The process of firing one of these guns may thus be summarised. The turntable is swivelled to right or left until the gunners, looking through the sights, which are really telescopes, see the object straight in front of them. Meanwhile the sights have been set according to the range--that is to say, they have been so set in relation to the gun itself that when they point directly at the target the gun will be pointed upwards at exactly the right angle for that range. The whole thing, therefore, gun and sights combined, is tilted upwards or downwards as may be necessary until the sights point directly at the object aimed at. Then at a signal the gun is fired by electricity. The shock causes the gun to slide backwards upon its supporting slide, but the buffers, having taken the shock automatically, return it to its position again; the aim is thus undisturbed and it is ready for the next shot. These enormous guns can be fired at the rate of one shot every fifteen seconds. Field guns are in principle very similar to these, only, of course, they are much smaller and are mounted upon carriages, so that they can be quickly moved from place to place. It must be borne in mind, however, that there are in the case of land guns two distinct types. Field guns, like naval guns, fire straight at their target; howitzers or mortars fire upwards with a view to letting the shell fall on the target from above. The latter are, generally speaking, short, fat, stumpy guns, as compared with the long, slender field guns. In the field all guns have to be loaded by hand. The elaborate system of hoists which enables the great naval guns to be loaded with such rapidity is obviously impossible. That has to be compensated for by the skill and quickness of the gunners themselves, and it is indeed astonishing to see with what deftness they can handle the heavy and dangerous projectiles. With all guns, of whatever kind, range-finding is of the utmost importance. No projectile, however fast it may travel, really moves in a straight line. It must be fired more or less upwards in order to compensate for the downward pull of gravity. If the elevation be insufficient the shell will fall short; if it be too much it may go beyond the mark, or it may fall short, according to circumstances. Just the right elevation is absolutely essential for good shooting. And for that to be achieved the range must be known with the utmost possible accuracy. There are various systems and instruments used for this purpose, but all depend upon the same principle. It is the principle underlying all surveying and all astronomy; indeed it is the only possible principle for measuring a distance when you cannot actually go and lay a measure upon it or by it.