560 Scientific Proceedings, Royal Dublin Society. 
current-drift it will remain stationary at the spot where it is 
launched. As will be seen later, current-drift of the signal will 
not affect the value of the method of making observations now to 
be described, as the ship may be assumed to drift an equal amount, 
premising that we speak of ocean-currents. 
We will assume the mariner is desirous of making a stellar 
observation at night, when the horizon is obscured. To this end 
he looks for a recognisable bright star. If nearly astern of his 
ship so much the better as regards diminishing the time required 
to make the observation. But the mariner may optionally prefer 
to select a star near his meridian. 
Having decided on a suitable star, he takes its bearing, and, 
altering his course to the opposite bearing, he thus brings the star 
right astern. He now drops the rescue-signal overboard (having 
perforated it if necessary), and at the same time a reading of the 
log is taken. 
Having sailed or steamed a distance of about a mile from the 
signal, as indicated by the log, he reads the angular elevation of 
the star over the signal, using the sextant in the usual manner. 
In effecting this reading it will increase the accuracy of the obser- 
vation to heave-to; but this should not be necessary if the star is 
kept on the signal, as nearly as may be, by shifting the limb of 
the sextant gradually as the distance increases nearly to the mile ; 
diminishing the angle till the word is given that the knot is run. 
The reading being taken, the ship is put back on her course. 
In this operation it conduces to accuracy if, instead of running 
directly away from the signal on the exact bearing opposite to that 
first taken of the star, attention is paid to the fact that the star is 
not a fixed object, but is travelling east to west at the rate of 
about 1° in four minutes of time. The amount by which the 
ship’s course must be altered to counteract this motion and to pre- 
serve the signal in the vertical plane containing the star will 
depend on the direction of the ship’s course, being greatest when 
this is north and south. Unless the ship travels very slowly the 
error may be sufliciently corrected by keeping the ship’s course 
a very little to the eastward of the first bearing; in general 
something less than z; point. The inaccuracy introduced by this 
procedure into the determination of distance as effected by the 
log is negligible. 
