278 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
The indicatoi’S are of l)rass with steel shafts and spindles and glass faces; they 
are secnred with their backs to the after bulkhead of the i)ih)t-honse, as seen in cut 14 
with a circular hole in the wood large enough for the index arm to move in ; the arrows 
on the faces of the indicators 
are visible from the deck, or 
bridge, those on the backs 
from the interior of the pilot- 
house. 
The index arms revolve 
in the direction in which 
their arrows point when the 
engines are working ahead; 
upon reversing them the mo- 
tion of the arrows is also 
reversed. The index arms 
are mounted upon each end 
of a horizontal shaft carrying a toothed wheel, cut 14, which is turned by the revolu- 
tion of a fan or j)ropeller on a vertical spindle that has a worm or endless screw, the 
threads of which mesh with the teeth of the wheel. 
The propellers are rapidly revolved by an air curreiit when the engines move, 
transmitting motion through the spiral gearing to the index arms, the movement of 
which is moderate, but the speed is variable with the speed of the engines and 
incidentally affords a means of esti- 
mating by the eye their s]>eed as 
well as the direction in which they 
are moving. 
The air current is derived from 
small rotary blowers, cnts 10 and 17, 
placed near the main shafts and 
belted to them, as shown in cut 18. 
When the engines are moving ahead 
tlie blowers draw the air in and force 
it through the i>ipes to the propellers 
in the indicator case, thence it escapes 
to the atmosphere. 
When the engines are backing, 
the air is drawn in at the top of the 
case, reverses the pro])ellers, and, after 
traversing the connecting pipes and 
blowers, it is exhausted into the atmos- 
l»here as before. The connecting pipes are of drawn brass of commercial pattern. 
The indicators were installed early in 1880. They are simple and effective, and 
have greatly improved the juaneuvering qualities of the vessel. 
BOILBKS. 
There are two single-ended, horizontal return tubular boilers of the Scotch type, 
constructed of American chai’coal llauge iron and placed fore and aft on the midshiji 
line, with the fire-room athwartships between them. An annular steam drum is placed 
vertically over the fire-room, between the boilers, supported by wrought-iron girders 
which rest upon the latter, and is connected with an uptake common to both boilers. 
Indicator. 
Cut 18. 
