ASTRONOMICAL PHENOMENA AND PROGRESS. 



175 



the different parts of its flight, every comet 

 goes through changes of condition that contrast 

 extremely: not so, however, with the planets. 

 Halley's comet, at perihelion, is 56 millions of 

 miles from the sun ; at aphelion, 3,370 millions 

 of miles. Hence, its greatest being more than 

 60 times its least distance, the solar light and 

 heat received by it during a given time at peri- 

 helion is more than 3,600 times that received 

 during a like period at aphelion. Again, while 

 the comet is 6 years in passing over one helio- 

 centric degree of its orbit in aphelion, at peri- 

 helion it accomplishes the same distance in 

 15.7 hours. The comet, beginning to cool 

 faster than it is warmed by heat received, from 

 the time at which its distance from the sun first 

 begins to increase, must, in view of the feeble 

 heat received in all distant parts of its orbit, 

 continue cooling until in its return it has already 

 approached so near the sun that its velocity be- 

 gins to be greatly accelerated. Now, along 

 with this rapid approach toward the sun, there 

 is an augmentation (m duplicate ratio) of the 

 light and heat the nucleus or comet-mass re- 

 ceives; so that during the latter part of its 

 return course, and within a relatively small 

 part of the entire orbit, there occurs a violent 

 transition from the lowest temperature at any 

 time reached by the mass, to the highest. This 

 change, the author infers, is likely to develop 

 a powerful discharge and current of electricity, 

 which will carry off from the substance of 

 the nucleus myriads of extremely fine particles, 

 as in certain circumstances such currents are 

 known to do. That absolute smallness of the 

 comet's perihelion distance does not explain the 

 development of the luminous coma, is proved 

 by such facts as that some bright comets ap- 

 proach the sun no nearer than about the dis- 

 tance of Mars or Jupiter. But from the view 

 given, it follows that great splendor must result 

 when, along with extreme eccentricity of path, 

 there is combined a comparatively small peri- 

 helion distance ; since in such cases the change 

 of conditions during the approach will be cor- 

 respondingly intensified. And that such com- 

 bination of circumstances is attended with the 

 development of extraordinary brightness, is 

 shown by the facts, being remarkably exempli- 

 fied in the instances of the comets of 1843 and 

 1861. The ratio of aphelion to perihelion dis- 

 tance for Polyhymnia, the most eccentric of 

 known planetary bodies, is only 2 ; that of 

 some telescopic comets given is from as low 

 as 3.5 up to about 12 ; that of De Vice's comet, 

 just visible, is 52 ; that of Donati's comet (1858) 

 was 624; that of the great comet of 1843 was 

 18,673 ; and that of the comet of 1861, not less 

 than 137,173. Of the two last named, the pe- 

 rihelion distance was also extremely small. 

 Other consequences following from the theory 

 are. that brilliant comets should have long pe- 

 riods, and that they should begin to appear 

 bright at a great distance ; and both these re- 

 quirements are in accordance with observed 

 facts. If the theory be true, and the forms of 



the orbits of asteroids and comets could be ex- 

 changed, then, within a single set of revolutions, 

 each of these kinds of bodies would have as- 

 sumed the characters now peculiar to the other. 

 According to the view presented, the substance 

 of the cometary mass must slowly waste ; and 

 it is stated that Professors Pierce and Mitchell 

 had already been led to suspect that to a pro- 

 gressive diminution of its mass might be due 

 the known retardation of Encke's comet. It 

 will be seen, however, that the theory of Mr. 

 Marsh surrenders the idea of the planetary 

 spaces as really vacuous, or free from a dif- 

 fused ponderable matter. If the comets waste, 

 as he supposes, then all space or many parts 

 of space are filled with an actual atmosphere 

 of extremely rarefied comet-matter ; and the 

 planets are very sure to meet with obstruc- 

 tion accordingly. Moreover, the view of the 

 aurora involves a progressive exhaustion of 

 our own atmosphere, against which some of 

 the generations of a very remote future 

 might desire the opportunity of entering a re- 

 monstrance. 



Nebula. Toward the close of the year 1861, 

 Prof. D'Arrest, of Copenhagen, who has been 

 for some time engaged in a revision and cata- 

 loguing of the nebulfe, announced that a nebula 

 in the constellation Taurus, discovered by Mr. 

 Hind in 1852, had totally vanished from its 

 place in the heavens. By observations made 

 about Jan. 26, 1862, Le Yerrier confirmed this 

 declaration; although by other authorities it 

 has since been stated that the. nebula, invisible 

 to other telescopes, was still detected with the 

 great refractor at Pultowa. This much is 

 certain, that a nebula sufficiently bright to be 

 readily seen, between 1852 and '58, through 

 many telescopes in different countries, can now 

 no longer be seen through most of those in- 

 struments, if through any of them. The place 

 of this nebula was about 11 degrees from 

 f Tauri, in the group known as the Hyades. 

 Its diameter was about 1'; and its light being 

 more distinct toward the central portion, it 

 had an appearance which indicated that, if ever 

 resolved, it would prove to be a very remote 

 globular cluster of stars. It was seen by Cha- 

 cornac in 1854, while he was engaged in form- 

 ing a chart of the stars in this region; and 

 having also been missed by him during a sub- 

 . sequent search in 1858, it is now supposed that 

 its disappearance probably occurred about 1856. 

 From 1852 until the last named date, a star 

 showing the 10th magnitude, and first noticed 

 on the same night with the discovery of the 

 nebula, almost touched the latter at its edge. 

 From the circumstances of its discovery. Mr. 

 Hind had suspected that this star was variable ; 

 and in fact, since the nebula has ceased to be 

 visible, the star has dwindled to the 12th 

 magnitude. 



A second case of a variable or disappearing 

 nebula, is that of one in the Pleiades, and 4 

 distant from the preceding, discovered by 

 M. Tempel at Venice, Oct. 19, 1859, and which 



