26 Mr. Sidney G. Brown [Jan. 30, 



axis, however little, from this position of rest, and the action of the 

 earth comes in again to precess the wheel back again to the north. 



I have here a simple form of gyrostat with three degrees of 

 freedom. If I hold it in my hand and revolve on my axis it does 

 no! move the wheel, which still keeps pointing to the same part of 

 the room. 



On the other hand, if I restrain or clamp one of its degrees of 

 freedom so that I am able to tilt the axis of the wheel during my 

 revolution, I cause the wheel to precess and to set its axis parallel to 

 the axis on which I am revolving. Reversing the rotation, the wheel 

 also reverses. 



This is what takes place with the gyrostat on the earth's surface, 

 provided it is frictionlessly mounted. I have here such an instru- 

 ment, and I will try and demonstrate by its means the rotation of 

 the earth. 



A wheel is rotating inside this case at 15,000 revolutions per 

 minute. 



The case is constrained to move about this vertical friction- 

 less axis. 



Mere motion of translation will have no effect in changing the 

 direction of the axis of the wheel, but if this room rotates, the axis of 

 the wheel will tend to set itself parallel to the axis about which the 

 room is rotating. 



We all believe that this room is rotating about the axis of the 

 earth ; if so, the axis of the wheel will try to set itself parallel to the 

 axis of the earth, but it must be kept horizontal, and therefore it 

 will point north and south. 



Here it is pointing in an E. and W. direction ; it is held by a 

 string. I will now burn the string, and it will find for us the true 

 north. 



Observe that it is really the true north direction, whereas the 

 magnet points to the magnetic north. 



I will set it away from the north, but on the other side, and repeat 

 the experiment. 



Such a simple form of Gyro-Compass could not be of any use on 

 a moving ship, because the rolls of the ship would react too violently 

 on the spinning wheel and cause considerable deviations in the 

 readings of the compass. 



The use of a Gyro-Compass on land is very limited, and its great 

 value at the present time is on board ship. 



I have intimated to you that the spinning wheel is acted upon by 

 forces which tilt the axis. Now a rolling and pitching ship is about 

 the worst place to put a gyrostat to act as a compass, because the 

 ship's movements all tend to tilt the axis. 



The problem, therefore, is to make the compass insensible to the 

 moveme nts of the ship, and respond only to the slow angular rotation 

 of the earth. 



