=4. Ricochets.=--Bullets which rebound after striking the ground or any other obstacle, and continue their flight, are said to ricochet. Ricochets may occur from any surface, and bullets may ricochet two or even three times before their flight is finally arrested. At long range, they are less likely to ricochet from soft ground than from hard, smooth surfaces. =5. Firing Up and Down Hill.=--(i) When a shot is fired at a target placed on the same level as the firer, the forces acting on the bullet cause it to travel in its greatest curve, and the greatest elevation for any given distance must therefore be given to the rifle. If a shot is fired perpendicularly upwards or downwards, no elevation is required, for the bullet will travel in an approximately straight line until its impetus is exhausted. Hence it follows that, when shooting up or down hill, less elevation is necessary than when the object is on the same level. (ii) _For practical purposes, the effect of firing up and down all moderate slopes will be negligible, and may be disregarded by the soldier in taking aim on service._ Within close range, in firing up and down steep slopes, or slopes at an angle of 10 degrees and over, the decreased elevation necessary will usually be small, and may be allowed for by aiming down slightly. The correct elevation to be used in firing up or down hill at longer ranges in various circumstances is a matter for the judgment of fire-unit commanders, and can best be ascertained by careful observation of fire, when possible. Section =10=.--Effects of Barometric Pressure, Temperature, Wind, and Light. =1. Barometric Pressure and Temperature.=--Rifles are sighted for the following conditions: Barometric pressure, 30 inches (sea-level); thermometer, 60° F.; still air; a horizontal line of sight. =2.= The rise and fall of barometric pressure and temperature affect the flight of the bullet and elevation by changing the density of the atmosphere, and so increasing or decreasing its resistance to the bullet. For practical purposes, the soldier may disregard the effects of barometric pressure and temperature as negligible. In operations conducted at considerable heights above sea-level, orders as to allowance for barometric pressure will, if necessary, be issued by the Staff. =3. Effect of Wind=--(i) =Head Wind, or Wind from the Front.=--A head wind, or wind from the front, retards the bullet, and necessitates _more elevation_. (ii) =Rear Wind.=--A rear wind lessens the resistance of the air, and necessitates _less elevation_. (iii) =Side Wind.=--A side wind, or wind blowing _at right angles_ from either side across the front, acts on the greater surface of the bullet, and has, consequently, more influence on its flight than a wind blowing from the front or rear.