I cannot but regard myself as most fortunate in finding the first confirmation of my views (i.) coming from one of the most eminent astronomers and physicists of the day, (ii.) bearing upon one of the most definite and positive of my vaticinations, and (iii.) relating to one of the most interesting subjects in the whole range of recent astronomical research.

It will be in the remembrance of many readers of this magazine that, nearly four years ago, Dr. Huggins succeeded in showing that the bright star Sirius is travelling at an enormously rapid rate away from us. In other words, besides that rapid thwart motion which is shifting the place of this star upon the heavens, the star has a rapid motion of recession. In the paper called "Are there any Fixed Stars," in the "Popular Science Review" for October 1868, the nature of the means by which this discovery was effected was fully described and explained. It may be permitted to me to mention, also, that while Dr. Huggins's researches were still unannounced (or rather incomplete) I was so far fortunate as to indicate the possibility of employing the very method of research which Dr. Huggins was then engaged (unknown to me) in applying to Sirius. I propose here briefly to describe and explain the method, referring the reader who desires fuller information on these preliminary points to the paper of October 1868, mentioned above. I am the more desirous of doing this, because I find the principle of the method not readily grasped, and that I conceive the explanation I am about to offer may remove certain difficulties not uncommonly experienced.

Conceive that a person, standing on the edge of a steadilyflowing stream, throws corks into it at regular intervals-say one cork per second. These would float down the stream, remaining always separated by a constant distance. Thus, if the stream were flowing three feet per second, the corks would be a yard apart (supposing, for convenience of illustration, that each cork was thrown with exactly the same force and in exactly the same direction). Now, if a person a mile or so down the stream saw these corks thus floating past, he could infer that they had been thrown in at regular intervals; and, moreover, if he knew the rate of the stream, and that the corks were thrown in by a person standing at the river's edge, he would know that the interval between the throwing of successive corks was one second. But, vice versâ, if he knew the rate of the stream, and that the corks were thrown in at intervals of one second, he could infer that the person throwing

I am indebted for the illustration on which is based the explanation which follows, to my friend and college contemporary, Mr. Baily, greatnephew of the eminent astronomer, Francis Baily.

them was standing still. For let us consider what would happen, if the cork-thrower sauntered up-stream or downstream while throwing corks at intervals of one second. Suppose he moved up-stream at the rate of a foot per second; then, when he has thrown one cork, he moves a foot up-stream before he throws the next; and the first cork has floated three feet down stream; hence the second cork falls four feet behind the first. Thus the common distance between the corks is now four feet instead of three feet. Next suppose he saunters down-stream at the rate of a foot per second; then, when he has thrown one cork, he moves a foot down-stream before he throws the next; and the first cork has floated three feet down-stream; hence the second cork falls only two feet behind the first. Thus the common distance between the corks is now two feet instead of three feet. It is clear, then, that the person standing a mile or so down-stream, if he knows that the stream is flowing three feet per second, and that his friend upstream is throwing one cork in per second, can be quite sure that his friend is standing still if the corks come past with a common interval of three feet between them. Moreover, he can be equally sure that his friend is sauntering up-stream, if the corks come past with a common interval exceeding three feet; and that he is sauntering down-stream, if the common interval is less than three feet. And if, by some process of measuring, he can find out exactly how much greater or how much less than three feet the interval is, he can tell exactly how fast his friend is sauntering up-stream or down-stream. It would not matter how far down-stream the observer might be, so long as the stream's rate of flow remained unchanged; nor, indeed, would it matter, even though the stream flowed at a different rate past the observer than past the cork-thrower, so long as neither of these two rates were liable to alteration.

Now, we may compare the emission of light-waves by a luminous object to the throwing of corks in our illustrative case. The rate of flow for light-waves is indeed infinitely faster that that of any river, being no less than 185,000 miles per second. The successive light-waves are set in motion at infinitely shorter time-intervals, since for extreme red light there are no less than 458,000,000,000,000 undulations per second, and for extreme violet no less than 727,000,000,000,000; but these specific differences do not affect the exactness of the illustration. It is obvious that all that is necessary to make the parallel complete is that the flow of light-waves shall reach the observer at a constant rate (which is the actual case), and that he shall know, in the case of any particular and distinguishable kind of light, what is the rate at which the wave-action is successively excited, and be able to compare

with this known rate the rate at which they successively reach him. If they come in quicker succession than from a luminous body at rest, he will know that the source of light is approaching as certainly as our observer down-stream would know that his friend was sauntering towards him if the corks came two feet apart instead of three feet. If, on the contrary, the light-waves of a particular kind come in slower succession than from a body at rest, the observer will know that the source of light is receding, precisely as the river-side observer would know that his friend was travelling away from him if the corks came past him four feet apart instead of three.

Now, the stellar spectroscopist can distinguish among the light waves of varied length which reach him, those which have a particular normal length. He analyses star-light with his spectroscope, and gets from it a rainbow-tinted streak crossed by dark lines. These dark lines belong to definite parts of the spectrum; that is, to such and such parts of its red, or orange, or yellow, or green, or blue, or indigo, or violet portion. Thus they correspond to light having a particular wave-length. And many of these lines in stellar spectra are identifiable with the lines due to known elements. For instance, in the spectrum of Sirius there are four strong dark lines corresponding to the known bright lines of the spectrum of hydrogen. Thus the wave-length corresponding to any one of these dark lines is perfectly well known to the spectroscopist from what he has already learned by examining the bright lines of hydrogen. Now, if Sirius were receding very rapidly, the wave-length corresponding to one of these lines would be lengthened; it would correspond, in fact, to a part of the spectrum nearer the red end or the region of longer light waves, and thus the dark line would be shifted towards the red end of the spectrum; whereas, on the contrary, if Sirius were very rapidly approaching, the dark line would be shifted towards the violet end of the spectrum. All that would be necessary would be that the rate of approach or recession should bear an appreciable proportion to the rate at which light travels, or 185,000 miles per second. For, reverting to our cork-thrower, it is clear that if he travelled upstream or down-stream at a rate exceedingly minute compared with the stream's rate of flow, it would be impossible for the observer down stream to be aware of the cork-thrower's motion in either direction, unless, indeed, he had some very exact means of measuring the interval between the successive corks.

Now the spectrum of a star can be made longer or shorter according to the dispersive power employed. The longer it is, the fainter its light will be; but, so long as the dark lines can be seen, the longer the spectrum is, the greater is the shift due to stellar recession or approach; and, therefore, the more readily

may such recession or approach be detected. But, with the instrument used by Dr. Huggins four years ago, it was hopeless, save in the case of the brilliant Sirius (giving more than five times as much light as any other star visible in our northern heavens), to look for any displacement due to a lower rate of recession than some hundred miles per second (little more than the two-thousandth part of the velocity of light). What was to be done, then, was to provide a much more powerful telescope, so that the stellar-spectra would bear a considerably greater degree of dispersion. With admirable promptitude the Royal Society devoted a large sum of money to the construction of such an instrument, to be lent to Dr. Huggins for the prosecution of his researches into stellar motions of approach and recession. This telescope, with an aperture of fifteen inches, and a light-gathering power somewhat exceeding that usual with that aperture, was accordingly completed, and provided with the necessary spectroscopic appliances. Many months have not passed since all the arrangements were complete.

In the meantime, I had arrived at certain inferences respecting the proper motions of the stars, on which Dr. Huggins's researches by the new method seemed likely to throw an important light.

More than three years ago, I had expressed my conviction that whenever the recorded proper motions of the stars were subject to a careful examination, they would confirm the theory I had enunciated, that the stars are arranged in definite aggregations of various forms-star-groups, star-streams, star-reticulations, star-nodules, and so on.* Making leisure, in the summer of 1869, for entering upon such an examination I was led to several results, which not only confirmed the abovementioned theory but suggested relations which I had not hitherto thought of. Some of these results are discussed in the article called "Are there any Fixed Stars," already referred to; others are presented in an article called "Star Drift," in the "Student" for October 1870. The special results on which Dr. Huggins's recent discoveries throw light, were first publicly announced in a paper read before the Royal Society, on January 20, 1870.

I had constructed a chart in which the proper motions of about 1,200 stars were pictured. To each star a minute arrow was affixed, the length of the arrow indicating the rate at which the star is moving on the celestial vault, while the direction in

See "Notes on Star-Streams," in the "Intellectual Observer" for August 1867, "Notes on Nebula," in "The Student" for March 1868," and "A New Theory of the Universe," in "The Student" for February, March, and April 1869.

which the arrow pointed shows the direction of the star's apparent motion. This being done, it was possible to study the proper motions much more agreeably and satisfactorily than when they were simply presented in catalogues. And certain features, hitherto unrecognised, at once became apparent. Amongst these was the peculiarity which I have denominated "Star drift;" the fact, namely, that certain groups of stars are travelling in a common direction.* This was indicated, in certain cases, in too significant a manner to be regarded as due merely to chance distribution in these stellar motions; and I was able to select certain instances in which I asserted that the drift was unmistakable and real.

Amongst these instances was one of a very remarkable kind. The "seven stars" of Ursa Major-the Septentriones of the Ancients are known to all. For convenience of reference, let us suppose these seven divided as when the group is compared to a waggon and horses. Thus, there are four waggon-wheels and three horses. Now, if we take the waggon-wheels in sequence round their quadrilateral (beginning with one of the pair farthest from the horses), so as to finish with the one which lies nearest to the horses-these are named by astronomers in that order Alpha, Beta, Gamma, and Delta of the Great Bear. Thus, Alpha and Beta are the well-known pointers (Alpha nearest the pole), and Delta is the faintest star of the Septentrion set. The three horses are called in order Epsilon, Zeta, and Eta; Epsilon being nearest to Delta. Now when the proper motions of these seven stars had been mapped, I found that whereas Alpha and Eta are moving much as they would if the sun's motion were alone in question, the other five are all moving at one and the same rate (on the star-sphere, that is) in almost the exactly opposite direction. Moreover, a small star close by Zeta (the middle horse), a star known to the Arabian astronomers as the "Test," because to see this star was held a proof of good eyesight, is moving in the same direction and at the same rate as Zeta and the rest of this set. And besides this star (which has also been called Jack by the Middle Horse), Zeta has a telescopic companion which also accompanies him in his motion on the celestial sphere.

After a careful consideration of these circumstances, and an

I include this among "features hitherto unrecognised, though Michell had already noted the fact that the stars are arranged into systems. “We may conclude," he said, "that the stars are really collected together in clusters in some places, where they form a kind of systems; whilst in others there are few or none of them, to whatever cause this may be owing, whether to their mutual gravitation or to some other law or appointment of

the Creator."