The Adventures of Three Englishmen and Three Russians in Southern Africa/Chapter VIII
CHAPTER VIII.
THE TWENTY-FOURTH MERIDIAN.
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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.