128   THE GYROSCOPIC PENDULUM

of rotation of the ring. This torque also tends to produce precession of the spin-axle about the vertical axis.

The spin-axle should be kept as nearly parallel to the car axles as possible because this is the position in which the gyroscope is least affected by pen friction. For this reason, just as soon as the spin-axle is deflected from parallelism to the car axle by as much as two degrees, a torque is applied to the ring R about the horizontal axis and of sufficient magnitude to prevent the turning of the spinaxle about the vertical that otherwise would occur. The required torque is produced by a pull on one or the other of two soft iron cores, each having one end attached to the ring R and the other end within one of two solenoids D fastened to the frame of the apparatus. The circuit through one solenoid or the other is made or broken as the trolley T, moves from one contact plate across a strip of insulation to another contact plate on the contact arm C.

By adding another control pendulum capable of turning about a horizontal axis perpendicular to the one used in this apparatus, an apparatus could be produced that would furnish a horizontal plane which could be used wherever an artificial horizon is required.

80. Directed Gun-Fire Control. - It is a complex problem to make the observations and computations from which a gun on a pitching and rolling ship may be directed so that the projectile may strike another ship that is moving with respect to the first ship and that is invisible to the gunners. The target must be visible to an observer who is in communication with computers, and the latter must be able to transmit orders to the gunners. The observer may be on a mast above the smoke of battle, or he may be in an airplane at such a height that he can see the target which may be below the horizon of the gunners.

The observer notes the direction of the line from the target to the mother ship, the range, that is the distance between the target and the ship, and he estimates the course and speed of the target. These data are communicated to the computers. They know the course and speed of their own ship, the drift of the projectile from each gun (Art. 48), as well as the approximate deflection produced by the wind. There is an additional deflection when the axis of the gun trunnions is not horizontal. If the right end of the trunnion-axis dips, the projectile will be deflected to the right of the line of sight by an amount depending on the angle of dip. All angles in azimuth must be measured from a base line which is fixed relative to the earth. This base line is indicated by a gyro

GYRO-HORIZONTALS AND GYRO-VERTICALS   120

compass of the highest degree of precision known to the art. Such an instrument is called a " gun-fire control compass," whereas an instrument of a precision sufficient for navigational purposes, but not for gun-fire control, is called a " navigational compass."

These data are set up on the dials of a computing machine called a " range clock " or " range and bearing keeper." This instrument then indicates automatically and continuously what the range and the bearing of the target will be at any assigned subsequent instant so long as the data remain unchanged.*

The appropriate angles of elevation and of train are transmitted to indicating instruments situated at the guns. The gunners keep the gun pointed and trained in the directions given by the indicators. When the ship has rolled to the proper angle the gun is fired, either automatically or manually, as desired. An observer called a " spotter," stationed either on a mast or in an airplane, notes where the projectile strikes and informs the computers. The angles transmitted to the gunners are revised till the target is hit.

As the various gun-fire control computing machines do not depend on gyro-dynamics, they need not be described here. Various sighting instruments and automatic firing mechanisms, however, are stabilized by gyro-pendulums.

81. Gun-Fire Directorscopes. - For a given angle of gun elevation with respect to the deck of a ship, there is but one angle of roll of the ship at which a projectile fired from the gun will strike the target. The optical system of a sighting telescope on a ship may be stabilized by a gyro-pendulum so that the image of a distant target will remain stationary in the field of view however the ship may roll. A co-acting firing mechanism may be set so that when the ship has rolled to an assigned angle with respect to the horizontal, one or more guns will be discharged. If the guns were correctly loaded, correctly pointed with respect to the deck of the ship, and correctly trained with respect to the meridian, the projectiles will hit the target. An apparatus consisting of one or more sighting instruments combined with a co-acting firing mechanism, designed to discharge, in the proper direction and at the proper instant in the roll of the ship, the guns controlled by the apparatus, is called a gun-fire directorscope.

* U. S. Patents. Ford, No. 1370204, 1921; No. 1450585, 1923, No.

1472590, 1923; No. 1484823, 1924. Meitner, No. 1455799, 1923. E. Sperry, No. 1296439, 1919; No. 1356505, 1920; No. 1755340, 1930.