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It will be seen at once that there exist a number of well defined regions which show a surprising uniformity of motion. To make this more apparent I have outlined these regions on the drawing.

We notice that there is a conspicuous clustering along the path of the Milky Way. This is to be expected, since the stars in which stationary lines occur show the same tendency. More interesting, however, are the separate regions of uniform or nearly uniform motion. This is particularly well shown in a region in Cygnus which contains only positive velocities (at about 20h to 21h in a and +30° to +50° in 8). This region had been previously noticed by J. S. Plaskett. A similar, but more extended region of large negative velocities exists. in the constellations Perseus and Cassiopeia. Two small regions of uniformly negative velocity are shown near Cygni and in Lacerta. The large region in Orion and in Monoceros is characterized by a large dispersion in the velocities. This region should perhaps be divided into two at about the border line of the constellation Orion. Very interesting is the small region in Orion containing the stars X', λ" and 1 Orionis.

In Orion parts of the cloud coincide in position with the great nebula. The velocity determined from the nebula is the same as that obtained from the stationary Ca lines.18 Since the bright B and O stars are evidently situated within the nebula,19 we are led to the conclusion that the Ca cloud and the Orion nebula are identical in position in space and in motion. However, as we shall see later, the densest portion of the Ca cloud is in the northern part of Orion and does not coincide with the densest portion of the visible nebula. It is interesting that at least one of the stars, situated in the Orion nebula and showing stationary Ca lines, has a velocity that is quite different from that of the nebula or of the Ca cloud. This star is 43 02 Orionis.18 Assuming the Orion nebula to be stationary with respect to the stellar system, which is very nearly the case, I found that this star is moving with a velocity of nearly 20 km/sec. This motion is directed away from us. As the star is apparently at the present time near that surface of the nebula. which is directed toward us, we may expect that the star will continuously decrease in brightness as it is submerged into more and more dense portions of the nebula. Accurate determinations of the brightness of this star are, therefore, much needed.

Another group of mostly positive velocities is indicated in the constellation Scorpius. The existence of this region was first suspected by V. M. Slipher in 1909.20

The appearance of the map in Figure 1 strongly suggests the existence of separate clouds of calcium covering many square degrees, which show uniform or nearly uniform motion as a whole. Table III contains the more conspicuous clouds of this nature with their average velocities referred to the stellar system.

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north of declination -20° for 1925.0. Drawn by Dr. Fr. Goos from photographs by Professor Max Wolf, Heidelberg. Edition Henry Grand,
Hamburg, Germany, 1921. (Reproduced by permission of Dr. Goos.) The arrows on top of the drawing indicate some of the more con-
spicuous Milky Way clouds: (1) Cygnus cloud, between Gamma and Beta Cygni. (2) Cassiopeia cloud, extending toward Cygnus. (3) Per-
seus cloud. Faint portion of Milky Way. (4) Milky Way in Monoceros.

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POPULAR ASTRONOMY, No. 330.

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Since the distribution of these velocities cannot be due to an error in the assumed value and direction of the solar motion, it is distinctly. shown that systematic motions of the calcium clouds exist.

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FIG. 1. Distribution of residual velocities obtained from stationary calcium lines.

If we compare our map in Figure 1 with that of the Milky Way, we find a definite relation between some of the regions enumerated in Table III and some of the more distinct Milky Way clouds. The Milky Way drawings of Dr. F. Goos are best suited to such a comparison.21 It is obvious that the Ca region in Cygnus is identical with the brilliant Milky Way cloud between y and ẞ Cygni. The region. farther north, extending to nearly Oh in a, corresponds to the Milky Way cloud extending from Cassiopeia to a Cygni. The Perseus

Cassiopeia region may be identical with the faint portion of the Milky Way in these constellations. To be sure, there are some very distinct Ca clouds, like those of Orion, Lacerta and Scorpius, which do not coincide with any of the brilliant patches of the Milky Way. They are probably closely associated with some of the dark and the bright diffuse galactic nebulosities.

Calcium Clouds Outside the Milky Way.

If the Ca clouds are related to the Milky Way, or to some of its bright or dark nebulae, as was suggested also by H. D. Curtis, 22 we are confronted with the question whether the "stationary" or "sharp” Ca lines are purely galactic phenomena. Plaskett2 has shown that this is not the case, since there is at least one star, namely p Leonis, which is situated far from the Galaxy (Galactic latitude 53°), and contains stationary Ca lines.24

The question of the existence and distribution of stationary Ca clouds outside the Galaxy is important; we would not necessarily expect every B star to be surrounded by a cloud of calcium, especially if such a star lies outside the Milky Way.

However, this question is greatly complicated by the possibility that in addition to the stationary lines, there are H and K lines present in the spectra of the stars, which are produced by stellar atmospheres.

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The Stellar Ca Lines.

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Various methods may be applied in the determination of intensity curves for spectral lines. Fowler and Milne25 have approached the matter from a theoretical point of view. Their results show that the intensity curve of the ionized Ca lines depends greatly on the value of Pe, i. e., the partial pressure of electrons from all sources. The two curves which they give for ionized calcium at P. 10-" atmospheres and at P.=1.31 X 10 tmospheres show very different fractional concentrations of ionized Ca atoms for stars of spectral type BO. The value of P. cannot be ascertained with sufficient accuracy to interpolate the true curve. Furthermore, as pointed out by Fowler and Milne, the marginal appearance of a line depends upon various other factors, such as the relative abundance of a given element, the actual value of that "very small fraction of atoms" which is necessary to produce a line visible in our spectra, etc. All of these sources may produce large uncertainties in the lower branches of the theoretical intensity curves. As an example of this uncertainty it may be mentioned that Plaskett believes that the curves of Fowler and Milne indicate that the ordinary lines H and K of stellar origin should not disappear until about type BO or earlier.20 At the same time Kienle,27 also from theoretical considerations, concludes that they should not exist in any stars earlier than B3. Apparently, the theoretical method is not sufficiently accurate to solve this problem.

I have found it more satisfactory to use observational data. The

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