Let us suppose two such stations to be chosen on or nearly on the same meridian of longitude, and 6,000 miles apart. From each of these stations the planet is seen to traverse the disk of the sun, like a dark spot moving steadily across an illuminated circular dial‐plate. The lines as seen from stations so far apart are sensibly different. What the observers first seek to know is the apparent distance between these lines, the angle they form, when seen from the earth. Were both visible at once from the same station, through the same telescope, it would not be difficult for a skilful observer to measure the angle directly. But at each station only one line is seen, if, indeed, we may properly give that name to the course of the dark spot that passes on and leaves no trace behind. Each observer must determine correctly the position of his line on the face of the sun, in order that it may be afterwards compared with the other line similarly determined at the other, and the apparent distance between them is then determined by calculation. How to determine the true position of such a line is the delicate and difficult task. One mode is to take the measurements in two directions on the face of the sun, northward and eastward, from the position of the planet to the edge of the solar disk. This must be done for a number of positions which the planet occupies successively as it moves onward. But such measurements are very hard to be obtained with the desired precision. The edge of the sun, viewed in a large telescope, appears always tremulous, on account of the action of solar heat on our own terrestrial atmosphere. The better and larger the telescope, and the brighter the day, the greater and the more embarrassing does this tremulousness appear. Such measurements are difficult, and are open to too much uncertainty.