Advantages 
of star obser¬ 
vations. 
Instruments 
Sextant. 
Adjustments 
Index error. 
By sun. 
By star. 
540 ASTRONOMICAL OBSERVATIONS. 
II.— Finding- the Longitude. 
(a) By finding Apparent Time at place by Altitude of the Sun, and 
thence the Longitude by chronometer . 
(b) Ditto by Altitude of a Star or Planet, and thence Longitude by 
chronometer . 
III.— Finding Variation of Compass by Amplitude. 
In estimating generally, for Latitude and Longitude, tlie relative 
advantages of Sun and Star Observations, tlie latter are practically 
found to be— 
(1) Greater exactitude obtained by tbe smallness of the objects 
reflected. 
(2) Tbe observations can be taken at leisure without exposure to 
the glare of the mid-day sun. 
(3) The brilliancy of stars in tropical climates renders their use 
favorable in those latitudes. 
(4) Near the Equator, where the Declination and Latitude are nearly 
the same, it is impossible to take the double arc by sextant, while a 
suitable star can always be selected. 
The more complicated but not less serviceable methods by Lunar 
Observations must be omitted in this paper for want of space. 
Instruments. 
It may here be well to very briefly note the most important instru¬ 
ments necessary in these expeditions. 
A sextant of 6 inch or 4 inch radius. The former, reading to 10 
seconds, is scarcely necessary, unless the traveller intends to take 
lunar observations. The 4 inch, graduated to 20 seconds, is more 
portable, and gives all requisite accuracy. 
The principal adjustment is to see that the reflectors are vertical to, 
and that the line of collimation is parallel to, the plane of the instru¬ 
ment. The mode of adjusting the horizon glass varies in different 
instruments. The index glass should never be touched, as this should 
always be put into proper adjustment by the maker, and is not liable 
to shift. The line of collimation is adjusted by the small screws in the 
collar, into which the telescope is screwed. 
The index error, which has to be applied to all observed altitudes, 
can be found by measuring the sun's horizontal diameter, noting the 
reading on the arc. Then reverse the images by moving the index the 
opposite way, and take the measure again, this time reading off the 
arc; half the difference of the two readings will be the index error, 
minus if the reading on the arc be the greater, plus if the reading off 
the arc be the greater. To test the accuracy of the observation, one 
fourth of the sum of the two readings should be equal to the sun's 
semi-diameter, as given in the nautical almanack for date in current 
year. 
Another method, by means of a star, is this. Set the vernier to 
zero, and, looking through the inverting telescope at the star, make; 
