192 
HINTS TO TRAVELLERS. 
cotangent of the latitude; their sum (rejecting 10 in the index) will be 
the log tangent of arc I. 
To find the true Altitude .-—Add together the log sine of the Lati¬ 
tude, the log secant of arc I., and the log cosine of the difference 
of arc I. and the Polar Dist.; their sum will be the log sine of the 
true Alt. 
N.B.—When the hour angle is more than 6 hours, or 90°, take the log 
cosine of the sum of arc I. and the Polar Dist. 
From the True Altitude to find the Apparent Altitude 
The corrections must be applied in reverse order, and with contrary 
signs to those with which the true is derived from the Apparent Altitude. 
For the Sun or for a Planet .—Subtract the Parallax in Altitude, and 
add the Refraction. 
For a Star .—Add Refraction. 
For the Moon .—Compute the parallax in altitude first by adding 
together the cosine of the true altitude and the log of the horizontal 
parallax (in seconds); the result will be the log of the parallax in 
altitude (nearly). Subtract this parallax from the true altitude, and 
with this corrected altitude again recompute the parallax in altitude; 
the parallax thus found must now be subtracted from the true altitude; 
with the remainder take out the refraction, which correct for tempera¬ 
ture and barometer, and add it to the corrected altitude; the result is 
the apparent altitude. 
