not homogeneous as to the consistence of its material, the steepest partsshall be where the rock or soil is hardest, and thus meet the most violent action withthe greatest resistance. Nevertheless, the best course will generally be between suchlimits as are given by a b, a' b, of figures 3,4, respectively ; as, though much latitudemay be allowed the line adb, fig. 2 in plan, very little can be admitted in section.—See ‘ Levelling.’

Fig. 1. Section of Fig. 2.

ICVEl AT HICHEST WSTER