=3. Back-Reckoning.=--Back-reckoning may be defined as deduction as to distance made from any known range. For instance, in advancing on a position the range of which is ascertained on commencing the advance, the distance gained by each forward movement may be deducted from the original range of the position in estimating its range for firing on it at successive halts. Again, the range of an objective further away than another objective, of which the range is known, may be estimated by adding to the known range the estimated distance of the former from the latter objective. An example of back-reckoning is shown in Fig. 44. =4. Ranging by Gun-Flash and Sound.=--Light travels at the rate of 186,000 miles a second, so that a gun-flash is practically seen as it issues from the muzzle, irrespective of the distance of the observer. Sound, on the other hand, travels about 1,100 feet, or about 365 yards, a second. By timing the interval between seeing the flash of a gun and hearing the report, its approximate distance may therefore be deduced. This, of course, will only be possible when the position of the gun is betrayed by the flash, and when guns are fired singly at appreciable intervals, or simultaneously, so that there is practically one flash and one report at a time. =5. Range-Finding Instruments.=--A certain number of officers, N.C.O.’s, and men in each unit will be trained to use the One-Man Range-Finder or the Mekometer instrument, full directions regarding the use of which will be found in the official handbooks dealing with these instruments. Section =40=.--Range-Cards and Range-Marks. =1. Range-Card.=--A range-card is a device for giving ranges to various points in a field of fire for use in attack or defence. Range-cards should be prepared by fire-unit commanders for their own use and for use by N.C.O.’s and observers. When possible, the best way to prepare a range-card is to use a piece of a large scale map of the country on which to draw the range circles and direction lines. There will usually be more time for preparing range-cards in defence than in attack, and consequently range-cards prepared for defensive positions will as a rule be more elaborate than those made for use in attack. [Illustration: =Fig. 44.=--SIMPLE RANGE-CARD FOR ATTACK. _Note._--The distances on the left are those from the enemy’s position. Those on the right in brackets are only for the range-taker’s information, and should not be shown on the card to avoid confusion.]