1920] on The Gyrostatic Compass 27 



To indicate the severity of the ship's movements I may recall a 

 recent trip of this G-yro-Compass on board a fast destroyer. * During 

 a severe gale the ship was recorded to roll over 50° of total angle. 

 Many of the crew were forced to lie on the decks, the lockers emptied 

 their contents, and even some of the oil-lamps suspended from the 

 ceiling were unseated by the pitching of the vessel, and yet the Gyro- 

 Compass maintained its accuracy and allowed the ship to be safely 

 steered into harbour, to which she had to run for safety. 



In all this whirlwind of movement the Gyro-Cornpass heard 

 and only responded to the still small voice of the earth's rotation. 

 How small this force is I shall try to indicate to you a little 

 later on. 



For use on board ship the compass must be mounted on a 

 pendulum in gymbal rings, and its period of oscillation is lengthened 

 to something like 85 minutes, which is usual in practice, so that the 

 rolls, which are of the order of 7 to 15 seconds' period, have but 

 small effect on the compass. 



In this case the rotation of the earth does not act directly upon 

 the gyro wheel, but by means of the force of gravity through the 

 pendulous weight. Unfortunately this form of mounting introduces 

 troubles of its own. 



Suppose we study our simple gyrostat and see what happens 

 when we attach a weight to the end of the horizontal spindle : this 

 will give us some idea of what occurs when the force of gravity is 

 acting through the pendulum trying to tilt the gyro wheel. 



We know from our law that the wheel will precess under the 

 tilting action, but as the new direction of rotation that we are trying 

 to produce by means of the weight, unlike that produced by the 

 earth, which is always in one direction, is in this case continually 

 carried round by the precessing wheel, the precession is permanently 

 maintained. We also find that if we hurry the precession the 

 spindle rises, lifting the weight ; while, on the other hand, if we 

 delay the precession the spindle drops, and the weight falls. The 

 rate of precession is proportional to the weight ; halving the weight, 

 for instance, halves the rate at which the wheel rotates round the 

 vertical support. 



Coming back again to our pendulous mounted Gyro-Compass. 

 Suppose the spindle is pointing west, and is horizontal, then the 

 earth as it rotates will leave the wheel pointing in this one direction 

 in space, but the weight will try and follow the earth's rotation, and 

 will start precessing the gyro towards the north. The rate at which 

 the wheel comes to the north depends upon the weight attached to 

 the casing. All the time the wheel is coming to the north the earth 

 is adding to the rate of the precession, and the spindle is as a con- 

 sequence tilted, and is deflecting the weight at the north position. 

 Under these conditions the effect of the weight is to continue the 

 precession, and the gyro wheel will swing through the north position, 



