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Regard must also be had to libration, which affects to a greater or less degree every object; carrying out of the range of observation regions near the limb at one time, and at another bringing into view others beyond the limits of the maps, which represent the moon in the mean state of libration.
In the first place, we see on the extreme border of the moon, when the libration turns one side the farthest around toward the earth, the edge of a number of the great walled pits such as are so plenty on the visible area; it is fair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side.
For instance: he proposes to take a photographic image of the moon, at one of the periods of her libration, and a second one about fifteen months afterwards, at the next libration, which, as you know, would be in the opposite direction to the first.
In these paroxysms of fevers, which consist of the libration of the arterial system between the extremes of exertion and quiescence, either the fits become less and less violent from the contractile fibres becoming coming less excitable to the stimulus by habit, that is, by becoming accustomed to it, as explained below XII. 3. 1. or the whole sensorial power becomes exhausted, and the arteries cease to beat, and the patient dies in the cold part of the paroxysm.
"Peter," he said, turning to me, "you know what libration means; well, it's libration that is going to save us. As Venus travels round the sun she turns just once on her axis in making a complete circuit, the consequence being, as you already know, that she has one side on which the sun never rises while the other half is in perpetual daylight.
The action in reference to the phases is clearly shown in the small diagram on the right. The one on the left illustrates the manner in which the libration in longitude is made apparent. It will be noted that the center of M is not directly over the axis of the bearing of the crank, B, so that after half a revolution the moon will be farther from the earth than she is here shown.
LAGRANGE. A larger but similar formation, 100 miles in diameter, associated with the last on the N.E., with a complex terraced border, including peaks of 9000 feet, a bright crater on the W., and a ring-plain on the N.W. The inner slope of the E. wall is a fine object at sunrise, when libration is favourable. The floor is dark and devoid of detail.
The effects of libration on Venus were not new to me, but they were to Jack and Henry, who had never studied such things, and they expressed much doubt about Edmund's plan, but I had confidence in it from the beginning, and it turned out just as he had predicted, as things always did.
By the effects of ``libration'' parts of the opposite hemisphere of the moon which is turned away from the earth are from time to time brought into view, and their aspect indicates that that hemisphere resembles in its surface features the one which faces the earth.
Taking, then, an opportunity when the moon had just swung to the extreme limit of her balancing, or, to use technical terms, when she had attained her maximum libration in longitude, the observers approached the level opening to Lake Langrenus, as the narrator calls this fine walled plain, which, by the way, is fully thirty degrees of lunar longitude within the average western limit of the moon's visible hemisphere.
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