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Earth, as seen by a distant spectator, would be in some degree greater than that which it would have if it had no atmosphere, because the rays which pass from the periphery of the globe* itself to the position of the supposed spectator must have a curved course concave to the straight line joining that position and the Earth's centre. The amount of this augmentation of the disc of the Earth, supposing the spectator to be placed at the Moon, has been calculated by Prof. Challis to be, almost exactly, the very small one of 1"-one second of arc or of 'angular space—that is, 13200 of the apparent diameter of the Earth as seen from the Moon.
It follows from this that a mountain five miles high-an elevation which is exceeded by only four or five peaks in the Himalayan range, the highest in the world—situated at the Earth's apparent border, would subtend at the Moon an angle of 41" only, or equal to 2933 of the apparent diameter of the earth as there seen, and its top would be somewhat less elevated by refraction, in proportion, than the base, on account of being higher up in the atmosphere. The atmospheric refraction, therefore, would only produce the effect of a slight apparent depression or diminution in altitude of the mountain; not augmenting the earth’s diameter to a perceptible amount, nor materially altering the forms of the inequalities on the periphery when supposed to be seen by a telescope from a distant station in the solar system. A
ray of light from a star or other object seen from the earth passes through the atmosphere in a course which grazes, or is a tangent to, the interior globe itself. The same of course is true of the reverse case, of a ray from a point on the earth seen from the moon or other locality in space outside the atmosphere. It is evident that in such a case the curvature of the path of the ray is less than the curvature of the surface of the globe. “ But it is conceivable," observes Professor Challis, -and in this conception of the effect of the refraction of the atmosphere of a heavenly body he is not aware that he has been anticipated by any previous investigator, " that there may be such a relation between the gradation of density of the atmospheric strata, and the curvature of the globe, that this condition cannot be fulfilled. For instance, it is reasonable to suppose that this is the case in the Sun's atmosphere, when the vast magnitude of the globe is considered," and the mathematical result is taken into account, which shows that the decrement of density of the atmosphere corresponding to a given increment of height above the globe varies conjointly as the density and the Sun's gravity.
“ It seems necessary to suppose,” continues Prof. Challis, “that an atmosphere has an upper boundary like that of an ocean, because, the density continually decreasing with the height, a point must at length be reached at which the upward repulsive force of an atmospheric stratum is just equal to the force of gravity, in which case there can be no downward repulsive force, and therefore no further extension of the atmosphere.” In this conception of the necessary existence of an upper boundary to an atmosphere, it may
* Throughout this explanation of the unbroken circularity of the edge of the sun's disc the term globe is used in reference to a planet, or other heavenly body, as distin. guished from the atmosphere surrounding it, or within which it is contained.
be remarked, Prof. Challis agrees with the late Dr. Wollaston, and also with Sir J. Herschel, who, in adopting the inferences of the former relative to the finite extent of the Earth's atmosphere, says: Arguments are not wanting to render it, if not absolutely certain, at least in the highest degree probable, that the surface of the aërial, like that of the aqueous ocean, has a real and definite limit, beyond which there is positively no air.” *
* This conclusion is opposed to the idea which was formerly prevalent that the earth's atmosphere had in fact no termination-an idea which is still retained by certain physicists. Even those who, by comparing the calculable density of the air at certain assumed heights with that to which it would be reduced in the receiver of an air-pump, have ascribed an extent of 40 or 50 miles to the atmosphere, and others who have implicitly accepted that inference have maintained the same belief. They have supposed that the atmosphere becomes indefinitely rare beyond a certain but unknown distance from the earth, ceasing to affect the rays of light, having neither refractive nor reflective power, graduating or evanescing into nothing it may be said-into that the existence of which it would not only be impossible to recognise by any means we possess were it on the surface of the earth, but is really not conceivable by the mind-à condition of matter (!) devoid of properties, but still supposed to extend through spaceand from this attenuated condition of virtual monentity to become gradually condensed again into the tangible and ponderable condition of the air as we know it,-around other bodies of the solar system or of the spaces beyond. Accepting, on the contrary, as proved, the doctrine that atmospheres generally have definite boundaries, at which, mathematically speaking, their densities, though small, have finite values, it will be as near a representation of the truth as the present condition of science enables us to arrive at, to say, that as the surface or upper boundary of the aqueous ocean of the earth is succeeded by the aërial ocean or atmosphere, so the upper boundary of the latter, “beyond which there is no air,” is succeeded by the luminiferous ether, which extends through and fills the interplanetary spaces, surrounding all the heavenly bodies of the solar system, with their respective aërial atmospheres; and indeed, as the visibility of the stars evinces, extending, with certain modifications, throughout all known space. But wbile the aërial atmosphere of the earth rests upon the aqueous nearly as it does upon the land, the ether does not rest upon, but pervades the former, together indeed with the waters and the solid earth, the ether thus resting upon itself alone; independently of the ponderable matter, whether solid, liquid, or aëriform, constituting the earth and other planets, and at least the outer regions of the sun also, which it permeates.
Though the ether, when in equilibrium, thus rests upon itself alone, if we identify it with the resisting medium in the planetary spaces, which Professor Encke, the Director of the Observatory of Berlin, bas sbown to retard the motion of the comet bearing his name, we must ascribe to it a sort of friction with the molecules of certain forms of ponderable matter, of great tenuity, of which comets consist.
The subject of the termination of the atmosphere above, necessarily involves that of the medium which succeeds it, occupying the space beyond, and hence the consideration of the ether is inseparable from that of the nature of atmospheres in general, as known or believed to encompass the earth and other bodies of the solar system. On this ac. count it is here noticed; but it has no further immediate bearing on the subject of this paper.
It has not yet been recorded in the history of science, though occasionally mentioned by the writer in lectures on the atmosphere, that Wollaston had been anticipated by Henry Thomas Colebrooke, F.R.S., sometime President of the Asiatic Society of Bengal, and an eminent contributor to its . Transactions,' called “the Asiatic Researches He was one of the ornaments of our Indian Empire, whom that empire itself in great measure contributed to form, and by whom as its reward it was maintained and extended. The remarkable paper which contains his theory of the atmosphere will be found in the Quarterly Journal of Science' (vol. ix., p. 57, 58) for April, 1820. Dr. Wollaston's paper was read before the Royal Society, January 17th, 1822, and published in the Philosophical Transactions' for that year. But the two pbilosopbers were led independently to the same important conclusion by distinct roads. Similar views had been vaguely entertained before ; but, until the modern limitation of the theory of the infinite divisibility of ponderable matter by the discovery that its elements would unite only in fixed and definite proportions, and the conception of an imponderable ether at once pervading and extending beyond the aërial atmosphere consisting of such elements had been expressed, the finite extent and termination of the latter, it may fairly be said, even if they could be definitely conceived of, could not be intelligibly described, nor securely maintained. In the use of the last phrase the objections urged by Dr. Whewell against the conclusions of Dr. Wollaston are not ignored; even he admits the height of the atmosphere to be finite, though he denies that this involves the consideration of atoms, supposed to be inseparable from the truth that bodies combine only in definite proportions.
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Recent Observations and Researches on the
No ray of light which comes through such an atmosphere to the eye of a distant spectator can be inclined to the surface of the interior globe at a less angle than that made with the surface of the globe by the course of a ray, which, after traversing the atmosphere and being refracted at the boundary, issues out in the direction of a tangent to the refracting surface. Now, a ray proceeding from a point on the surface of the globe, in a direction making an angle with the surface less than this, will not pass out of the atmosphere, but will either reach a maximum distance from the point whence it proceeds, and return by a like path to the surface again, or will be reflected internally from the upper atmospheric surface or boundary.
“From these considerations it follows,” says Professor Challis, “ (1.) that rays proceeding from points of the sun lying beyond the surface which contains all these limiting courses cannot reach the eye of a distant spectator; (2.) that rays proceeding from points within that surface appear to come from points within the sun's periphery ; (3.) that rays proceeding from any points on that surface eventually are tangential to the boundary of the atmosphere, and appear to come from the sun's periphery. Consequently"--and this is the important result—"consequently, if the boundary of the atmosphere be spherical, the periphery of the sun will be an exact circle, notwithstanding any actual inequalities of the surface of the globe. Also, if we may suppose that any objects like clouds are suspended in the solar atmosphere, all the points lying on the limiting courses will be brought by the refraction to the same level—that is, to the level of the upper boundary of the atmosphere. The augmentation of the sun's apparent semidiameter, as seen from a distant point, will be just equal to the apparent height of the atmosphere.” With this latter subject, however, we are not at present concerned.
• The foregoing theory," in the words of its author, “fully explains why the contour of the sun is generally observed to be entirely free from inequalities.” But in certain rare instances, as in that noticed above, the possibility of still seeing inequalities in the sun is evinced, showing that the solar atmosphere itself may be liable, under abnormal conditions, to disturbances and changes of level sufficient to account for their visibility.
This particular subject is so important and instructive with respect to the constitution of the sun that it becomes requisite to give in some detail the observations in which these seemingly exceptional phenomena were detected, two of which Prof. Challis has referred to.
The Rev. W. R. Dawes, a most accurate and accomplished astronomical observer, to whose solar researches we shall have repeatedly to return, employing instrumental means of the highest character, aided by a peculiar eye-piece already mentioned, thus describes the faculæ and an observation made by him of the appearance in question :—“The bright streaks or faculæ are best seen near the east and west edges of the sun's disc, where they give the impression of narrow ridges, whose sides are there presented to view. They usually lie nearly in the direction of a circle of latitude on the sun's surface, and are rarely high enough to be seen as actual projections from his limb. On one occasion, however, the 22nd of January last (1852], I
had an opportunity of observing a satisfactory confirmation of the idea, that they are ridges or heapings-up of the luminous matter ; and as the requisite circumstances are extremely rare, I will advert more particularly to the observation. A large bright streak, or facula, was observed to run, as usual, nearly parallel to the sun's edge for some distance, and very near it
, and then to turn rather abruptly towards the edge and pass over it. The limb was at times very well defined ; and when it was most sharp and steady, the bright streak was seen to project slightly beyond the smooth outline of the limb, in the manner of a mountain ridge nearly parallel to the sun's equator.”
Mr. Dawes, also, noticed on the 22nd of October, 1859, about noon, near the south-eastern edge of the sun, an unusually large mass of faculo. He satisfied himself that a bright streak, which
formed the very edge of the sun, projected irregularly beyond the circular contour of the edge ; reminding him of a ridge of low hills often seen at the enlightened limb of the moon. (Precisely the lunar appearance alluded to in p. 26, when considering the supposed necessary effect of the solar spots, if cavities, in causing inequalities of the sun's periphery.) Nearly parallel to this, and within a very small distance of it, ran another streak nearly as bright; and the space between them was observed to be darker than is the ordinary limb of the sun, but not so black as the umbra or nucleus of a spot. He supposed it to be the penumbra* of a large spot just entered on the disc—that is, brought into view by the sun's rotation, or by currents in the photosphere, as made known by the researches of Mr. Carrington, or some combination of both agencies—the umbra of which was as yet hidden by the inner bright streak, either superposed upon it or also interposed in the line of sight. At two, P.M., Mr. Dawes satisfactorily saw an excessively narrow black line, a little broken in two or three places, as if by irregularities in the inner bright streak, the top or outer edge of which was projected upon it, either by superposition or interposition, as already mentioned. On October 24th a fine spot was visible, with a remarkably extensive penumbra. On the 26th the umbrá had changed its form in a very extraordinary way. The successive changes to November 3 were more extraordinary than Mr. Dawes had ever before observed, and some of the forms which the umbra assumed were curiously grotesque. In the night of November 4th it passed off, and when it came round again, and up to November 21st, the date of the communication, beyond the richness of its vicinity in faculce, there was nothing remarkable in the group into which it had resolved itself.
“ It appeared to me very remarkable,” Mr. Dawes continues, "that in so violent and extensive a disturbance of the luminous envelope, I could not find, in any part of the umbra, a really black nucleus. In spots of not one-tenth part of the area I have, in numerous instances, perceived a decidedly black portion, not always central, but comparatively well-defined, and totally devoid of light, visible through the
* This distinction of umbra and penumbra arises from Mr. Dawes's prior telescopic researches on the spots, which we shall consider hereafter: it will be sufficient now to apprise the reader that he uses the former term to denote a region of a spot intermediate in 'darkness between the nucleus and that designated as usual by the term penumbra,
darkening glass rendered necessary by the solar illumination of our own atmosphere. The umbra of this large spot was, however, of various depths of shade in different parts. It appears probable that the upward force by which the luminous envelopes were thrown aside must, in this instance, have acted upon them very obliquely. This inference, of course, is drawn on the assumption that the black appearance of the nuclei of spots, as held by Wilson and Dr. (the late Sir William) Herschel, results from the partial exposure of the interior solar surface, beneath the mediums of the penumbræ and of the faculæ or photosphere. “To obtain ocular demonstration of the bright streaks being really elevated ridges or waves of the exterior luminous envelope is, of course, a very rare occurrence; but in the present case the evidence was as complete as could be desired,” part of it being afforded by the circumstance that a bright streak or facula was precisely at the sun's edge.*
A magnificent group of spots, which, as seen near the centre of the sun's disc, was in its principal dimension about 122,000 miles—and which, therefore, could it have been placed between the Earth and the Moon, would have extended half-way-was observed from July 25th, 1862, to August 4th, when it passed off the disc, by the Rev. F. Howlett. When first noticed this group had already advanced a considerable distance upon the sun's surface. It then included a remarkable mass of brilliant photospheric matter, which measured at least 12,000 miles in length by 6,000 miles in breadth, embracing, therefore, a superficial area of not less than seventy-two millions of square miles, which is greater than that of the Pacific and Indian Oceans taken together, and not much less than double the extent of all the dry land on the globe. This lay amidst the various associated nuclei of the principal spot, and was completely insulated from the contiguous photosphere by a well-defined penumbra. On the following day, July 26th, the muclei had run into each other, and be come considerably enlarged, at the expense apparently of the bright patch, which had become very much reduced in magnitude, as it was
* Among sonie observations on the solar spots by the (late?) Rev. J. B. Emmett, published in the 'Annals of Philosophy for the year 1825, which appear to have remained unnoticed by astronomers, is the following, in which also is described a similar phenomenon to that twice observed by Mr. Dawes : “In 1818 I traced a spot which was surrounded with a fine umbra to the very edge, when there was a fine line of light beyond the spot; both the nucleus and umbra were very distinct; about half the nucleus projected beyond the umbra towards the sun's centre.” Mr. Emmett here denotes by umbra what is at present understood by the penumbra of a spot.
How rare this phenomenon is, of the projection beyond the limb of any portion of a spot may additionally appear from the circumstance, that Mr. Emmett, who observed the spots for some years, and gave his attention to them especially as seen on the limb, should have noticed only one occurrence of it, as just described; while bis observation of that evinces that his instrumental means, though doubtless greatly inferior to those of the present day, were yet adequate to the task. He also noticed, apparently, one instance of another rare phenomenon, the extension of the penumbra of a spot to the limb (see p. 26).
It seems probable that if the remarkable outburst of intense white light appearing to exceed in brightness that of the sun's surface, and though over a very large group of spots, apparently unconnected with and high above it, wbich was simultaneously observed on September 1st, 1859, by Mr. Carrington and Mr. Hodgson at their respective observatories, had taken place on the limb, it would have been seen as a bright projection beyond it. But even this would have made only a fifth or sixth case in the annals of the telescope, and we know it to have been an extraordinary one from its effect, as yet unique, upon the instruments recorţing the variations of terrestrial magnetism,