1920] on The Gyrostatic Compass 31 



bath of mercury ; in the " Sperry," Fig. 6, by a suspended wire, the 

 twist, if any, being taken out by a follow-up motor, through an 

 electric contact which switches on the current to the motor ; and in 

 the " Brown," Fig. 7, by a hydraulic system of support. The lower end 

 of the vertical spindle acts as a ram and stands upon a column of oil. 

 The oil is under great pressure, some 500 lbs. per square inch, and is 

 kept pumping up and down, and thus raising and lowering the 

 vertical axis continually some 180 times every minute. 



The continual movement of the spindle results in a practically 

 frictionless vertical support, so that the total moving part, some 

 7 J lbs. in weight, can be carried round in "azimuth" by the smallest 

 force, due to the earth's rotation ; in fact, so small is the friction that 

 the compass, if deflected, will always come back again to its true 

 north position, certainly within y^th of a degree. I think I am safe 

 in saying that it is the most perfect frictionless support yet given to 

 the vertical spindle of any Gyro-Compass, or indeed of any machine. 



I have mentioned in an earlier part of this lecture that the period 

 of oscillation given to a Gyro-Compass is of the order of 85 minutes. 

 I will now try and explain why this is so. 



The earth has no angular movement from south to north, but 

 has one from west to east, due to the daily revolution on its axis. 



A ship, however, sailing to the north at, say, 20 knots an hour 

 introduces an angular movement in that direction, because it is 

 moving over the curved surface of the ocean, and would complete a 

 revolution of the globe in 45 days. 



If there were a Gyro-Compass on the ship, the instrument would 

 be sensible of these angular movements, and would set itself so as to 

 make a compromise between them, and would as a consequence point, 

 not to the true north, but one or more degrees west of the actual 

 pole ; this division is termed the " North Steaming Error." 



Knowing the latitude, the speed of the ship, and its direction 

 towards the north and south, the extent of the error can be accurately 

 calculated, and speed correction tables have been prepared so that 

 this error can be determined for any latitude, speed and heading of 

 the ship, and can be allowed for. 



Automatic means have also been devised to make these necessary 

 corrections in the reading of the compass. For instance, this, my 

 special form of repeater, has been designed with the card set 

 eccentric, so that, when once set, the correction will be automatically 

 applied without any further reference to the tables. 



If a ship is in harbour, then a Gyro-Compass on board would be 

 pointing due north, but when the ship starts steaming to the north, 

 the compass will begin an oscillation so as to bring the axis of the 

 wheel into the new resting position to include the north steaming 

 error into the reading. 



Getting up speed will, however, have another effect on the com- 

 pass. AYe~know that the gyro wheel is acted upon by a pendulous 



