Page:Popular Science Monthly Volume 76.djvu/99

Rh the pole, to remain twenty-four or even eighteen hours at one camp for the purpose of exactly determining his position. By making daily observations on the sun, at different hours, so as not merely to fix his successive positions on a series of parallel lines, but on lines having different directions; by keeping his direction with the compass and estimating the drift of the ice and his rate of travel, he could always know where he was without too large an error. But when he was in the immediate neighborhood of the pole he should make as many observations, with the sun in different directions, as circumstances would permit.

Fatigue, severe cold, the condition of his commissariat, and the anxiety to return after having succeeded in his bold undertaking, might prevent him from making as many observations as would be desirable; but nevertheless they might be sufficient to be convincing that he had been within a few miles of the pole; it would surely be a quibble to dispute with an explorer the honor of having reached the pole if his observations showed, without reasonable doubt, that he had been within ten or fifteen miles of it.

There are two kinds of instruments used for measuring altitudes; the transit-theodolite and the sextant. The former consists of a telescope so mounted that it can turn in a vertical and in a horizontal plane; it is provided with vertical and horizontal graduated circles, to measure the angle turned through, and with leveling screws and spirit levels to adjust it in position. It is supported by a tripod, and after being properly leveled, the reading of the vertical circle gives the altitude of the object sighted through the telescope. It is by far the best instrument for an explorer on land, because it is very easy to use, and its adaptation to measure horizontal angles enables the explorer to carry on an ordinary survey.

The sextant was originally invented for use at sea, where a steady support can not be found. It consists of a telescope mounted on a frame, which is held in the hand. To measure the angle between two objects, one of them is sighted directly through the telescope, and the image of the second is reflected into the telescope by means of two mirrors, one fixed rigidly to the frame in front of the telescope, and covering half its field, and the other movable around an axis fastened to the frame. The movable mirror is turned by an arm, which moves along a graduated arc on the frame, and its reading, when the two objects appear in the telescope superposed upon each other, gives the angle beteweenbetween [sic] the objects. In determining the altitude of the sun at sea the edge of the sun is made to touch the horizon; a movement of the ship moves the sun and the horizon together and the contact is not destroyed. On land, when the sea horizon is not available, a so-called artificial horizon must be used. The ordinary mercurial artificial