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THE POPULAR SCIE2^CE MONTHLY. 



is, that the tail which is to be seen to the 

 west of the sun is composed of entirely dif- 

 ferent matter from that which was seen to 

 the east. The former matter has been re- 

 pelled so far from the sun, and has been so 

 expanded, that it has become invisible ; and 

 new matter has been repelled from the nu- 

 cleus, forming a new tail upon the western 

 side. The law of repulsion will not only ac- 

 count 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 com- 

 ets of 1680 and 1843. The magnitude of 

 the comet's tail in those instances was stu- 

 pendous ; but its velocity was no less as- 

 tounding. 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 retard- 

 ing 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 acceler- 

 ating force which they receive will gradu- 

 ally diminish as the distance from the sun 

 increases. There is no known instance of a 

 comet coming into our system with a velo- 

 city 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 mat- 

 ter forming a comet's tail is so thoroughly 

 diffused in space that it can never be re- 

 united." 



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 I'ZSY, 

 thus making its period of revolution 137 

 years. But the opinion of Prof. Hind, as 

 lately expressed in Nature^ is that, not- 

 withstanding 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 3Iammalia. — 



According to the researches of Prof. Marsh, 

 the larger mammals of the Tertiary period, 

 as compared with their existing representa- 

 tives, 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 remark- 

 able. 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 RhinocerotidcB. The Pliocene rep- 

 resentatives 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 Orohippns, through MioMppus and 

 AnchitJiet'ium of the ]\tiocene, Pliohippus 

 and Hipparion of the Pliocene, to the re- 

 cent Eqniis. 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 Musenm of Xatnral Hi?- 

 toryt — "We learn from the College Coiirant 

 that the Peabody Museum of Natural His- 

 tory, 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, cost- 



