18 MANUAL OF CURRENT OBSERVATIONS 
which is known as the ‘‘index mirror,” is attached to the head of an index bar that is 
free to turn on a pivot. The index bar is provided with a clamp and tangent screw 
for accurate setting and carries a vernier that may be moved over a fixed graduated 
are of a circle. The construction of the instrument is such that the angle bemg meas- 
ured is twice as great as the angle between the two mirrors, and the scale of the instru- 
ment is graduated accordingly. Thus an are of one-fourth of a circle is graduated 
and marked for 180°. The instrument is equipped with a telescope which may be 
used when sighting upon distant objects. 
44. To measure the angle between two objects the observer looks at the one on 
the left over the top of the horizon mirror and moves the index bar until the reflection 
of the right-hand object appears in the horizon mirror directly under the left-hand 
object, the clamp and tangent screw being used to secure a fine setting. The angle 
may then be read directly from the graduated scale. When used to determine the 
position of the vessel, angles are measured between three fixed objects whose positions 
are known and plotted on a chart. When used to determine the direction of the cur- 
rent, it is necessary to have a reference object whose bearing from the vessel is known. 
Then, immediately after a velocity observation has been taken with current pole and 
log line, the angle between the reference object and the current pole is measured and 
entered in the record with the letter “R” or “Z” according to whether the current 
pole is to the right or left of the reference object. 
45. The principal adjustments of the sextant which may require attention in the 
field consist in placing the two mirrors perpendicular to the plane of the instrument 
and in placing them parallel to each other when the vernier reads zero. ‘The position 
of the index mirror is first checked by holding the instrument so that the reflection of 
the graduated arc can be seen in this mirror and then, moving the index arm slowly, 
observe if the arc and its reflection appear to form a continuous and unbroken are. 
If not, the inclination of the mirror should be corrected by an adjusting screw provided 
for the purpose. For the adjustment of the horizon mirror set the index arm at zero. 
Then, holding the sextant in a vertical position, sight at the sea horizon and observe 
if the horizon and its image in the horizon mirror coincide; if not, correct by means of 
the screw at the side of the mirror. Next, rotate the instrument about 45° and see 
if the horizon and its image remain in coincidence; if not, correct with the screw at 
the back of the mirror. As the latter correction may disturb the previous one, the 
operation should be repeated until the horizon and its image in the horizon mirror 
remain in coincidence for every position of the instrument as it is rotated. 
_ Bifilar Direction Indicator 
46. The bifilar current indicator is an instrument which has been used to determine 
the direction of subsurface currents. It consists essentially of a pipe with rudder 
attached to one end, suspended horizontally by two wires from a bar which is free to 
turn in azimuth by virtue of a ball-bearing joint at its connection with an outrigger 
or davit on a vessel. A triple form of the indicator, which was used to determine 
simultaneously the direction of the current at three different depths, is described in 
Coast and Geodetic Survey Special Publication No. 124, Instructions for Tidal Current 
Surveys. Because of the difficulty of managing the apparatus from a vessel in rough 
water, its use has been discontinued. 
