12 



SCIENCE. 



[Vol. II., No. 22. 



irregular way, with a tendency very often toward the 

 tangential direction at the lower parts of the rifts. 

 The photographs extend about a diameter and a half 

 from the sun's limb, and a comet appears on the 

 plates about a solar diameter and a half from the 

 sun's centre. It must have been very bright, as it 

 appears clearly in tlie photographs. Measurements 

 seem to indicate a small shift iu its position during 

 the interval between the first photograph and the last. 



Turning now to the photographs taken with the 

 camera and prism in front, — an instrument which 

 gives an image of the prominences as oft repeated 

 as there are rays in the prominence, — the plates 

 employed were sensible to the infra-red as well as 

 violet rays. One prominence gave a great number 

 of lines iu the ultra-violet. The fact was brought 

 out in this eclipse, that the brightest lines in the 

 prominences are due, not to hydrogen, but to cal- 

 cium. Besides these and the hydrogen lines, there 

 is the line D, in the yellow, and the C line of 

 hydrogen in the red, and also a photograph of two 

 prominence-lines in the ultra-red. In addition to the 

 prominences, there are visible in the photographs 

 certain short rings round the moon, which mean that 

 at these places the light sent out by tlie gaseous part 

 surrounding the moon is not confined to the prom- 

 inences. It is, as would be expected, the green 

 coronal line which chiefly corresponds to one of 

 those rings. This green line, K 1474, is a true coronal 

 .line, and is only very faintly traceable in one of the 

 prominences. 



In considering the results obtained with the com- 

 plete spectroscope, it is a strilj;ing fact that some of the 

 lines cross tire moon's disk, and especially the two 

 lines H and K. This proves tliat the calcium-lines, 

 H and K, were so strong in the prominences that the 

 light was scattered iu our atmosphere, and reflected 

 right in front of tlie moon. 



The prominence-lines are very numerous : thirty 

 such lines appear in the photograph. The hydrogen- 

 lines are there, including those in the ultra-violet 

 photographed by Dr. Huggins; also H and K, and 

 other calcium-lines; and still others, chiefly un- 

 known. 



Close to the sun's limb we can only trace a con- 

 tinuous spectrum, a very strong one, going up to 

 about a quarter of a solar diameter. The photo- 

 graphs bear out the distinction between the inner 

 and the outer corona, the former being much 

 stronger in light. The boundary at which this con- 

 tinuous spectrum ends corresponds to the extension 

 of the inner corona. The continuous spectrum is 

 stronger on the side where the prominences are 

 weaker. In the corona we first of all see a very faint 

 continuous spectrum, and in that continuous spec- 

 trum one can trace at G the reversal of the dark 

 Frauuhofer lines. In addition, a series of faint true 

 coronal lines can be traced in the outer regions of the 

 corona. We have not traced any known substances 

 in the solar corona. The greater luunber of the 

 prominence-lines in the ultra-violet are also un- 

 known, but they seem to be present in Dr. Huggins's 

 photograph of the spectrum of a Aquilae. 



LETTERS TO THE EDITOR. 



*«* Correspondents are reque fled to be as brief as possible. The 

 writer's name is in all cases required as proof of good faith. 



The relative ages of planets, comets, and 

 meteors. 



The theory that the sun was once a gaseous mass 

 extending beyond the most distant planet, and that 

 it has contracted to its present dimensions by the 

 continuous action of gravity, and is still so cimtract- 

 ing, is now very generally accepted by astronomers. 

 It is well known, moreover, that the C'ind«usation ot 

 a gaseous body produces heat, and that the impact 

 of solar matter in consequence of its motion towards 

 the centre of gravity is one cause, at least, — perhaps 

 the principal one, — of the sun's high temperature. 

 The modern law of the conservation of energy af- 

 fords data for determining the amount of heat" pro- 

 duced by the couden^ation of the sun's ma^s from 

 one voUime to another. It is thus found that the 

 contraction to its present dimensions, from a primi- 

 tive volume extending indefinitely beyond the orbit 

 of Neptune, would have kept up a uniform supply of 

 heat equal to the present for twenty millions of years.^ 

 The age of the solar system, however, may be great- 

 er or less than this, as the sun's radiation may not 

 have been constant. 



In any form of the nebular hypothesis, Neptune is 

 the oldest planet known, and the innermost of the 

 number has had the most recent origin. 



A inajority of comets probably move in hyperbolas, 

 and visit the solar system but once. Some orbits 

 have been changed into ellipses by planetary pertur- 

 bation. 



For any thing we can know to the contrary, com- 

 etary matter has been falling towards the centre of 

 our system in all ages of its existence. Whenever 

 the perihelion distance has been less than the radius 

 of the solar spheroid, the comet's orbital motion 

 must have been arrested, and transforriied into heat. 



As the limits of geological dates are determined by 

 the strata of the earth's crust, so the superior limits 

 of the age of periodic comets are fixed by the plane- 

 tary orbits next exterior to their perihelia. Of the 

 comets known to be periodic, the perihelion distances 

 of thirteen are less than the earth's distance from the 

 sun. The ages of all these must therefore be less 

 than that of the earth. In like manner the ages of 

 others are shown to be less than that of Venus, 

 while those of a few are found to be less than the 

 age of Mercury. We may conclude, then, in general, 

 that the ages of comets, as members of the solar sys- 

 tem, are less than those of planets. 



But as meteoroids, partly at least, are derived from 

 comets, their origin as separate bodies in connection 

 with our system must be still more ri'cent: in fact, 

 meteoric matter is being constantly detached from 

 comets at each successive return to perihelion. The 

 indications of this process were unmistakable in the 

 case ot the great comet of 18S2, and many meteoroids 

 of the Biela group have been separated from the 

 comet in our own day. Daniel Kirkwood. 



Bloomiugton, lud. 



First use of ■wire in sounding. 

 Professor Verrill is quite right in supposing that I 

 was unaware that any report of the souitding expedi- 

 tion of Walsh had been published. A casual ri'fereiice 

 to Walsh in the ' Depths of the sea ' led me to inquire^ 



^ A contraction of tlie r.idiiis equal to a hundred and twenty- 

 nine feet per annum woutd yield the present supply of heat. 

 See Monthly notices of the R. A. S., April, 1S72. 



