entire vessel to look so small, and certain portions, the size of which he is familiar with, to become indistinguishable. He is guided, also, by the amount to which, on account of the earth’s curvatures, the vessel seems to be sunk below the horizon. These are data from experience. It is wonderful with what accuracy they enable him to judge. A landsman by the seaman’s side, and without such aid, could give only the most random guesses as to the distance of the vessel. That we really do make this use of both eyes in judging of the distance of bodies near us will be evident if we bandage one eye and try to determine their distances, only using the other. It will require caution to avoid mistakes. We knew an aged painter, who had lost the sight of one eye, but still continued to play, at least, with his brush. He had to use the finger of his left hand to ascertain by touch whether the tip of his brush, loaded with the proper color, was sufficiently near the canvas or not. If he relied on his eye alone, it often happened that when he thought it near, not the eighth of an inch away, it failed in reality by an inch and a half to reach the canvas. He would ply the brush, and, noticing that the color was not delivered, would smile sadly at what he called his effort to paint the air. So long as he had retained the use of both eyes, this mishap, of course, had never occurred to him. When a surveyor desires to ascertain the distance of a visible object which he cannot approach, he must avail himself of the same principle of nature. He measures off on the ground where he is a suitable baseline, and takes the angle of the object from each end of it, not vaguely by his unaided eyesight alone, but with a well‐graduated instrument. It is, as it were, putting his eyes that far apart, and taking the angles accurately. From the length of the measured base‐line and the size of the two angles he can easily calculate the distance of the object. In taking such measurements, the surveyor must make his base sufficiently large in proportion to the distance sought. If the base be disproportionately small, the angles at the extremities will not serve. Their sum will be so near 180° that the possible errors which are ever present in observations will more than swallow up the difference left for the third angle, and the distance is not obtained. In our excellent Coast Survey, which, in exactness of working, is not surpassed anywhere in the world, the bases carefully measured may be five or seven miles long, and angles under 30° are avoided when possible.