CHAP. nences. on the appearance of an English hedge-row, the prominences looking like luxuriant elms. That definition stiil, to my mind holds perfectly good. The structure is filamentous. When a disturbance is not going on, instead of getting the eruptive form, in which the prominences are straight and dense, we get a nebulous appearance. Now Corona, the whole phenomena of the corona may be defined in cool promi- two words, cool prominences. I examined the corona with first-rate optical power, and under first-rate atmospheric conditions; I examined a large arc of it, and there was no part of the corona which I saw, which could not have been described as cool prominences; but in one part of the corona the line form, so to speak, predominated, and in another part the coronal masses got more and more to resemble those strange conglomerations which we see not only in some of the prominences, but also in some of the brighter nebulæ, so much so that although I was not very much excited, I still did exclaim, perhaps more loudly than Its nebular I need have done, that the corona looked like the nebula appearof Orion; and there was one especial part of the corona which reminded me most vividly, even amongst all the excitement of those two minutes, of that exquisite drawing made by the late Sir John Herschel of the nebula round Argûs. Passing then from one part of the corona to another, we still remain in the region of cool prominences, but in one part we get the nebulous prominences represented, and in the other we gradually put on the jetty appearance, so often chronicled by Respighi. ance. THE CHROMOSPHERE. You may ask, What is the relation of the chromosphere to this solar atmosphere which we are now discussing? My reply is that that is a question of definition. What we have to do now we know that the photosphere floats in the solar atmosphere, is to define the region above the photosphere. In the early days when the name was given, it looked as if hydrogen alone or almost alone extended above the photosphere, and the name was given to denote the bright line region in which a new world of phenomena was daily revealed to us by the new method. We now know that out of the reach of the new method there is a region of cooler hydrogen and something else—what that else is we do not yet know-but we have now to take it into consideration. I proposed, therefore, that the term photosphere should hold for all the solar material outside. the chromosphere, as its continuity was undoubted, leaving the word corona for the mixed phenomenon, which we see when we can see it-that is, during total eclipses. But both Janssen and Respighi have considered this nomenclature inadequate, and propose to restrict the term chromosphere to the solar surroundings visible by the new method, naming the exterior portion atmosphère coronale or chromosphère extericure. I willingly yield, with the remark that the last of these terms seems to me to be the better. Accepting this nomenclature, then, we have Exterior. CHAP. XXV. Revised nomen clature. What, then, is the interior chromosphere? It is un doubtedly a region which as a rule bounds the convection currents of the sun; it is a region where there is a sharp break in temperature: its hairy or cloudy outline probably depending somewhat upon the upper incandescent air. THE MOVEMENTS OF THE SOLAR ATMOSPHERE. I have now to pass as rapidly as may be to another increase of our knowledge-as great as that we have obtained from those two eclipse expeditions,-I refer to some magnificent work done in the clear sky of Italy by Professor Respighi, which has taught us more about the movements of the sun's atmosphere, by means of the new E E Interior chrome sphere. СНАР. XXV. Spot zones. method, and in a couple of short years, than we could probably have learnt without it in thousands of centuries. What do we know already about the sun's atmosphere? In what has already been done we are limited especially to the base of that atmosphere. We have been limited, so far as a complete and careful study of it is concerned, to the phenomena of the spots. We may say roughly that the salient phenomena of the spots are something like these: sometimes we have a great many spots in the sun, sometimes we have very few: the time from a minimum to a minimum or from a maximum to a maximum is somewhat over eleven years, but there are longer periods still. But this is not all that astronomers have been able to tell us about the distribution of the spots. The work of seven long years is recorded in Mr. Carrington's book on the sun, and his plates show very clearly a fact that was gathered by the first discoverers of the spots, namely, that they affect certain regions. Let us draw a diagram of the sun, and represent on it the solar equator and the parallels of thirty north and south latitude. As the time of minimum sun spots-when there are no spots or very few on the sun-passes by, what happens? We begin to see the spots putting in an appearance at about latitude thirty north, and thirty south. As the maximum is more nearly reached they gradually approach the equator; the zone widens, and the gradual increase in the range of the spots becomes very apparent: the spot zone, that is to say, gets wider as it gets nearer the equator. We have therefore two spot zones-one to the north, and the other to the south of the equator; the amount of spot frequency increases rapidly, and at the same time the zone in which the spots mostly appear varies considerably. Probably future investigation will show that these zones are not absolutely symmetrical with the sun's equator. Here, then, we have roughly the outcome of a great many years of work on the sun's spots-those sun-spots lying at the base of the solar atmosphere. Now it is clear to you that if we could get anything higher up in the solar atmosphere, (where the pressure must be much less, and where we know that the changes are much more rapid) to render evident to us exactly what is going on there; and if from these records we found that there is a method in all the apparent irregularity; it is obvious that we should have in this way a much greater chance of being able to get bodily into the secrets of these solar regions than we should in any other way. If in the region of the spots it is extraordinary for a spot to change very much from day to day, and if in the region of the prominences it is extraordinary if they do not change from hour to hour, it is evident that a very little labour will go a great way towards showing us first of all whether there is a law, or whether there is not a law, which regulates the apparent irregularity with which the prominences make their appearance; and if there be a law, letting us know what that law is. CHAP. XXV. work. Now, pre-eminent amongst the men who have worked at Respighi's this branch of research, Professor Respighi must be mentioned. I shall give you some idea of his wonderful assiduity when I tell you that since the discovery of this new method, he has already mapped more than 8,000 prominences, and when we come, as some of you do, to know exactly what that means; that every day, or sometimes two or three times a day, you have to bring the sun's image on to the slit-plate of your spectroscope, and then carefully go round the whole limb of the sun bit by bit, carefully recording the position and form of all the prominences, be they big or be they little; most carefully measuring their heights at the same time; it will be perfectly clear to you that a labour of this kind is one of enormous magnitude. I have here a diagram which will show you the way in which such observations are recorded. The prominences may be mapped on a circle, representing the profile of the sun, with the north, south, east, and west points and the CHAP. position of the poles of rotation; or they may be mapped along a line in each case beginning at the northern point of the sun, inserting the prominences at the proper degree, and then afterwards noting the exact position of the sun's equator and poles. In this graphic way the history of every prominence is written down from day to day, if the weather be fine. You have now an idea of the enormous work which has been done by Professor FIG. 140.-Diagram showing how the prominences are daily recorded. (Respighi.) |