348 Mr. G. K. B. Elphinstone on the Gyrodatk Gom]jass. [Feb. 23, 



Therefore it is travelling in space about some axis which it sets its 

 own axle and its N. and S. line on the compass card, parallel to which 

 is not the North and South axis of the earth. 



The speed of the ship, the course and the latitude, come into this 

 correction, for which tables are made out, the maximum correction 

 which has to be considered being some 3° ; for all manoeuvring this 

 correction can be neglected. 



The second correction which has to be taken into account is due 

 to the existence of the air blast used in damping, the damping check- 

 ing the precession whenever this takes place. 



The precession varies with the cosine of the angle latitude ; the 

 air blast is constant in its effects in all latitudes, depending only on 

 the speed of rotation of the Gyro — therefore there is a varying cause 

 and a constant retarding force, and in consequence a varying result. 



The effect is that for every 10" of latitude a correction of about ^° 

 has to be applied — 1° in a distance of 600 miles. 



The Gyro Compasses in use in the English navy are adjusted to 

 be correct at 50° north latitude, so that for all cruising in the Channel 

 and say up to the Firth of Forth, this correction does not require 

 consideration. 



Both these corrections can be treated arithmetically by adding to 

 or deducting from the reading on the card the same quantity all the 

 ivaij round ; it does not vary in different parts of the card, as is 

 the case when applying a deviation correction to a Magnetic Compass 

 reading. 



The worst difficulty which the Gyro Compass is faced with is the 

 effect produced by violent rolling or great vibration in a ship. 



This has been receiving a great amount of attention from the 

 inventors during the last eighteen months, since practical experience 

 at sea showed the necessity of some improvements in this respect. 



Fortunately the results of the investigations have led to con- 

 siderable improvement, and to a complete cure of the trouble 

 experienced in this way, so that it will shortly be possible for Gyro 

 Compasses to be installed in ships which are quite independent of the 

 rolling motion or vibration of the vessel. 



[G.K.B.E.] 



