114 ANNUAL REPORT 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. 
Tt 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 (pl. 3, fig. 2). It may be noted that it has at its center a 
