In this mode, two stations are necessary, east and west, or, rather, along that line on the earth’s surface from all points of which the transit will show the same line on the solar disk. The further apart the stations are, the better; for the base between them will be larger. To know the distance between them, we must know their longitudes as accurately as their latitudes. From the longitudes we ascertain with precision the difference of time between them. At one of those stations, the first exterior contact is seen, and the exact time is noted. As Venus moves on, the shadow of this first contact flies along that line of the earth’s surface like the shadow of a cloud in spring traversing the fields. It is only after the lapse of a certain length of time that the contact is seen and timed at the other station. This certain length of time is the key to the solution. It may be determined by observations on any one or on all the contacts, or by the observation of any other points of the transit examined and timed at both stations. It is obvious that the contacts, being the most unmistakable in their character, will be all used to check and control each other; the more so, as they serve also, as we saw, for Halley’s method. The most careful use of the telescope will be supplemented by the photograph and the spectroscope. Let two such stations be chosen which, by their longitudes and latitudes, we know to be 5,000 miles apart. It will be found that the transit, or any special point of it, will be seen at the second station about three minutes of time later than at the first. This means that the shadow of Venus travels 5,000 miles in three minutes on the earth’s surface or at the earth’s distance from the sun. Applying Kepler’s formula, we find that, to produce this effect, Venus herself must have travelled about 3,860 miles in those three minutes. There‐fore in 224.7 days—her solar year—she would travel about 416 millions of miles, supposing that, during the transit, she was moving at her mean velocity. This, then, is the length of her periphery of her orbit around the sun. Observations have determined its shape. Now that we know its size, it is not difficult to ascertain what her mean distance from the sun must be. It is about 66,300,000 miles. From this, the usual formula leads us to the earth’s distance from the sun—91,650,000 miles. We merely indicate the salient points of the process, and that with summary numbers. An astronomer would enter into minor questions: how far the earth had travelled in her orbit during those three minutes, and what had been the special motion of the second station during the same time, on account of the diurnal revolution of the earth on its axis. He would carefully establish the proportion of the distances between the sun and Venus and the earth, during the transit, to their mean distances as contemplated in Kepler’s law, and he would compare the velocity of Venus at that time with her mean velocity. Other