158 
HINTS TO TRAVELLERS, 
Equal Altitudes of the Sun , Star , or Planet .—In consequence of instrtl* 
mental errors, time obtained by absolute altitudes is sometimes com 
siderably in error. 
To eliminate these, it is necessary to observe equal altitudes of the 
heavenly body—that is, to note the time when it is at the same altitude 
east, and when west, of the meridian. 
This necessitates a halt of some hours, and, in the case of a star, 
observation in the night and early morning; but when time and circum¬ 
stances are favourable, the result will always be more satisfactory than 
absolute altitudes. 
This observation must be commenced when the heavenly body observed 
is three or four hours east of the meridian. Having placed the artificial 
horizon in its proper position, bring down the reflected image of the 
object with the sextant until it is in contact with the image in the 
horizon, then advance the index until it points to a whole degree—for 
example, 40°—and, looking through the telescope at the image reflected 
by the sextant mirrors, wait until it attains this altitude, note the time, 
advance the index 20', to 40° 20', and wait until this altitude is reached, 
note the time; again advance the index 20', to 40° 40', and in like 
manner wait till this altitude is attained, note the time. Repeat this 
operation as often as convenient; nine such observations will be ample. 
The heavenly body observed will, of course, at some time, have the same 
altitude when it is west of the meridian, and this will be the case when 
it is about the same interval, in time, from it. The observer must there¬ 
fore watch until the last altitude taken is again furnished, note the time 
when this takes place, and couple it in his note-book with the time when 
the heavenly body had the same altitude on the other side of the meridian; 
move the index bach 20' and wait until this altitude is furnished, note the 
time, and again couple it with the time when the same altitude was 
before taken, and so on through the set, moving the index back after each 
sight by the exact amount it was moved forward when the object was 
east of the meridian, or rising. When an artificial horizon is used, equal 
altitudes of a star should be taken in preference to those of the sun, for 
as the images of the star are but small luminous points, there cannot be 
any great error in the observation if they are made to touch, while in the 
case of the sun, exact contacts are by no means so easy to make. The 
computation necessary to find the error of the w^atch, by equal altitudes 
