Page:The Moon (Pickering).djvu/87

Rh of water. Figures 11 and 12, Plate D, represent the planet Mars. The narrow markings in Figure 11 are the canals. The coarser markings or bands in Figure 12, which were discovered at Arequipa in 1892, seem to be analogous to them, and are sometimes referred to as the "canals in the dark regions." These two sketches were made with the eighteeninch telescope of the Lowell Observatory at Flagstaff, Arizona, in 1894. They are drawn on a scale of $1⁄100,000,000$, or about 4.4″ or 1,600 miles to the inch. Since two observers, seated at the same instrument, may represent what they see quite differently on paper, it is important in making a comparison of the surface markings of the different heavenly bodies that all the drawings should be by the same observer.

In the course of my observations of the Moon made in Jamaica in 1901, special attention was paid to the crater 9E [2.5, 5.4] Eratosthenes. This was due to the fact that it was known to contain extensive variable spots, and that being near the Moon's equator, in latitude 14° N., it was considered highly probable that at some time in the course of the lunar day these spots would be subject to rapid changes. Moreover, being near the centre of the Moon's disk, in longitude 11° E., the crater could be well seen, and its appearance at different times would be unaffected by the slight apparent shiftings of the Moon about its central position due to libration. On Plates E and F are given four drawings of this crater, made on a scale of $1⁄2,000,000$, or about 28" or thirty-two miles to the inch.

After the drawings were finished and in the hands of the printer, it occurred to me that it would be interesting to compare them with photographs taken at about the same intervals after lunar sunrise. The photographs would thus give a very interesting independent check on the accuracy with which the drawings were made, and would show at the same time, in general, the kind of errors into which a draftsman is liable to fall.

Since one of the main objects of the Jamaica expedition was to obtain the pictures used in the present Atlas, we had negatives to select from in abundance, although none were taken with special regard to Eratosthenes itself. The advantage of photography as applied to the lunar surface is that it gives with absolute accuracy the size, shape and relative positions of the various formations. Unfortunately, however, in the representation of the finer details it is at a hopeless disadvantage as compared with the eye. The best photograph of the Moon ever taken will show nothing that cannot readily