[1153]. Notches and loopholes, [Figs. 20]–22, are alike in all respects, except that the latter have a roof or top and the former have not. The bottom, also called floor or sole, is a part of the original superior slope. The sides, sometimes called cheeks, are vertical or nearly so. The plan depends upon local conditions. There is always a narrow part, called the throat, which is just large enough to take the rifle and permit sighting. From the throat the sides diverge at an angle, called the splay, which depends upon the field of fire necessary.
The position of the throat may vary. If on the outside, it is less conspicuous but more easily obstructed by injury to the parapet and more difficult to use, since in changing aim laterally the man must move around a pivot in the plane of the throat. If the material of which the loophole is constructed presents hard surfaces, the throat should be outside, notwithstanding the disadvantages of that position, or else the sides must be stepped as in [Fig. 22]. In some cases it may be best to adopt a compromise position and put the throat in the middle, [Fig. 22]. [Figs. 23] to 26 show details and dimensions of a loophole of sand bags.
A serviceable form of loophole consists of a pyramidal box of plank with a steel plate spiked across the small end and pierced for fire. [Fig. 27] shows a section of such a construction. It is commonly known as the hopper loophole. The plate should be 3/8 in. thick, if of special steel; or 1/2 in., if ordinary metal. [Fig. 28] shows the opening used by the Japanese in Manchuria and [Fig. 29] that used by the Russians.
The construction of a notch requires only the introduction of some available rigid material to form the sides; by adding a cover the notch becomes a loophole. Where the fire involves a wide lateral and small vertical angle, loopholes may take the form of a long slit. Such a form will result from laying logs or fascines lengthwise on the parapet, supported at intervals by sods or other material, [Fig. 31], or small poles covered with earth may be used, [Fig. 30].
[1154]. Overhead cover. This usually consists of a raised platform of some kind covered with earth. It is frequently combined with horizontal cover in a single structure, which protects the top and exposed side. The supporting platform will almost always be of wood and may vary from brushwood or light poles to heavy timbers and plank. It is better, especially with brush or poles, to place a layer of sods, grass down, or straw, or grain sacks over the platform before putting on the earth, to prevent the latter from sifting through.
 Fig. 32 |  Fig. 34 |
 Fig. 33 |  Fig. 35 |
| Types of overhead cover. | |
The thickness of overhead cover depends upon the class of fire against which protection is desired, and is sometimes limited by the vertical space available, since it must afford headroom beneath, and generally should not project above the nearest natural or artificial horizontal cover. For splinter proofs a layer of earth 6 to 8 ins. thick on a support of brush or poles strong enough to hold it up will suffice if the structure is horizontal. If the front is higher than the rear, less thickness is necessary; if the rear is higher than the front, more is required. For bombproofs a minimum thickness of 6 ins. of timber and 3 ft. of earth is necessary against field and siege guns, or 12 ins. timber and 6 ft. of earth against the howitzers and mortars of a heavy siege train, not exceeding 6 inches in caliber.