Now this regulation of the revolutions of the ship's engines has a most important part to play in warfare. One of the most essential things in naval manoeuvring is that ships shall keep a certain distance from one another. It avoids collisions, and preserves regularity in fire and in changing positions at critical times. It is as essential as that soldiers shall present a solid line to the enemy in battle on land. A helter-skelter fleet would be beaten from the start in a fight. It is most difficult for ships to keep at regular intervals. The engines of one turn just a little faster than the engines of another, and little by little a ship creeps up or drops away from its fellows. Sometimes the distances are preserved by guess-work. Lately a little instrument has been invented by which the Captain can see at a glance how far he is from the ship ahead of him. It is a modification of the sextant. The height of a certain object on the ship in front is known. That is the base of a triangle. The size of the angles at the end of that base are seen at a glance by the observer, and by the manipulation of a screw or two the Captain of a ship can see on a sliding-scale whether he is going too slow or too fast. In either case he signals a change or two in the number of revolutions he wishes the engines to make, and he preserves his required distance. Accuracy in this matter may win a battle. So great is the din and confusion on war-ships in time of battle that what are called "visual signals" are demanded. This has brought several contrivances into operation that are new. For example, we have a transmitter of orders. It tells the gunners when and what guns to load, and with what kind of shot; when to stand ready to fire; when to fire, and when to cease firing. In the old days, and even in the present days, such orders are conveyed in speaking-tubes or by telephone on most ships. In the great noise an order may be mistaken, but with a visual signal in the shape of an indicator, operated by the mere pressing of a button, orders from the Captain may be conveyed clearly and instantly.