If the shell went straight it would only be necessary to point the gun in the right direction and the object would be hit no matter how far away it might be. The more curved the course is, the more likely the shell is to fall either too near or too far, in the one case dropping into the water, in the other passing clear over the opposing ship. Let us look at it another way. Suppose the vital parts of a ship rise 20 feet out of the water and the shell arrives at such an angle that it falls 20 feet in 100 yards: then, if the ship be within a certain zone 100 yards wide it will be hit in a vital spot. If it be nearer the shell will pass over, if it be further the shell will fall into the water. That 100 yards is what is called the "danger zone." If the shell is falling less steeply, say, 20 feet in 200 yards, then the danger zone is increased to 200 yards and so on, which gives us the rule that the flatter the trajectory, or the more nearly straight the course of the shell the greater is the danger zone and the more likely is the enemy ship to be hit. We have established two facts, therefore, first, that the trajectory must be as flat as possible and, second, that to make it flat the velocity must be high. We can also see another reason for high velocity, namely, to give penetrating power. To obtain a high velocity the gun must be long, and consequently naval guns are always long, a fact which is very noticeable in the photographs of warships. The reason for this is quite obvious after a little thought. You could not throw a cricket ball very far if you could only move your hand through a distance of one foot. To get the best result you instinctively reach as far back as ever you can and then reach forward as far as you are able, so that the ball shall have as long a journey as possible in your hand. Perhaps you do not know it but all the time you are moving your hand with the ball in it you are putting energy into that ball, which energy carries it along after you have let go of it. And it is just the same with the shell in the gun. So long as it is in the gun energy is being added to it but as soon as it leaves the muzzle that ceases. After that it has to pursue its own way under the influence of the energy which has been imparted to it. The powder which is employed as the propellant or driving power is of such a nature and so adjusted as to quantity that as far as possible it shall give a comparatively slow steady push rather than a sudden shock, so as to make full use of the gun's length, the expanding gases following up the shell as it goes forward and keeping a constant push upon it. On the other hand, a gun can be too long, for no steel is infinitely strong and stiff, so that beyond a certain limit the muzzle of the gun would be likely to droop slightly of its own weight and so make the shooting inaccurate. The limit seems to be about 50 calibres or, in other words, fifty times the diameter of the bore.