which the reader is referred. The main object of the present article is to place before those American readers of this Journal, who may not have access to foreign Journals, such views and important geological facts as Profs. Marcou and Capellini obtained in their field investigations, as well as those of Prof. Heer from the study of the fossil plants. University of Pennsylvania, Philadelphia, Jan. 8th, 1867. ART. XIX.-Researches on Solar Physics;* by WARREN DE LA RUE, Esq., Pres. R.A.S., BALFOUR STEWART, Esq., Superintendent of the Kew Observatory, and BENJAMIN LOEWY, Esq., Observer and Computer to the Kew Observatory. First Series. On the Nature of Sun-spots. 1. THERE is a marked difference between our luminary and our satellite, as far as regards our knowledge of their physical aspect and constitution. Many parts of our own globe are not so well known, or so correctly mapped, as certain regions in the moon; and could we imagine an observer transported into the neighborhood of Tycho or Copernicus, he would probably be better prepared for the appearance presented to him, than he would be if placed suddenly in equatorial Africa or central Australia. But with regard to the sun the case is very different; for although the progress of science has enabled us to detect the presence of certain familiar substances in the atmosphere of our luminary, it has hitherto only shrouded in deeper mystery than ever the origin of that wonderful outpouring of light and heat which is the sun's most prominent characteristic, and to this very day it has not been finally decided whether this luminosity proceeds from the sun's solid body, or from an envelope which surrounds it. Indeed so strange and so unaccountable are many of the features presented to us, not only by our own sun, but by many of the stars, that it has even been conjectured that these bodies exhibit instances of the operation of some force of the nature of which we are yet ignorant. If we accept this view of the case, the study of our luminary becomes one of very great importance, but one in which we must be very careful to be guided by observation alone. We must obtain numerous and accurate representations of the sun's surface, and study these carefully and minutely, before we attempt to generalize. SI. Methods of observation. 2. There are two methods of accomplishing this. (1.) Eyeobservations of the sun's surface may be made by means of a * From a memoir printed for private distribution. telescope, and the appearance carefully mapped by the observer. (2.) Or we may call to our aid that art which has already proved of signal service in many branches of science, and, by means of photography, obtain autographs of our luminary, which we may measure and examine carefully at our leisure. Each of these has its advocates, but it is not our design to discuss the comparative merits of the two methods; on the contrary, as each has its own special advantages, we are willing to adopt them both, and to avail ourselves of all those materials. which our own observations or the kindness of friends may have put into our hands. § II. Historical sketch. 3. The most important knowledge which we possess regarding the physical appearance and structure of our luminary is derived from the following sources. 4. Sun's rotation.-We are, in the first place, indebted to Galileo, if not for the first discovery of sun-spots, at least for the first attempt to ascertain through their means the period of rotation of our luminary. 5. Nature of sun-spots.-The next great advance in solar physics is due to Alexander Wilson, Professor of Astronomy at Glasgow, who in 1773, communicated a paper to the Royal Society, describing certain phenomena with regard to spots, which, in his opinion and in that of many others, appear to indicate that spots are cavities in a luminous photosphere which surrounds the sun. The accuracy of this conclusion has recently been questioned; but whatever may be said regarding the theory, there can be no doubt regarding the importance of the fact which was first revealed by Wilson. 6. Their periodicity. The next step is due to Hofrath Schwabe, of Dessau, who has shown, as the result of nearly forty years' laborious observations, that the number of spots which break out on the sun's surface is not the same from year to year, but has a maximum about every ten years-a remark which led General Sabine to observe that the various epochs of maximum spot-frequency were also those of maximum magnetic disturbance in our own globe. 7. Their proper motion, &c.-Carrington is the next observer who has greatly extended our knowledge of this subject. In a large and most remarkable work recently published, and containing the result of many years' observation, he has shown that sun-spots have a proper motion of their own, those near the solar equator moving faster than those near the poles; and he has also made interesting remarks on the distribution of spots in solar latitude for different years. In addition to these new facts, he has furnished us with very accurate data regarding the sun's rotation. 8. Gradations in their luminosity.-We ought also to mention the discovery by Dawes, that what is regarded as the umbra of a spot consists in many cases of two well-defined and separate parts, the exterior part being less luminous than the interior; so that we have often connected with the same phenomenon not less than five degrees of luminosity: these are-(1.) The faculæ. (2.) The ordinary photosphere. (3) The penumbra. (4.) The borders of the umbra. (5.) The very dark central nucleus. Mr. Dawes's discoveries are mainly due to his employing, with an eye-piece of his own invention, the full aperture of the telescope; but it is necessary to recall the fact that Sir William Herschel, in earlier times, was fully aware of the importance of not contracting the aperture of the objective. Moreover, we must not forget that Sir W. Herschel contributed to solar physics a theory which still holds its ground. 9. Red flames.-But there is another phenomenon connected with our luminary, not less curious than solar spots. We allude to the red flames, or protuberances which are seen to surround the sun's disk on the occasion of a total eclipse. Airy and Arago were the first to conjecture that these belonged to the sun. In the total eclipse of 1851, the former of these observers, by combining his observations with those of O. Struve, showed it to be probable that these flames do not change during the moon's motion. Great credit is also due to this observer for organizing the Spanish expedition of 1860; and it was here that Mr. De la Rue, by means of the Kew heliograph, set the matter completely at rest. Mr. De la Rue, from the pictures which he obtained, was able to show that the flames only change apparently, not really, by the moon's motion over them, that is, by covering one portion and disclosing another, and do not otherwise undergo any alteration; so that when the clock of his instrument was adjusted to the sun's motion, that portion of the flames not covered by the moon stood still. He also showed that the angular motion of the red flames, with respect to the moon, corresponds to the theory of their fixation in the sun. These results were verified by Secchi, who also obtained photographs of the same phenomenon, which were compared in Rome by Mr. De la Rue and Father Secchi with Mr. De la Rue's photographs. The forms of the red prominences were found identical in both, so that no change occurs in their form during an interval much longer than the duration of totality observations in a solar eclipse. * In some cases, however, it is fair to assume that the appearance of lighter portions of the umbra may be caused by the floating across of portions of the brighter part of the sun's surface. AM. JOUR. SCI.-SECOND SERIES, VOL. XLIII, No. 128.--MARCH, 1867. - 10. Willow-leaves. We may be allowed to mention here that very lately Mr. James Nasmyth, during the course of his observations of the sun's surface, has come to the conclusion that, when the circumstances of observation are very favorable, the whole surface will be found to be composed of separate luminous bodies, of a great similarity of figure, interlacing one another; and he has given the name of willow-leaves to these appearances. The existence of these is still disputed; but some of our best observers in this country have seen them under very favorable atmospheric conditions, and they have been seen more frequently by Secchi and other Italian observers. 11. Other observations of the sun's surface.-Chacornac, the eminent French observer, has noticed a behavior of those portions of the sun's surface around a spot, which seems to imply the existence of a downward current. More recently Lockyer, in this country, has made a very important observation of a similar kind. A tongue of faculous matter projecting over a spot was observed to lose its brilliancy very rapidly, so as ultimately to seem less brilliant than any portion of the penumbra. At the same time it seemed to be "giving out," as it were, at its end, and a portion of the umbra between it and the penumbra appeared to be veiled with a stratus cloud evolved out of it. We ought likewise to mention the excellent and numerous observations of Pastorff, preserved in the library of the Royal Astronomical Society, also those of Captain Shea, both of which the Council of that Society have placed at our disposal. Professor Wolf, of Zurich, has collected data for establishing the periodicity of sun-spots before the commencement of Schwabe's observations. Also, the Rev. J. Howlett, in this country, has produced a large series of drawings of the sun's surface on a large scale, and of exquisite delicacy of delineation, which will no doubt prove of much value; and, finally, this field of research is one that has been occupied by many observers in all parts of the world, so that we may hope with some confidence for a speedy increase of our knowledge in this very important branch of physical astronomy. 12. Composition of solar atmosphere.-Before concluding this very brief historical sketch, we ought to allude to the discovery of Kirchhoff and Bunsen, who, by means of the spectroscope, have proved that many familiar substances, such as sodium, iron, magnesium, &c., exist in the atmosphere of our luminary in the state of vapor. § III. Materials at the author's disposal. 13. We now proceed to describe what materials we have at our disposal for the purpose of these investigations. In the first place, Mr. Carrington has very kindly put into our hands all his original drawings of sun-spots. These extend from November, 1853, to March, 1861; and in them the sun's disk is represented on the scale of one foot in diameter, while, for each spot, the apparent position on the disk, as well as the proportion in size to the whole surface, is accurately delineated. We hope, in our investigations (as far as spots are concerned), to make much use of these pictures by Carrington; they do not, however, afford us any information with regard to facula. More recently we have received into our hands the magnificent collection of drawings of the sun made by Hofrath Schwabe, of Dessau, during the course of about forty years,-this distinguished observer having generously placed these in the possession of the Royal Astronomical Society, for the use in the meantime of the Kew Observatory. Our materials are, moreover, derived from the pictures taken by the Kew heliograph. This instrument, with its various adjustments, has already been described by Mr. De la Rue in the Bakerian Lecture for 1862; and it is therefore unnecessary to give a further description of it here. A few pictures were taken by this instrument at the Kew Observatory in the years 1858 and 1859. In July, 1860, it was in Spain, doing service at the total eclipse. In 1861, a few pictures were taken at Kew; while, from February, 1862, to February, 1863, the instrument was in continuous operation at Mr. De la Rue's private observatory at Cranford; and from May, 1863 until the present date, it has been in continuous operation at Kew, under Mr. De la Rue's superintendence. It is right to mention that for the perfection of these pictures much credit is due to the late Mr. Welsh and to Mr. Beckley, under whose immediate supervision the pictures at Kew have been taken by a qualified assistant, Miss Beckley. § IV. Method of reduction. 14. These are the materials at our disposal; and it may here be desirable to state in a few words the principle by which we shall be guided in our reduction of these materials. In the progress of this branch of knowledge, observers have been led to recognize certain laws, which represent the average behavior of sun-spots; but to each of these laws there are individual exceptions. In this state of things it is probable that our knowledge of the subject will ultimately be advanced, not only by a study of those groups which behave in a normal manner, but also by a study of those which are exceptions in their behavior to the general rule; and on this account it has been thought desirable to publish the results in such a way that any one may be able as far as possible to study the appearance and behavior, in fact, the whole history of any one group. Setting aside, in the meantime, Schwabe's drawings for future consideration, we propose |