At six o'clock Colonel Everest gave the signal for starting, and passengers and crew went on board the "Queen and Czar." The hunter, who was acquainted with the course of the river, followed, leaving the two Bochjesmen to take the waggon back to Lattakoo. Just as the vessel was slipping its cable, Colonel Everest turned to the astronomer, and said,-- "By-the-bye, Mr. Emery, you know why we have come here?" "I have not the least idea, Colonel." "It is very simple, Mr. Emery: we have come to measure an arc of meridian in South Africa." CHAPTER IV. A FEW WORDS ABOUT THE "MÈTRE." The idea of an invariable and constant system of measurement, of which nature herself should furnish the exact value, may be said to have existed in the mind of man from the earliest ages. It was of the highest importance, however, that this measurement should be accurately determined, whatever had been the cataclysms of which our earth had been the scene, and it is certain that the ancients felt the same, though they failed in methods and appliances for carrying out the work with sufficient accuracy. The best way of obtaining a constant measurement was to connect it with the terrestrial sphere, whose circumference must be considered as invariable, and then to measure the whole or part of that circumference mathematically. The ancients had tried to do this, and Aristotle, according to some contemporary philosophers, reckoned that the stadium, or Egyptian cubit, formed the hundred-thousandth part of the distance between the pole and the equator, and Eratosthenes, in the time of the Ptolemies, calculated the value of a degree along the Nile, between Syene and Alexandria, pretty correctly; but Posidonius and Ptolemy were not sufficiently accurate in the same kind of geodetic operations that they undertook; neither were their successors.