those of Proboscidians. The atlas and axis are very similar to those of the elephant, but the rest of the cervicals are proportionally longer. The dorsal and lumbar vertebræ have the articular faces nearly flat, and the lumbars have an inferior ridge on the median line. There are four sacral vertebræ, the last being quite small. The anterior caudals have long, depressed, transverse processes. The ribs resemble those in Mastodon. The segments of the sternum were well ossified, and most of them were flattened vertically.

The scapula, in its general form, is similar to that of the elephant, but there is much less constriction above the glenoid fossa. The latter is elongate, deeply concave longitudinally, and nearly flat transversely. The spine extends downward nearly to the glenoid border. The coracoid portion is a rugose protuberance, separate from the margin of the articular fossa. The humerus is short and massive, and in its main features resembles that of the elephant. One of the most marked differences is seen in the great tuberosity, which does not rise above the head, and is but little compressed. The condylar ridge, moreover, of the distal end is tubercular, and not continued upward on the shaft. The lower extremity of the humerus is much like that of the rhinoceros, and the proportions of the two bones are essentially the same. The radius and ulna are nearly of the same size. The head of the radius rests on the middle of the ulnar articulation, and hence the shaft of this bone does not cross that of the ulna so obliquely as in the elephant. The ulna has a small face for articulation with the lunar, as in the elephant.

There are five well developed toes in the manus, which is well shown in Plate VI, figure 2. The carpal bones are eight in number, and form interlocking series, as in Perissodactyls. The scaphoid resembles that bone in the elephant, but is shorter and stouter. Its proximal end is rounded, forming about onefourth of a sphere. On its distal end, the articular faces are confluent. It supports the trapezium and trapezoid. The pyramidal sends down an outer angle to articulate with the fifth metacarpal, as in Elephas. The trapezoid is the smallest bone in the carpus. The magnum is supported by the lunar, and not at all by the scaphoid. The unciform is the largest carpal bone. It has the usual metacarpal faces well marked, and separated by ridges. The metacarpals are of moderate length, and the third is about equally supported by the magnum and unciform. The articulations for the phalanges are nearly flat, indicating but little motion. The phalanges are very short, and the distal ones rugose.

The pelvis is much expanded, as in Proboscidians. The ilium is suboval in outline. The pubis is slender and short,

and the ischium has less posterior extension than in the elephant. 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 has 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.)

EXPLANATION OF PLATES.

Plate 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

size.

THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[THIRD SERIES.]

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 which 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, varying 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 had 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 * From the Proceedings of the American Academy.

AM. JOUR. SCI.-THIRD SERIES, VOL. XI, No. 63.-MARCH, 1876.

it does under the envelope of the chromosphere, could be better seen through the thin veil formed by the greatly attenuated chromospheric gases.

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 have 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 somewhat 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 have 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 perhaps 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.