Another chamber contains the compressed air which furnishes the motive power. This also serves to give buoyancy. Another chamber, again, contains the engines, beautiful little things of the finest workmanship almost exactly like the finest steam-engine, but of course very small in comparison. In the early stages the range of the torpedo was limited by the amount of compressed air which it could carry. At first sight there seems no reason why any limit should be placed upon this, but in practice there are often limitations in engineering matters which are not apparent on the surface. For example, to increase the air chamber would mean enlarging the whole torpedo, calling for more propulsive power and larger engines, and these larger engines would call for more air, thus defeating the object in view. Forcing more air in by using a higher pressure, in a similar way would necessitate a thicker chamber, to resist the higher pressure. This would add weight, calling for more buoyancy. Thus there seemed to be a practical limit beyond which it was impossible to go. The difficulty was overcome, however, in a very cunning way. When the engines have used some of the air, and the store is somewhat exhausted, chemicals come into action which generate heat, which is imparted to the air which is left. This heat expands the air, producing in effect a larger supply of it, and enabling the torpedo to make a longer journey. Steering in a horizontal direction--that is to say, to left or right--is done by a gyroscope. The action of a rotating wheel is discussed in the last chapter, and it is not necessary here to say more than this: a rotating wheel always tries to keep its axle pointed in the same direction. Just at the moment of starting such a wheel is set going inside the torpedo, and its arrangement is such that, should the torpedo swerve to the left, the gyroscope operates the rudder and steers it back. In the same way, if it tends to turn to the right, the ever-watchful gyroscope brings it to its true course once more. The effect of the gyroscope, therefore, acting upon the rudder, is to keep the torpedo faithfully to the direction upon which it is started. The up and down rudders are likewise controlled quite automatically, but in a different way. Their function, clearly, is to keep the thing at a certain uniform level. Without such control a torpedo would be equally likely to jump out of the water altogether, or to go downwards vertically and bury its nose in the mud. The depth at which it is to move is determined beforehand, certain necessary adjustments are made, and the torpedo then pursues its even way, neither coming to the surface nor driving beneath its target.