Popular Science Monthly/Volume 5/September 1874/Miscellany


Coggia's Comet.—The comet which lately made such a grand display in our northern heavens was discovered by Coggia, at Marseilles, on April 17th. When first seen, the nucleus and coma together had a diameter of 100,000 miles, the comet being then 133,000,000 miles from the earth, and 153,000,000 from the sun. It travels round the sun in the same direction as the planets, but in an orbit the plane of which is very much inclined. Of all the planetary orbits, excepting those of the asteroids, that of Mercury is most oblique, having a slant of seven degrees; but the inclination of the orbit of Coggia's Comet is nearly ten times as great, being set down at 66°. Its perihelion passage, or nearest approach to the sun, occurred on July 8th, when it came within 62,000,000 miles of that orb, and it was then moving at the rate of 160,000 miles an hour. It continued to approach the earth until July 20th, coming, on that date, within 26,000,000 miles of us, when it appeared at its brightest, or, according to Prof. Parkhurst, 140 times more brilliant than when first discovered. Of its tail, the same authority says: "On June 25th the observed length of the tail was computed as 3,000,000 miles; on July 1st, 5,000,000 miles; on July 13th, 12,000,000 miles—an increase, after the first of that month, of one-twelfth per day. The tail continued, from its first appearance till the head of the comet ceased to be visible, to point from the latter directly toward the stars Beta and Gamma of the Lesser Bear. Afterward it moved slowly to the westward, so that it covered the dipper of the Greater Bear. The speed of the particles leaving the head to form the tail was estimated as over 3,000,000 miles per day. This brings the particles leaving the comet on July 4th to a distance from its head on July 24th of about 26,000,000 miles, corresponding at the latter date with the distance of the head of the comet from the earth. But, though the tail thus sweeps over sufficient space to cover the interval between the nucleus and the earth, the direction of the tail is such that it fails to reach us; its central line being distant about 4,000,000 miles, and the edge of the tail about 1,500,000 miles, from the earth."

Concerning the theory which accounts for the formation of comets' tails by a repulsive action of the sun on the matter of the nucleus. Prof. Parkhurst also writes, in the Tribune of July 23d: "The existence of a repelling force was suggested by the fact that a comet's tail, pointing eastward when the comet is east of the sun, points northward and westward as the comet itself moves around to the north and west of the sun. Yet, as there is no coherence in the tail, it is evident that no repelling force from the sun, when it is to the east of the sun, can have any tendency to bring it around to the west of the sun. The fact is, that the tail which is to be seen to the west of the sun is composed of entirely different matter from that which was seen to the east. The former matter has been repelled so far from the sun, and has been so expanded, that it has become invisible; and new matter has been repelled from the nucleus, forming a new tail upon the western side. The law of repulsion will not only account for this, but for the formation of a new tail at the rate of 30,000,000 miles per day, as recorded in the case of the comets of 1680 and 1843. The magnitude of the comet's tail in those instances was stupendous; but its velocity was no less astounding. It commenced to move at the rate of 30,000,000 miles per day; but, unlike the motion of the comet itself, the motion is accelerated so long as the repelling force continues to operate. There is no retarding force, as in the motion of a receding comet. Whatever velocity has once been reached is retained, and the particles are constantly receiving an accelerating force from the time they leave the head of the comet, although the amount of the accelerating force which they receive will gradually diminish as the distance from the sun increases. There is no known instance of a comet coming into our system with a velocity approaching this; and, as the tail of a comet is chiefly formed after it has passed its perihelion, when each successive addition to the tail is impelled with less velocity than that which started before it, there seems to be no alternative to the theory that the matter forming a comet's tail is so thoroughly diffused in space that it can never be reunited."

Astronomers appear to be agreed that Coggia's Comet is an entire stranger to us, if not to these regions of space. It was at first surmised that it might be the one seen by the French missionaries in China in 1737, thus making its period of revolution 137 years. But the opinion of Prof. Hind, as lately expressed in Nature, is that, notwithstanding similarity of orbit, the two are not identical. The orbit of this latest comer has not been definitely determined, but is pronounced either a parabola or hyperbola. As the comet will be visible in the Southern Hemisphere until the end of September, more on this interesting point will probably be learned before its final disappearance.

Brain Development in the Mammalia.—According to the researches of Prof. Marsh, the larger mammals of the Tertiary period, as compared with their existing representatives, were sadly deficient in brains. Their later remains, however, afford evidence of steady improvement in this particular. The mammals of the Eocene all had small brains, being little better provided in this respect than the higher reptiles. The type genus of the largest of the Eocene mammalia, Dinoceras, nearly equaled the elephant in bulk, but had a brain only about one-eighth the size of that in existing rhinoceroses. The smallness of the cavity in the other genera of this order was equally remarkable. The Brontotherium of the American Miocene (a later division of the Tertiary), while equaling the dinoceras in size, had a decidedly larger brain-cavity; and, in the still later strata of the Pliocene, a species of mastodon was found which likewise exhibited increase of brain-dimensions, the cavity approaching but not equaling that of existing proboscidians." The Tapiroid ungulates of the Eocene had small brain-cavities, much smaller than their allies the Miocene Rhinocerotidæ. The Pliocene representatives of the latter group had well-developed brains, but proportionally smaller than living species. A similar progression in brain-capacity seems to be well marked in the equine mammals, especially from the Eocene Orohippus, through Miohippus and Anchitherium of the Miocene, Pliohippus and Hipparion of the Pliocene, to the recent Equuus. In other groups of mammals, likewise, so far as observed, the size of the brain shows a corresponding increase in the successive subdivisions of the Tertiary. These facts have a very important bearing on the evolution of mammals, and open an interesting field for further investigation."

The Peabody Museum of Natural History.—We learn from the College Courant that the Peabody Museum of Natural History, connected with Yale College, is to be commenced at once. The building, when complete, will be 350 feet in length. At present, only one wing is to be built, costing $160,000, with a ground-plan of 115 feet by 100. It will be of brick, with cut-stone trimmings, and strictly fire-proof; of three stories, with high basement, making, virtually, four stories. This basement will be largely devoted to the exhibition of fossil footprints. The first story will be devoted to a lecture-room and a mineralogical cabinet; the second to geology, with the fossil vertebrates from the Rocky Mountains; the third to zoology, and the attic to archaeology and ethnology. The funds for the erection and maintenance of this institution were furnished, in 1866, by the late George Peabody, who, by deed of gift dated October 22d of that year, gave $150,000 to Profs. J. D. Dana, O. C. Marsh, B. Silliman, G. J. Brush, and three others, in trust "to found and maintain a Museum of Natural History, especially of the departments of zoology, geology, and mineralogy, in connection with Yale College." The present curators of the several departments of the Museum are Prof. Brush, of the mineralogical, Prof. Marsh, of the geological, and Prof. A. E. Verrill, of the zoological department.

The Movement of Storms.—The American Journal of Science for July contains Part I. of an able paper, by Prof. Loomis, entitled "Results derived from an Examination of the United States Weather Maps for 1872 and 1873," read before the National Academy of Sciences in April last. The weather-maps which furnished the data for his examination exhibit storm-paths for 314 days. These he has carefully tabulated and classified. The course and velocity of the storms for each month are given, showing that the average velocity in forward movement was 26.6 miles per hour, that the greatest average velocity in any month was in February, it being 31 miles per hour; the lowest was in August, when the rate was 17.7 miles an hour. It also appears that their forward movement is greater in winter than in summer. But some atoms move with exceptional velocity. Thus, in May 15, 1873, a storm-centre advanced 1,200 miles in twenty-four hours, while, in other cases, there was no forward movement, and the storm-centre remained stationary for twenty-four hours. The average direction of the storm-paths for two years was 8° north of east; in summer, nearly due east; in winter, more northward; but most northward in fall and spring. In October the direction was 21° north of east. Instances occurred, however, in which storms moved north-northwest; and, on the 6th of April, 1873, a storm in the Mississippi Valley moved in every direction in a little more than twenty-four hours.

Prof. Loomis carefully studied the causes which appear to influence the velocity and direction of storms. Of these, rainfall is important. It is found that the area of rainfall extends farther on the eastern than it does on the western side of a storm-centre; so that the rain-area is a long oval, the longer diameter of which is in or nearly in the direction in which the storm is moving. This is true of most of the storms which traverse the United States. This rain-area extends to an unusual distance on the eastern side of a storm when it is advancing—the average extent being about 500 miles.

By the condensation of vapor eastward of the storm, it, in a measure, makes its own way. Thus the barometer continually falls in advance of it, announcing its approach, but rises as the storm-centre is past. The conditions by which a storm is sustained, and which are present before or in front of it, cease to exist in its wake. Instances occur, however, in which increased velocity and condensation in the western quadrant of a storm set back the storm's centre, and give it, for a time, a retrograde motion. The wind on the western quarter of a storm usually blows with greater velocity by about 22 per cent. than it does on its eastern quadrant, and this is a means by which the forward motion is retarded; and it is found that, when the wind's velocity in the western quadrant is 44 per cent. greater than in the eastern, the storm's forward motion is seven miles an hour less than its average rate of progress.

The atmosphere in the storm circuit moves inward, but also upward, to the central region of the storm, which is supposed to be from one to two miles above the earth's surface. At this elevation atmospheric movements are greatly increased in velocity. Thus, at the summit of Mount Washington, the velocity was 29 miles an hour, or, compared with that at the level of the sea, was as 5.5 to 1. By comparing the velocity of the wind at this elevation in the direction which the storm advances with the velocity of the storm's advance, Prof. Loomis is enabled to deduce a measure of the force of the upward movement of the centre of a storm.

Storms are divided, by Prof. Loomis, into two classes. Those of the first class traverse the continent northward of the fortieth parallel; many from the remote west reach the great lakes Superior and Huron, and show a decided preference for that region. Some of these have their origin among the Rocky Mountains, and some come from the mountains of Oregon. Those of the second class originate chiefly westward of the mouth of the Mississippi, and move nearly northeast. These comprise only about one-sixth of the whole number, but include some of the violent cyclones which traverse our coast.