into clouds; in attaining to a greater height; and lastly, in being seen all round the sun's limb, whereas the jets are limited to the neighbourhood of the spots.

Plumes are sometimes simple, sometimes compound. Some forms of simple plumes are presented in figs. 5 and 6 (Plate LXXXI.). Amongst other forms, Secchi notes plumes terminated by diffuse clouds, or crossed by two or three sets of clouds, or doubled down upon themselves, or attached to a cloud by a tail. (Most of these forms have been already described and illustrated.) Near the poles, "evidently on account of the absence of directing currents, they take an almost vertical form, with a diffused cloudy summit as in fig. 6." Compound jets form appearances not readily classified. Some are reticulated as in fig. 7, an arrangement due to the interlacing of distinct plumes. "Thus, in fig. 8, the masses of streamers diverging from three centres join and form arcades, through which appear black spaces. When these streamers are more numerous and extended, they appear as in fig. 9, where the crossing of the threads in all directions produces cornered interstices." But where the crossing streamers are diffused the interstices assume an oval or rounded appearance, as in fig. 10. "These masses attain the enormous heights of from 150 to 240 seconds. Their summits are generally very much broken up, and strongly resemble the masses of cirro-cumuli which we see at the borders of rain clouds. One fact with regard to them is very interesting; it is, that however distinctly marked and well defined the separation of the streamers may be at their base, after a certain height they become completely mingled with each other, and form a mass which appears to be quite uniform in structure."

Secchi gives some interesting particulars respecting solar clouds,—as he terms those masses which float above the chromatosphere, fig. 11, Plate LXXXI. "One class of clouds," he remarks, "is produced by the breaking up of plumes; others appear to be plumes which have ceased to be fed by the chromatosphere, and therefore become detached. The very curious phenomenon is sometimes presented of a cloud suddenly forming itself into plumes (figs. 12 and 13), showing that these plumes can take their origin from gaseous matter, and do not require an orifice of projection for their formation. M. Tacchini, of Palermo, has also made this observation, and we have both seen the jet directed downwards like a fiery rain.”

Secchi's remarks upon the physical distinction between plumes and jets, as well as upon the association between prominences and the phenomena of the sun's surface, are of extreme interest and importance. "In distinguishing between jets and plumes," he says, "I have no intention to decide as to whether

plumes are not also jets. The real distinction appears to be that in jets a part of the photosphere is lifted up, while in the case of plumes it is only the chromatosphere which is disturbed. It does not appear to be established as a fact that all prominences require an orifice of projection, and still less that the height of protuberances can be taken as a measure of the pressure which has projected the gaseous stream, since plumes have been seen to form themselves in the masses suspended in the free atmosphere, far above the possibility of a liquid origin. The persistence of plumes is very remarkable as compared with the continuance of the sheaves. In spite of the great mobility of the former, they may be found for two or three days in the same place; towards the poles their existence lasts still longer. On the other hand, the most beautiful sheaves generally last but a few minutes, in very rare cases a few hours. This confirms me in the opinion that sheaves are due to a veritable eruption, taking place at a great depth, the matter composing them having an exceedingly high temperature, and being propelled with immense velocity. The presence of jets and sheaves is the most certain sign that a spot is imminent. As to the connection between protuberances and the faculæ, it may be stated that jets, whatever may be their shape, are invariably accompanied by facula; but that plumes, more particularly if they are small, are often seen where there are no faculæ. A peculiarity worthy of notice is the feebleness of the light from prominences near the pole-an indication, as I have before stated, of less activity and a less powerful propelling force. The protuberances, both as to number and size, are in accordance with the solar activity as manifested by the spots; the fewer the spots the less numerous and the less extensive are the protuberances likewise. The dimensions of the protuberances are very variable. The largest that I have seen for some time have not exceeded four to four and a half minutes, from which it may be concluded that from the origin of their mass, their measure would be at least five to six minutes, being the height assigned by eclipse-observers to the highest part of the corona. The jets are in general not so high, seldom exceeding one to three minutes."

It will be evident that Secchi's observations bear in a most important manner on the question of an association between the prominences and the solar spots, though they do not make quite clear the nature of the connection. Further evidence is wanted before we can be sure that the eruption-prominences are directly connected with the outbreak of spots on the photosphere. Now Professor Young's researches have supplied (as it appears to me) just the evidence which was required. He has actually witnessed the eruption of matter from the sun, and he

has afforded us the means of measuring the energy of ejection and the velocity with which the ejected matter rushes through the lower strata of the solar atmosphere.

On September 7, at noon, he had been examining with the telespectroscope an enormous hydrogen cloud on the eastern limb of the sun. "It had remained," he says, "with very little change since the preceding noon," a long, low, quiet-looking cloud, not very dense or brilliant, nor in any way remarkable except for its size. It was made up mostly of filaments nearly horizontal, and floated above the chromatosphere with its lower surface at a height of some 15,000 miles, but was connected with it, as is usually the case, by three or four vertical columns brighter and more active than the rest. In length it measured 3′ 45′′, and in elevation about 2" to its upper surface" (that is, it was 100,000 miles long by 54,000 miles

high). At half past twelve Professor Young was called away for a few minutes. At that time, "there was no indication of what was about to happen, except that one of the connecting stems at the southern extremity of the cloud had grown considerably brighter, and was curiously bent to one side; and near the base of another at the northern end a little brilliant lump had developed itself, shaped much like a summer thunderhead."

On returning, though less than half an hour had passed, Professor Young found to his great surprise that "in the mean time the whole thing had literally been blown to shreds by some inconceivable uprush from beneath." "In place of the quiet cloud I had left," he says, "the air, if I may use the expression, was filled with flying débris-a mass of detached vertical fusiform fragments, each from 10" to 30" long by 2" or 3" wide, brighter and closer together where the pillars had formerly stood, and rapidly ascending. When I first looked some of them had already reached a height of nearly 4'(100,000 miles); and while I watched them they rose, with a motion