Page images

and the ischium has less posterior extension than in the ele. phant. The thyroid foramen is an elongate oval. The femur is proportionally about one-third shorter than that of the elephant. The head of this bone has no pit for the round ligament, and the great trochanter is flattened and recurved. There is no indication of a third trochanter. The distal end of the femur is more flattened transversely than in the elephant, and the condyles are more nearly of the same size. The corresponding articular faces of the tibia are consequently about equal, and also contiguous, with no prominent elevation between them. When the limb was at rest, the femur and tibia were nearly in the same line, as in the elephant and man. The patella is elongate, and oval in outline. The fibula is slender, and entire, with articular faces well marked at each extremity. The astragalus bas no distinct superior groove. Its anterior portion has articular faces for both the navicular and cuboid, thus differing from Proboscidians, and agreeing with Perissodactyls. The calcaneum is very short, its longitudinal and transverse diameters being about equal. It does not articulate with the navicular, as in Elephas, and has only a small face for the cuboid. There are four well developed digits in the pes, and a rudimentary or small hallux. The metatarsals are much shorter than the metacarpals. The phalanges and sesamoid bones are smaller, but otherwise similar to those of the manus. The hind foot is shown in figure 1 of Plate VI. None of the bones of the skeleton are hollow

The known species of Dinoceras nearly equalled the elephant in size, but the limbs were shorter. The head could reach the ground, and there is no evidence of a proboscis. All the remains of the genus yet discovered are from the Eocene of Wyoming Yale College, New Haven, Jan. 18th, 1876.

(To be continued.)


Plato II.- Dinoceras mirabile Marsh. Figure 1, side view of skull; figure 2, front

view; figure 3, top view. One-eighth natural size. Plate III.Dinoceras mirabile. Superior premolar and molar teeth; bottom view.

Three-fourths natural size. Plate IV.-Dinoceras mirabile. Cast of brain cavity. Figure 1, side view ; figure

2, top view; figure 3, bottom view. One-half natural size. Plate V.Dinoceras laticeps Marsh. Lower jaw. Figure 1, front view; figure

2, side view; figure 3, top view. One-fifth natural size. Plate VI.-Dinoceras. Figure 1, hind foot; figure 2, fore foot. One-third natural






ART. XIX.—On the Veiled Solar Spots ;* by L. TROUVELOT.

[Read before the American Academy by William A. Rogers, Oct. 12, 1875.]

It is now pretty well established that the visible surface of the sun is a gaseous envelope called “the chromosphere ;" mainly composed of incandescent hydrogen gas, with wbich are occasionally associated some metallic vapors, usually occupying the lower strata. To all appearances, the granulations called “rice grains,” the faculæ and the protuberances, are phenomena belonging to the chromosphere; in fact they are the chromosphere itself seen under the particular forms and aspects peculiar to it. Ordinarily this envelope has a thickness of 10" or 15". This thickness, however, is by no means constant, vary. ing from day to day within certain narrow limits.

At no time since I have observed the sun have I seen the chromosphere so thin and shallow as during the present year, and especially between June 10 and August 18. I bad before quite often observed local depressions and upheavals of the chromosphere, sometimes extending over large surfaces, but I had never before observed such a general subsidence.

So thin was the chromosphere during this period that it was sometimes very difficult to obtain its spectrum by placing the slit of the spectroscope tangent to the limb of the sun. This was especially the case on the afternoon of August 9.

This unusual thinness of the chromosphere could be easily recognized without the assistance of the spectroscope. Indeed, the phenomenon was even more interesting seen through the telescope, as, with it, the structure of the photosphere, lying as it does under the envelope of the chromosphere, could be better seen through the thin veil formed by the greatly attenuated chromospheric gases.

* From the Proceedings of the American Academy. AM. JOUR. S01.—THIRD SERIES, VOL. XI, No. 63.- MARCH, 1976.

That the gases forming the chromosphere are sometimes thin enough to become transparent is a phenomenon which I have observed hundreds of times; as is abundantly proved by the numerous drawings of protuberances which I bave made at the Harvard College Observatory, in which the limb of the sun is seen through the base of the protuberances in front of it. In plate X, figure 3, there occurs a very striking instance, where two small prominences are seen through a large protuberance nearer the observer.

During this period of general subsidence, the granulations appeared to be smaller and farther apart than usual, and consequently the light-gray colored background upon which they are seen projected was more distinct, as it occupied more space than formerly. During this period, the light-giving element would appear to have been less than usual.

I am not aware that the phenomena of which I shall speak in this communication have been before observed; but I cannot speak positively on this point, owing perhaps to the some. what confused nomenclature of solar physics.

Ever since I have observed the sun with instruments of a large aperture, I have noticed that the light-gray colored background seen between the granulations is by no means uniform, as it is generally stated to be. On the contrary it is greatly and strikingly diversified. Aside from the very small black dots called "pores,” patches of a darker gray are irregularly distributed all over the surface of the sun. But partly owing to the effect of perspective, and partly on account of the thicker strata of the chromospheric gases through which they are necessarily seen near the limb, they disappear gradually as they approach the border.

These dark spots have been so remarkable during the present year, and so conspicuous during the period of the greatest subsidence of the chromosphere, that I bave availed myself of every favorable opportunity to study them. So strongly were they marked that when one had passed the field of view, it could be easily found again among many others, even after the lapse of several hours. Of the most striking and complicated, I have made sketches.

In order to be able to count how many of these gray spots could be seen in different heliographic latitudes, and also to estimate their area with respect to the whole surface of the sun, Mr. W. A. Rogers, assistant at the Harvard College Observatory, kindly ruled for me on glass a reticule of small squares. Though the problem is apparently a simple one, it nevertheless presented many difficulties ; partly owing to the minuteness and delicacy of these objects, partly on account of the unsteadiness of the atmosphere, and partly to the many defects caused by the great amount of heat concentrated at the focus of the objective. However, the observations show clearly that though the number of gray spots varies but very little in different latitudes, in general the spots become larger and more complicated as they approach the equatorial zones.

The most marked characteristic of the gray spots is their vagueness of outline. They are never sharply defined like ordinary spots, but they appear blurred and diffused like an object seen through a mist. As I shall endeavor to show presently, these objects are really seen through chromospheric gases which are spread as a veil over them, causing this vagueness of outline. For this reason, I propose for them the name of Veiled Solar Spots.

The veiled solar spots, especially in the lower latitudes, have a remarkable tendency to assemble into small groups after the manner of ordinary spots. Sometimes three or four are seen in contact, while there are comparatively large intervals where none are to be seen. I have in several instances seen the actual formation into groups of distinct veiled spots.

The granulations of the chromosphere are seen projected upon the veiled spots, just as anywhere else, but they are not there so regularly distributed ; some being closely crowded together, while others are widely scattered. Small faculæ are often formed in this manner by the aggregation of several granules into one mass. Once in a while the granulations appear as if they were under the power of a propelling force by which they arrange themselves in files, and sometimes in capricious figures which are very remarkable.

In many cases I have observed that the granulations projected upon the veiled spots have an extraordinary mobility, to be seen nowhere else, except perbaps in the immediate vicinity of ordinary spots in full activity. Often their form and position are totally changed within a few minutes, and sometimes even within a few seconds. This was especially the case June 21. At 8h 30m on that day, I was observing a group of veiled spots not far from the center of the sun, when my attention was drawn to the extraordinary mobility of the granulations covering this group. In an instant they changed their form and position, some crowding together as though briskly attracting each other, while others would fly apart as if repelled by an invisible force. Under this tumultuous conflict of forces, new veiled spots would appear and disappear in an instant, faculæ would form and vanish ; in fact, all was in motion and confusion on that particular part of the sun. It was evident that immense forces were in conflict under the chromosphere.

At 2b Om P. M., on the same day, several small black spots had opened through the chromosphere upon the group of veiled spots observed in the morning. At gh Om on the following morning, the group of small black spots was considerably increased, having quite a large spot on the preceding side, followed by twelve or fifteen smaller ones. On June 24, this group had attained to its maximum size. It was then very large and complicated. In fact, it was the largest group of sun spots observed thus far during the present year.

On August 8, I noticed a group of veiled spots a little south of the sun's center. The following morning at 7h Om, there was at the same place a small group of half a dozen black spots disposed in a crescent shape. At 250m P. M., the black spots had vanished, but the veiled spots still remained, having retained the characteristic crescent form of the black spots and many other details observed in the morning; and, as a proof that the chromosphere covered this spot, the granulations could be plainly seen upon the whole, indicating clearly that this spot was seen through the veil of the chromospheric gases.

On August 24, the same phenomenon took place. Just fol. lowing the principal spot of the only group then to be seen on the surface of the sun, there was a fine group of veiled spots. The following day some black spots bad made their appearance upon them. On August 27, the black spots had vanished, but in their place the veiled spots seen at first still remained, and they continued to be seen there for several days.

To all appearances, the black spots which I had seen disappear under the chromospheric gases, and which continued as veiled spots, were exactly alike and undistinguishable from the many other veiled spots scattered all over the sun; and, had I not seen the opening of the photosphere, with the black spots, I could not have had any idea of the true nature of the veiled spots.

So far, I have only spoken of veiled spots observed in the zones where the ordinary sun spots usually make their appearance; but, as I have said, the veiled spots are scattered all over the surface of the sun.

During this period, I had many occasions to observe very remarkable and characteristic veiled spots in very high heliographic latitudes north and south. On July 15, within a few degrees of the north pole of the sun, I observed a remarkable veiled spot, unusually large and dark. Upon it were several bright slender faculæ projected in crest shape to very high altitudes. These faculæ appeared to be precisely like those observed in lower latitudes near ordinary sun spots. Upon this veiled spot could unmistakably be seen a small black spot, not a pore ; a real opening of both chromosphere and photosphere.

« EelmineJätka »