114 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1911. 



velocity of rotation frees it from any danger of deformation due to 

 shocks. 



A small level is attached to the compass card to assure that the 

 instrument is horizontal. The whole system is mounted on gymbals 

 and attached to the binnacle with springs after the usual manner with 

 marine magnetic compasses. 



The damping of the oscillations of the compass is effected in a very 

 ingenious manner. Near the center and on both sides of the gyrostat 

 box are bored small holes. A third hole is bored in the surrounding 

 case. The rotation of the gyrostat produces a current of air within 

 and a pressure toward the exterior. 



The jet of air at its exit is cut by a small blade attached to a 

 pendulum. When the compass is exactly horizontal, the jet is divided 

 equally on both sides of the blade. If it is not horizontal, the pen- 

 dulum displaces the blade with reference to the aperture; then the 

 divided portions of the jet of air are no longer equal and a damping 

 couple is produced. 



As with other compasses, this, too, is subject to disturbing influences. 

 But a great advantage of the gyrostatic compasses lies in the fact 

 that in all cases the causes of the disturbances are independent of the 

 special instrument and may be corrected by specially prepared tables. 



Another important property is that the directing force can be made 

 much greater than is possible with the magnetic needle. Generally 

 it is five times that of an ordinary well- constructed magnetic compass. 

 Further, since the axis remains both in the meridional and in the 

 horizontal planes, the dial can oscillate only slightly about the north 

 and south direction. This renders it easy to fix contact points at 

 the extremities of the east and west line for the electrical transmission 

 elsewhere of the indications of the compass. 



A gyrostatic compass equipment consists of a master compass, 

 provided with a transmitter, and secondary compasses connected 

 electrically with the master. The master compass, together with 

 the transmitter, is placed in a convenient, well-protected place, and 

 the secondary ones placed wherever they are needed; or two master 

 compasses may be used with two systems of secondary ones. 



The master compass, with its transmitter, of which a photograph 

 is shown in plate 3, figure 1, differs from the others in that the binnacle 

 is moved by a reversible electric motor controlled by contacts under 

 the gyrostat itself, so that it turns rapidly when necessary and follows 

 at all times the motions of the axis of the gyrostat. 



It is this moving binnacle which sends the currents controlling the 

 secondary compasses and keeps them in synchronism with the master 

 compass. The latter carries on its axis the necessary commutator. 



Special secondary compasses are employed, as shown in the illus- 

 tration (pi. 3, fig. 2). It may be noted that it has at its center a 



