In some cases these duties are all carried out by one man, but in others there are two: one the pilot who looks after the working of the machine, and the other the observer whose whole attention can thus be devoted to scrutinizing the enemy. Of course, when aeroplanes go on scouting expeditions like this they are apt to be attacked by the enemy both by anti-aircraft guns and also by other aeroplanes. The former can only be met by high speed and the steering of a somewhat erratic course so as to confuse the gunners and prevent them from taking good aim. The other aeroplanes, however, must be met by actual fighting. The only way to defeat them is to go for them and attack them, a machine-gun being the most usual weapon. Besides those who go up for definite scouting operations or to "spot," as it is termed, for the artillery, there are other machines whose sole duty is fighting. These go up for the purpose of driving off those machines of the enemy which may come prying, or to keep the ground, so to speak, for the scouting machines and enable them to do their work unmolested. Then there are, of course, still others whose function is to carry out bombing expeditions. All these different duties call for different types of machine, but I do not propose to go into the differences here since changes are so rapid in this particular field that only the general principles remain unchanged for any length of time. What has just been hinted, however, as to the different kinds of work which the aeroplane is called upon to do will enable the reader to see why different kinds of machines are needed. So far we have only spoken of aeroplanes. There is a kind of machine sometimes called a hydroplane but which we are gradually getting to call a sea-plane. The latter term is much to be preferred, since the former is also in use to denote a special kind of high-speed boat. Now a sea-plane only differs from an aeroplane in that it has floats instead of wheels. The aeroplane has wheels to enable it to alight upon and arise from the ground: the sea-plane has floats by which it can alight upon the water and arise from the water also. In some instances this float idea is made so pronounced a feature of the machine that it becomes a flying boat. Sea-planes are therefore really only aeroplanes specially adapted for a certain purpose. They are really just as much aeroplanes as those machines which go by that name. It is somewhat unfortunate, therefore, that a separate term is used to describe them. But there it is: names grow in a very curious way, not always in a logical way, and a name having once stuck to a thing in the mind of the public it is very difficult to make any alteration. Aeroplanes, then, may be said to include a subdivision known as sea-planes, and for the rest of this chapter what is said of aeroplanes will apply to sea-planes also.