A DRIFTING STAB.

EW of the statements made by Professor Stokes in the address with which he opened the recent meeting of the British Association attracted more attention than the assertion that Sirius is receding from the earth at the rate of nearly thirty miles in every second of time. Amazing as the fact is, it was not the fact that attracted so much attention ; for even such a tremendous rate of motion is no uncommon attribute of the orbs which deck our skies. Astronomers have shown that our own sun sweeps ever onward through space with a velocity altogether inconceivable by us. Our own earth speeds around its central luminary with a velocity of more than eighteen miles per second. Even minute bodies like the meteors which flash in momentary splendour across the heavens, and then vanish for ever, have a velocity of motion almost equal to that with which the stately orb of Sirius courses onwards through space. It was not, then, the enormous velocity ascribed to the fiery Dog Star that attracted men's notice. The wonder was how astronomers could measure the motion with which the star is rushing from us. Knowing that, vast as is the velocity of its motion, countless ages must pass before the star could seem to be diminished in splendour through its recession, it seemed indeed an amazing thing that any process we can apply could tell us anything respecting a motion whose primary effect is absolutely inappreciable.

As the time is approaching when the same method which has revealed to us the recession of Sirius is to be applied with increased instrumental powers under the able management of our leading spectroscopist, it may interest many to learn something of the strange mode of inquiry which can deal successfully with a pro- blem of so much difficulty. Already the new telescope is approach- ing completion, and before long a series of questions which Mr. Huggins had found beyond the powers of his 8f-inch telescope will be solved by means of the increased powers now placed at his disposal.

The new mode of estimating the stellar motions is in reality sufficiently simple, though the principle on which it depends is the result of a long series of labours by the most eminent physicists of the age. We must conceive our earth as placed within a wave-tossed ocean extending on every side into infinite space. The waves which traverse this ocean are the minute light-waves, and heat- waves, and chemical waves which every celestial orb is ever generating. Transmitted through the ethereal ocean with a velocity altogether inconceivable by us, these waves not only generate a myriad forms of force and motion, but tell us all that we can learn about the habitudes of the celestial bodies. Our earth is a part of the shore of the infinite ethereal ocean, and the waves which roll upon that shore bring from beyond the ever- tossing ocean waifs and strays of knowledge, which our astro- nomers are busily piecing together—waif by waif, and stray by stray—until a noble structure is rising under their hands, built though it be of the mere sand and shells brought to us by the ethereal waves that lave our shores.

Like the waves of our own seas, those which traverse the ethereal ocean of space are of unequal dimensions. From the long heat-waves which Tyndall has termed the rollers of the great ether ocean, to the billows of the light-waves, and so to the tiny ripples of the actinic waves, we have every gradation of length. But there is this peculiarity in the waves that come to us from any particular star, that while the same gradations of length are observed, waves of a certain definite length are wanting. Still, comparing ethereal with sea waves, it is as though the waves which travelled to our sea-coast before some particular wind had nearly every length, from that of the roller to that of the ripple, but that waves exactly ten feet from crest to crest, and waves of exactly certain other lengths, were invariably found to be wanting.

Now, let us conceive of our earth as a ship in the vast ocean of space, and no longer as a fixed part of that ocean's shore-line. As a ship speeds over a wave-tosssd ocean, there is an obvious apparent change in the length of the waves she crosses. If she is meeting a long series of rollers, for instance, she crosses them more quickly (that is, more pass her in a given time) than if she were at anchor ; and if she is moving in the same direction as the rollers, fewer pass her in a given time, and if those on board of her were not aware of her motion, they would think the rollers narrower or wider than they are in reality in the respective cases mentioned.

Supposing, however, that such a crew had some exact method of measuring the apparent length of the rollers and billows which passed under them, and that they knew beforehand that waves exactly ten feet long were wanting in the sea they were traversing, then they would be able to tell whether their ship was moving or not, and in what direction. For instead of waves of ten feet in length being absent, waves of exactly nine feet in length might seem to be wanting ; and then they would know that these were in reality the ten-feet waves, only that their ship's motion had reduced them to nine feet. So they would know that they were travelling one-tenth as fast as the sea-waves and meeting them. And if waves of eleven feet in length seemed to be wanting, they would know that their ship was travelling one-tenth as fast as the sea-waves and in the same direction.

One more illustration, and we shall be ready to show how cer- tainly astronomers have become assured of the recession of Sirius.

Suppose the reason why waves of all, save certain definite lengths, came from a particular direction, was that a number of buoys lying far away in that direction were tossing, each with its own rhythmic motion, only that no buoys were tossing with the motion which would supply certain definite waves. Then it is perfectly clear that if the fleet of buoys were suddenly to begin to move away from or towards the shore, a change would take place in the length of every order of waves. A tossing buoy, for example, which was generating a twenty-feet wave, would gene- rate a longer wave when travelling quickly away. When it was. at its highest it would mark the crest of a wave, and when next at its highest that crest would be twenty feet away if the.

buoy had not travelled, but if the buoy had travelle& a foot in the interval the crest would be twenty-one.

feet away, and all the waves generated by the buoy would be twenty-one feet from crest to crest. This being- true ()mantis mutandis) for all the buoys, instead of ten-feet waves.

being wanting (say), there would now be no eleven-feet waves. On the other hand, if the fleet of buoys ware approaching the. shore at a similar rate, there would be no nine-feet waves.. Thus in every case a motion of approach is indicated by the- shortening of wave-lengths, a motion of recession by the reverse.

Now, the waves which Sirius sends across the ethereal ocean are- of all, except certain, lengths ; and our physicists have recognized the missing waves as corresponding to those which certain known gases have the power of absorbing. When we look at- the spectrum of Sirius, we see the waves of different orders separately, and we see the gaps distinctly marked. These- gaps ought to correspond to the places where waves of a. certain length should fall. But if Sirius is not at rest there will not be this exact correspondence. Now, fortunately, we can tell whether this is the case or not. We can cause the light from the- very vapour which is absorbing certain of the light-waves of Sirius to produce a bright-line spectrum side by side with the spectrum.

of Sirius ; and the fundamental principle of spectroscopic analysis teaches that the bright lines should correspond with the dark gaps.

in the star's spectrum. If not, it must be because the recession or- approach of the star is lengthening or shortening all its light. waves,. and so displacing the dark gap.

Now, when the spectrum of Sirius is thus compared with the- spectrum of hydrogen, it is found there is not that exact corre- spondence which was to have been looked for if the star were at rest. The dark absorption-line of hydrogen in the star's spectrum? is shifted in a direction indicating that the wave-lengths have been increased. In other words, it is found that the star must be- receding from us. The indication is one of extreme delicacy, however, and nothing but the enormous velocity with which the star is really travelling away from us would have sufficed to.

render the motion accessible with the instrumental means applied by Mr. Huggins. Now that he is to be placed in possession of improved optical appliances, we may hope for information respect- ing the motions of many other stars. The knowledge thus acquire& cannot but have an important bearing on the theories which we- are to form respecting the sidereal spaces. Hitherto we have been forced to be content with the measurement of those apparent motions which our telescopists have been able to detect. Igno- rant of the stars' distances, we could form but the vaguest notion.

of the true significance of these movements. Now, however, we- have a mode of measurement which tells us of the actual velocity

of stellar motions, and will thus enable us to form much clearer- conceptions than we have yet been able to obtain respecting the- grand processes of cosmical evolution which are in progress. around us.