The length of a shadow may be obtained by the following method: the longitudinal wire of the micrometer (19) is adjusted so as to pass through the shadow whose length is to be measured, and the transverse wires are placed one at each end of the shadow, as shown in Fig. 133. The micrometer screw is then turned till the wires are brought together, so as to ascertain the length of the arc between them. We may then form the proportion: the number of seconds in the semi-diameter of the moon is to the number of seconds in the length of the shadow, as the length of the moon's radius in miles to the length of the shadow in miles. [Illustration: Fig. 133.] The height of the sun above the horizon is ascertained by measuring the angular distance of the mountain from the terminator. (2) We may measure the distance of a bright point from the terminator, and then construct a right-angled triangle, as shown in Fig. 134. A solution of this triangle will enable us to ascertain the height of the mountain whose top is just catching the level rays of the sun. [Illustration: Fig. 134.] _B_ is the centre of the moon, _M_ the top of the mountain, and _SAM_ a ray of sunlight which just grazes the terminator at _A_, and then strikes the top of the mountain at _M_. The triangle _BAM_ is right-angled at _A_. _BA_ is the radius of the moon, and _AM_ is known by measurement; _BM_, the hypothenuse, may then be found by computation. _BM_ is evidently equal to the radius of the moon _plus_ the height of the mountain. By one or the other of these methods, the heights of the lunar mountains have been found with a great degree of accuracy. It is claimed that the heights of the lunar mountains are more accurately known than those of the mountains on the earth. Compared with the size of the moon, lunar mountains attain a greater height than those on the earth. 115. _General Aspect of the Lunar Surface._--A cursory examination of the moon with a low power is sufficient to show the prevalence of crater-like inequalities and the general tendency to _circular_ shape which is apparent in nearly all the surface markings; for even the large "seas" and the smaller patches of the same character repeat in their outlines the round form of the craters. It is along the terminator that we see these crater-like spots to the best advantage; as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations into bold relief. They vary greatly in size; some being so large as to bear a sensible proportion to the moon's diameter, while the smallest are so minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them. [Illustration: Fig. 135.] The prevalence of ring-shaped mountains and plains willbe evident from Fig. 135, which is from a photograph of a model of the moon constructed by Nasmyth.