86 
HINTS TO TRAVELLERS. 
on the object, if accessible, equal to the height of the eye; set the 
index to one of the angles in the Table, and advance or go back¬ 
wards from the object, until, by reflection, the top of the object is 
brought by the mirrors to coincide with the mark first made. If 
the angle be greater than 45°, multiply the distance to the object by 
the number in the next column to the angle in the Table; if the angle be 
less than 45°, divide, and the result will be the height of the object from 
the mark; to which add the height of the eye. 
If the object is inaccessible, set the index to the greatest divisor angle 
in the Table that the least distance from the object will admit of; move 
backwards and forwards until the top of the object is reflected level with 
the eye; at this place set up a staff equal to the height of the eye. Then 
set the index to any of the lesser angles; go back in a line with the 
object, until the top is made to appear on the level with the top of 
the staff; fix here another mark; measure the distance between the 
two marks set up; divide this by the difference of the numbers corre¬ 
sponding to the angles made use of, and the quotient will be the height of 
the object from the top of the staff; to which add the height of the eye. 
For the Multiply the height of the object by the numbers 
against either of the angles used, and the product will be the distance 
of the object from the place where such angle was used. 
If the index is set at 45°, the distance is equal to the height, minus the 
height of the eye. 
At a given 'point to mark off a line perpendicular to any given directions 
If this direction is not sufficiently distinguished by some natural object, 
such as a tree, mark it by a flag set up as far off as convenient; then, 
standing at the given point, with the sextant set to 90°, make a man, 
bearing a flag, stand in a line estimated as the perpendicular. Motion him 
right or left until his flag can be seen, by reflection, to coincide with the 
other. There let him fix his flag, so marking the direction of the 
perpendicular. 
Of course any other direction can be marked in the same way, setting 
off the required angle on the sextant, instead of the 90°« 
