Page:Popular Science Monthly Volume 5.djvu/656

638 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,