3d. _Traverses._ Each gun is usually separated from the next by a traverse, whose thickness when subject to enfilade fire is the same as that of the parapet (30 feet); under other circumstances the thickness may be reduced if deemed advisable, but should, when practicable, be such that a shell bursting at any point within it will blow out at the top or on one side only. 4th. _Bomb_ and _splinter proofs_ sufficient to cover the gun detachment and reserves against vertical fire. The thickness of cover for these is to be regulated according to the principles laid down in par. 21. 5th. _Magazines_ which will hold at least 24 hours' supply of ammunition, besides recesses near the guns for shells and a few cartridges. 6th. Easy and direct _communications_ for bringing up the guns and placing them in position; including tramways, ramps, etc., etc. 7th. _Look-outs_ or _observatories_ from which the effect of the fire can be seen. These when possible will be placed in high sheltered places well on the flanks of the battery and preferably in advance of it. They may be connected with it by signals, telegraph, or telephone, when necessary. 8th. _Screens._--Earthen screens should when possible be thrown up in front of all exposed batteries. CONSTRUCTION OF BATTERIES. =44.= =Batteries for Field Guns.=--When the place is invested, the field artillery is placed in positions considered most advantageous for repelling attacks from the garrison upon the investing force. _Gun-pits_ (described in “Field Fortifications”) are usually made at once for cover for the guns and their detachments. When any of these sites are occupied during the siege the gun-pits may be connected and converted into a battery as indicated by Pl. IV, Figs. 42-45. A similar construction may sometimes be used during the siege when the artillery fire of the place is weakened, and it is desirable to place a field battery in position for reaching some point in the work. As a rule, however, batteries for field guns will during the siege be constructed in the same way as are those for siege guns and howitzers. =45.= =Batteries for Siege Guns and Howitzers.=--These may be _screened_ or _exposed_, _sunken_, or _elevated_. As a rule each battery has a magazine on each flank. The amount of powder necessary to serve two guns for 24 hours (150 to 200 rounds per gun = 2500 to 6000 lbs.) is as much as it is advisable to have in one magazine, in order to limit so far as possible the disastrous effects of an explosion. For this reason the number of guns in a battery is usually restricted to four. This number may be increased when necessary, or when howitzers firing small charges render it unobjectionable.