CHAPTER VIII.THE TWENTY-FOURTH MERIDIAN.[edit]
The measurement of the base occupied thirty-eight days,
from the 6th of March to the 13th of April, and without
loss of time the chiefs decided to begin the triangles.
The
first operation was to find the southern extremity of the arc,
and the same being done at the northern extremity, the
difference would give the number of degrees measured.
On the 14th they began to find their latitude. Emery
and Zorn had already on the preceding nights taken the
altitude of numerous stars, and their work was so accurate
that the greatest error was not more than 2", and even this
was probably owing to the refraction caused by the changes
in the atmospheric strata.
The latitude thus carefully
sought was found to be 27.951789°.
They then found the
longitude, and marked the spot on an excellent large scale
map of South Africa, which showed the most recent geographical
discoveries, and also the routes of travellers and
naturalists, such as Livingstone, Anderson, Magyar, Baldwin,
Burchell, and Lichtenstein. They then had to choose on
what meridian they would measure their arc. The longer
this arc is the less influence have the errors in the determination
of latitude. The arc from Dunkirk to Formentera,
on the meridian of Paris, was exactly 9°56'.
They had to
choose their meridian with great circumspection. Any
natural obstacles, such as mountains or large tracts of
water, would seriously impede their operations; but happily,
this part of Africa seemed well suited to their purpose,
since the risings in the ground were inconsiderable, and the
few watercourses easily traversed. Only dangers, and not
obstacles, need check their labours.
This district is occupied by the Kalahari desert, a vast
region extending from the Orange River to Lake Ngami,
from lat. 20° S. to lat. 29°. In width, it extends from the
Atlantic on the west as far as long. 25° E. Dr. Livingstone
followed its extreme eastern boundary when he travelled as
far as Lake Ngami and the Zambesi Falls. Properly
speaking, it does not deserve the name of desert. It is not
like the sands of Sahara, which are devoid of vegetation,
and almost impassable on account of their aridity. The
Kalahari produces many plants; its soil is covered with
abundant grass; it contains dense groves and forests;
animals abound, wild game and beasts of prey; and it is
inhabited and traversed by sedentary and wandering tribes
of Bushmen and Bakalaharis. But the true obstacle to its
exploration is the dearth of water which prevails through
the greater part of the year, when the rivers are dried up.
However, at this time, just at the end of the rainy season,
they could depend upon considerable reservoirs of stagnant
water, preserved in pools and rivulets.
Such were the particulars given
by Mokoum. He had often visited
the Kalahari, sometimes on his own
account as a hunter, and sometimes
as a guide to some geographical
exploration.
It had now to be actually considered
whether the meridian should
be taken from one of the extremities
of the base, thus avoiding a
series of auxiliary triangles[1].
After some discussion, it was decided that the southern
extremity of the base would serve for a starting-point. It
was the twenty-fourth meridian east from Greenwich, and
extended over seven degrees of latitude, from 20° to 27°,
without any apparent natural obstacle. Towards the north
it certainly crossed the eastern end of Lake Ngami, but
Arago had met with greater difficulties than this when he
applied his geodesy to connect the coast of Spain with the
Balearic Islands.
It was accordingly decided that meridian
24° should be measured, since, if it were afterwards prolonged
into Europe, a northern arc of the same meridian
might be measured on Russian territory.
The astronomers proceeded at once to choose a station
which should form the vertex of the first triangle.
This
was a solitary tree to the right of the meridian, standing on
a mound about ten miles away. It was distinctly visible
from each extremity of the base, and its slender top facilitated
the taking of its bearings. The angle made by the
tree with the south-east extremity of the base was first
observed, with the help of one of Borda's repeating
circles.
The two telescopes were adjusted so that their axes were
exactly in the plane of the circle, in such a way that their
position represented the angular distance between the tree
and the north-west extremity of the base. This admirably
constructed instrument corrects nearly all the errors of observation,
and indeed, if the repetitions are numerous, the
errors tend to counterbalance and correct each other.
The Commission had four repeating circles: two for
measuring angles, and two more with vertical circles for
obtaining zenith distances, and so calculating in a single
night, to the smallest fraction of a second, the latitude of
any station. And indeed, in this important survey, it was
not only necessary to obtain the value of the angles of the
triangles, but also to measure the meridian altitude of the
stars, that being equal to the latitude of each station.
The work began on the 14th of April, Colonel Everest,
Zorn, and Palander observed the angle at the south-east
extremity of the base, while Strux, Emery, and Sir John
Murray observed that at the north-west extremity.
Meantime the camp was raised, and the bullocks harnessed,
and Mokoum conducted the caravan to the first
station as a halting-place. Two caravans, with their drivers,
accompanied the observers, to carry the instruments.
The
weather was bright, but had the atmosphere been unfavourable
by day, the observations would have been made by
night by means of reverberators or electric lamps.
On the first day, the two angles were measured, and the
result inscribed on the double register; and the astronomers
all met in the evening at the camp which had been formed
round the tree which had served for their point of sight.
It
was an immense baobab, more than 80 feet in circumference.
Its syenite-coloured bark gave it a peculiar appearance.
The whole caravan found room beneath its wide branches,
which were inhabited by crowds of squirrels, which greedily
devoured the white pulp of its egg-shaped fruit.
Supper was prepared for the Europeans by the ship's
cook. There was no lack of venison, for the hunters had
scoured the neighbourhood, and killed some antelopes;
and soon the air was filled with an odour of broiled meat,
which still further aroused the appetite of the hungry
savants.
After the comforting repast, the astronomers retired to
their respective waggons, whilst Mokoum placed sentinels
round the camp. Large fires of the dead branches of the
baobab burnt throughout the night, and kept at a respectful
distance the tawny beasts, who were attracted by the odour
of the recking flesh.
After two hours' sleep, however, Emery and Zorn got up,
their observations not yet finished. They must find the
altitudes of some stars to determine the latitude of the
station, and both, regardless of the day's fatigues, stood at
their telescopes, and rigorously determined the change of
zenith caused by the removal from the first station to the
second, while the laugh of the hyena and the roar of the
lion resounded over the sombre plain.
1^ By
the aid of the accompanying figure, the
work called a triangulation may be understood.
Let A B be the arc. Measure the base A C
very carefully from the extremity A to the first
station C. Take other stations, D, E, F, G, H, I,
&c., on alternate sides of the meridian, and
observe the angles of the triangles, A C D,
C D E, D E F, E F G, &c. Then in the
triangle A C D, the angles and the side A C
being known, the side C D may be found.
Likewise in the triangle C D E, C D and the
angles being known, the side D E may be
found; and so on through all the triangles.
Now determine the direction of the meridian
in the ordinary way, and observe the angle
M A C which it makes with the base A C.
Then in the triangle A C M, because A C and the adjacent angles are known,
A M, C M, and the angle A C M, may be found, and A M is the first portion
of the arc. Then in the triangle D M N, since the side D M = C D − C M,
and the adjacent angles are known, the sides M N, D N, and the angle
M N D may be found, and M N is the next portion of the arc. Again, in
the triangle N E P, because E N = D E − D N, and the adjacent angles are
known, N P, the third portion of the arc, may be found. By proceeding
thus through all the triangles, piece by piece, the whole length of the arc A B
may be determined.
