Holden reported in 1892 that the small figure included in the paper was made from a negative of the Moon taken in the focus of the great telescope on August 1890, at 8 hours, o minutes. The original picture of the was about five and one-half inches in diameter. A small the original showing the lunar crater Copernicus has been slightly enlarged and is given in the cut.
The scale of the picture that the diameter of the whole moon would be about fourteen inches. The diameter of the crater itself is fifty-six miles. The cardinal points, north, south, etc., are indicated on the picture. Thé original picture is very satisfactory. The representation given here is far less so, but it is the best available. It will probably serve its purpose, however. The other cuts are not enlarged, and show, more or less satisfactorily, the surroundings of this crater.
The walls of the crater ring-form are not perfectly circular, and they vary somewhat in height at different points. Their general elevation above the floor of the crater is about eleven thousand feet, rising in places to twelve thousand or even thirteen thousand feet. The slope of the interior terraced wall is far more steep than that of the exterior; and this is a general rule in all such formations on the Moon. The average exterior slope of lunar craters is six degrees to seven degrees, while the average interior slope is thirty-five degrees; that is, they are really very gently sloped mounds with a steep-sided pit in the midst.
The terraces of the walls deserve careful attention; and, if the cut is examined with a common hand-magnifier, they can be seen a little better. The floor of the crater is by no means smooth ; and from it rise two groups of central peaks, the highest of which is some twenty- four hundred feet. Like all the central peaks of lunar craters, they are much lower than the bounding walls. Copernicus is surrounded by a mass of mountains, hills and ridges of highly complex structure, and by a marvelous system of brilliant streaks radiating from the crater as a center, and extending in some cases for four hundred miles, or even more, till they meet similar streaks from other craters, - from Kepler, Aristarchus, etc., which are shown on the cuts following. This famous lunar mountain has been drawn and described many times. The very best drawings show a number of minor features which are much too small to be represented in the engraving; but no drawing has ever given anything like the true plastic effect, and even the very best drawings fail to show details which are evident on the original photograph.
............................ The author concludes that in fact every feature which he sees in the crater Copernicus seems to have its analogue if not its counterpart on this small island of Hawaii. I think it is very pertinent to remark that the lavas of Hawaii are much more liquid than those of Italy, for example; and it seems almost safe to hazard the guess that the lavas of the Moon resemble the former rather than the latter. If I am straining the analogies, I beg the pardon of my confrères, the geologists;. - and I am aware that one of the very best observers of lunar topography has stated most emphatically that such an explanation as this will not serve. It, nevertheless, seems to me to be the true one for the region we are considering, while it certainly will not explain other phenomena of a somewhat similar character on other parts of the moon. Such analogies will surely strike any astronomer who travels in Hawaii.
The only serious question to my mind is in regard to the difference in scale. In Hawaii we have central craters or calderas of two and three miles in diameter, and lava-flows from them of forty miles long which would be much longer if they did not end in the sea. On the moon we have the caldera of Copernicus, which is fifty-six miles in diameter, with lava-flows of four hundred miles or so. Having regard to the immensely greater effect of volcanic forces on the moon (where the force of gravity, for example, is not more than one-sixth of that on the earth), I confess that I see nothing over-strained in drawing the conclusion that in the volcanoes of Hawaii we now have before our eyes something like 'a working model of what Copernicus once was. This, then, is what seems to me to be the key to the landscape shown in our engraving; and it gives a kind of unity to its complex confusion and wild variety. There are other regions on the moon far more difficult to understand ; but here, at least, it seems that a kind or order can be made to arise out of the chaos.
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