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almost perceptible to the eye, until in ten minutes (1 h. 5 m. P.M.) the uppermost were more than 200,000 miles above the solar surface. This was ascertained by careful measurements; the mean of three closely accordant determinations gave 7' 49" as the extreme altitude attained ; and I am particular in the statement because, so far as I know, chromatospheric matter (red hydrogen * in this case) has never before been observed at an altitude exceeding 5'. The velocity of ascent also, 167 miles per second, is considerably greater than anything hitherto recorded. A general idea of the appearance presented when the filaments attained their greatest elevation may be obtained from fig. 2.
“ As the filaments rose they gradually faded away like a dissolving cloud, and at 1 h. 15 m. P.M. only a few filmy wisps, with some brighter streamers low down near the chromatosphere, remained to mark the place. But in the meanwhile the little thunder-head' before alluded to had grown and developed wonderfully, into a mass of rolling and ever-changing flame, to speak according to appearances. First it was
Prof. Young probably means that he was observing the red image of the cloud and uprushing matter-i.e. the image formed by rays corresponding to the C-line of hydrogen. Father Secchi mentions that he finds the indigo image (i.e. the image formed by rays corresponding to the G-line of hydrogen) the most perfect and the fullest in details.
almost pyramidally 50,000 miles in height; then its summit was drawn out into long filaments and threads, which were most curiously rolled backwards and downwards like the volutes
had vanished like the other. Figs. 3 and 4 show it in its full
development; the former having been sketched at 1 h. 40 m., and the latter at 1h. 55 m."* " The whole phenomenon," he adds,“ suggested most forcibly the idea of an explosion under the great prominence, acting mainly upwards, but also in all directions outwards, and then after an interval followed by a corresponding inrush; and it seems far from impossible (the
turn out to be truly solar, as now seems likely, may find their origin and explanation in such events.”
Now, it is to be noticed in the first place, that although the explosion thus described is the only one of the kind that astronomers have yet witnessed, we cannot safely infer that it was an exceptional solar disturbance. It is to be remembered that the sun is not always under spectroscopic surveillance, even in
• Prof. Young mentions that his “ sketches" do not pretend to accuracy of detail, except the fourth, the three rolls in which are nearly exact.
to obnt the likes of day of the bless the But
suitable observing weather, at American and European stations. Professor Young in America, and in Europe Lockyer, Janssen, Secchi, Respighi, and Zöllner, with the few others who take a more or less systematic part in the work, are unable to devote the whole of the day-or probably even a large portion of the day—to observation of the sun. But apart from this we must take into account the occurrence of unfavourable observing weather, and Lockyer speaks of days seemingly fine, when certain indications in the appearance of the prominence-lines assure him that observation is useless. Doubtless the experience of other observers resembles his in this respect. But this is not all. During a great part of the 24 hours the sun is not above the horizon at any of the European or American observing stations. And then, lastly, even when he is above the horizon, solar outbursts of enormous importance might take place without any possibility that terrestrial observers could become cognisant of the fact; simply because any outbursts in the central parts of the face turned towards the earth and of the half turned directly away from the earth, could not produce prominence-phenomena outside the solar limb. The spectroscope gives us an account indeed of disturbances taking place on the sun's face; but the account can be by no means so easily interpreted as in the case of prominences seen in the ordinary manner.
When we combine these considerations with the circumstance that a solar eruption lasts but a few minutes, and that the observer is unable to examine more than one portion of the sun's limb at a time, so that many important eruptions might occur even while he was engaged in the most attentive observation, we see that outbursts like the one witnessed by Professor Young may occur very frequently and yet be very seldom seen. Again, the jet prominences seen by Respighi, Secchi, Zöllner and others, though not appearing to extend to the height reached by the hydrogen wisps watched by Young, may (many of them) have reached to an even greater height, being reduced by simple foreshortening; and as these are phenomena frequently observed, we may not unsafely infer that eruptions really as important as the one witnessed by Professor Young are by no means uncommon.
But let us consider what the facts observed by Professor Young really imply. This is precisely one of those cases where an observation requires to be carefully discussed in order that its full value may be educed.
Now the main point of the observation is this—that glowing hydrogen was observed to travel from a height of less than 100,000 miles to a height of more than 200,000 miles in ten minutes. To be safe, let us take the limiting heights at 100,000
ke the linan 200,08ht of lestrowing
miles and 200,000 miles; and let us assume that there was no foreshortening. These assumptions both tend, of course, to reduce our estimate of the velocity with which matter was ejected from the sun.
Now we need not trouble ourselves by inquiring whether the
were themselves ejected, or whether their motion might not have been due to the ejection of other matter impinging upon these wisps and forcing them upwards. Some matter must have travelled at the observed rate—or (if the hydrogen was not itself ejected, then) at a greater rate.
The question which we have to deal with is therefore thisWhat must be the velocity of ejection in order that matter may pass between the observed heights in the observed time?
But it may seem that the problem might be simplified by inquiring what must be the velocity of ejection in order that a height of 200,000 miles should be reached. This, however, introduces the question whether that was really the limit of the hydrogen's upward motion. The wisps seemed to dissolve away at that elevation ; but we cannot assume quite safely that the hydrogen there ceased to move upwards. On the contrary, it seems more likely that it neither diffused itself (so as to be come invisible) nor ceased to ascend, at that level; but simply became invisible through loss of temperature, and therefore of brilliancy. It will be better, therefore, to take simply the flight between the observed levels; for then we shall be attending solely to observed facts. We may, however, inquire as a preliminary process what would be the velocity of ejection necessary to carry a projectile (moving as if in vacuo) from the sun's surface to a height of 200,000 miles.
The calculation is not difficult. The formula for our purpose may be thus expressed. Let R be the sun's radius, or 425,000 miles; H the extreme height reached by a projectile from the sun; V the velocity of projection. Then a mile being the unit of length and a second the unit of time
(379 miles per second is the velocity which would be required to carry a projectile away from the sun altogether); and we have only to put for H 200,000 (miles) and for R 425,000, to deduce the required velocity. We find thus that a projectile must have an initial velocity of about 213 miles per second to reach the height certainly attained by the hydrogen wisps watched by Professor Young.
Now the time in which a projectile with this initial velocity
reach the an initial velocity. miles) and together): required
reach the in initial velocitt%: We find thuer R 425,000, we
would traverse the upper half of its path is not so readily determined-in fact the formula is not altogether suited to these pages.* I must, therefore, ask those readers who do not care to make the calculations for themselves, to accept on trust my statement that 25m. 56s. would be the time required for the upper half of our projectile's course.
It is already obvious, therefore, that the matter watched by
having 200,000 miles as the limits of its upward course. It traversed a space in 10 minutes which such a projectile would only traverse in about 26 minutes.
Now two explanations are available. We may suppose that the real limit of the upward flight of the hydrogen was greater than 200,000 miles, and that, therefore, the 100,000 miles next below that level were traversed with a greater velocity than would correspond to the case we have just been considering ; or
* Following, however, the plan adopted in my treatise on 'The Sun,' I give the formula for all such cases in a note, so that those readers whose tastes are mathematical may make the calculation for themselves, if they wish to. It runs thus :
R being the sun's radius, D the extreme distance of a projectile from the sun's centre, X its distance at time t after starting from rest at distance D (from centre, be it remembered), then,
V D In the course of my examination of Prof. Young's observation, finding the application of this formula rather wearisome (especially as the formula had to be applied tentatively in dealing with the main problem, for it tells us nothing as to the extreme height, when this is to be determined frcm the observed time between certain levels), I was led to consider whether a geometrical construction might not be found which would at least afford a test of the calculative results. (For this, be it noticed, is the great value of geometrical constructions; they prevent any serious errors of calculation, by affording a tolerably close approximation to the truth; and in calculation -crede experto-great errors are most to be feared).
I presently lighted on the following construction, which may be applied with singular ease, rapidity, and accuracy to all problems such as the one we are upon. Let KQEC be a carefully constructed half cycloid, K being a cusp, E the vertex, and EC the axis. (The same cycloid is to be used for all problems, the remaining constructions being pencilled.) Divide CE in A so that CA represents the sun's radius, AE the flight of a projectile. About centre C draw half circle ADL, cutting half circle on EC as diameter in D. Draw DM square to AC, and let MmL, a half circle on ML, cut KC in m. Then the time of descent from E to any point P in EA is represented by the ordinate PQ (parallel to KC), where mC represents 18 m. 40 s., which is the time in which CA would be traversed, with a velocity of 379 miles per second,