The ordinary meaning of nap is to doze off during the day. The secondary meaning is to be offguard. Hence the phrase Catch them napping . Flying by the nap of the earth refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or napping).
the tribunes The Tribunes were elected representatives that protected the plebeians from mistreatment. The Tribunes attended meetings of the consuls or Senate in an attempt to bring justice to the plebeians.
Where operations permit, welders or other persons adjacent to the welding areas shall be protected from the radiant energy and spatter of welding and cutting arcs by non-combustible or flame-resistant screens or shields, or shall be required to wear suitable eye/face protection and protective clothing.