Astronomical Register: A Medium of Communication for Amateur Observers and ...

COMETARY INTELLIGENCE.

BIELA'S COMET.-The following translation of some remarks by Dr. Weiss, of Vienna, on the failure in the attempts hitherto made to re-discover Biela's Comet (Ast. Nach. 1577), has been made by W. T. Lynn, B.A., F.R.A.S., of the Royal Observatory, Greenwich, expressly for the Astronomical Register:

"Since November last year I have looked in vain for Biela's Comet on every favourable evening. Yet I would not draw the conclusion, from the failure of the searches hitherto made, that its matter has been dissipated; for, independently of the consideration that the total dispersion of a comet, especially of such a one, without great development of tail, in the space of a few years, is scarcely conceivable, it appears to me that our present knowledge of other periodic comets by no means speaks for a rapid dispersion of those bodies. Professor d'Arvest mentions indeed (Ast. Nach. No. 1571), that his investigations prove a successive diminution of light in Faye's and Encke's Comets; but this must, to judge by a superficial comparison of the amounts of light stated in the different appearances, be, especially in the latter, of an extremely slow kind. To me, therefore, it appears more probable that the ill-success in re-finding Biela's Comet is due to the uncertainty in the assignments of its places, connected with the circumstance that it has hitherto, so far as I know, been sought for only with large refractors. If we reflect that it was not possible, from the observations of the years 1846 and 1852, to identify the separate nuclei with certainty, and that the predicted place in the year 1852, when the comet was at a great distance from the earth, differed by more than 6° from the actual, and that in the year 1859 the passage was not observed, we shall hardly expect any near agreement at the present appearance, when it is so near the earth. Now, if, besides the time of perihelion passage, the other elements of the orbit necessary for the calculation of an ephemeris also require considerable corrections, the comet will pass at a great distance from the zone indicated by the sweeping ephemerides, and consequently all the labour expended in looking for it with refractors must be fruitless, and only a survey of the sky with telescopes of large field of view can lead to a successful result. In the month of January I had intended to carry this out, but was prevented by the unfavourable weather. Although it has now vanished from the northern hemisphere, I think that all hope must not yet be given up of finding it at this appearance, even if it is not seen in the southern hemisphere, because it will again, in April-at least for the observatories of the south of Europe-come into a visible position with a brightness still considerable. For this reason I have requested Herr von Franzenau to calculate, with the elements of Santini, a continuation of the ephemeris of Michez, and will, as soon as it is completed, send it immediately by letter to Herr Director Schmidt at Athens, that it may come at once into his hands."

[According to the calculations, the brightness of the comet, even at the end of April, will be at least as great as it was at its second discovery by Secchi, in 1845.-W. T. L.]

NOTES AND GLEANINGS.

a ORIONIS, α HYDRÆ, a CASSIOPELE.-Concerning these stars, J. F. J. Schmidt writes--"Numerous observations have failed to produce any satisfactory proof of variability. Either there is none at all, or it is excessively feeble. a Hydræ I have had under my notice for 23 years, and never could detect a trace of variability."-Ast. Nach., 1570 (Feb. 3rd, 1866.)

THE VELOCITY OF LIGHT.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-As the current theory, as to the velocity with which light is supposed to travel, owes its origin to Astronomy, and affects so many astronomical conclusions, I beg leave to raise a few questions with respect to it, through the medium of your columns. On a former occasion, (about two years ago), when one of your correspondents raised some questions relating to light, he was assailed somewhat unpolitely, which probably was the cause of the discussion being prematurely extinguished. I trust, on the present occasion, that no unphilosophical heat will be generated; and at any rate, so far as Í am concerned I intend that nothing shall be said calculated to draw forth remarks not thoroughly gentlemanly and urbane.

Having adverted to this previous discussion, I may add, that then some of your correspondents were very positive as to the accuracy of the modern notion that light is invisible,-bearing upon which, I think the following remarks by the President-elect of the British Association will be read with interest by others. Speaking of Black's theory of "latent heat" -Mr Grove goes on :"I will mention the analogous doctrine of 'invisible light'; and I do this, meaning no disrespect to its distinguished author, any more than indiscussing the doctrine of latent heat, I can be supposed in the slightest degree to aim at detracting from the merits of the illustrious investigators of the facts which that doctrine seeks to explain. Is not invisible light' a contradiction in terms? Has not light ever been regarded as that agent which affects our visual organs? Invisible light, then, is darkness, and if it exist, then is darkness light." (Correlation of Physical Forces, 4th ed. pp. 47, 48.)

་

I confess I thoroughly agree with this; but now I will proceed with my remarks on the velocity of light. In a paper read by the first Herschel before the Royal Society in 1802, he says:

"When we see an object of the calculated distance at which one of these very remote nebulæ may still be perceived, the rays of light which convey its image to the eye must have been more than 1,910,000, that is, almost two millions of years on their way; and consequently, so many years ago, this object must have already had an existence in the sidereal heavens, in order to send out those rays of light by which we now perceive it."

Of course all this follows legitimately from the theory, if we take the velocity of light as 192,000 miles per second (or as reduced now to about 184,000) and if the nebulæ are at the supposed distance. The same notion has been amplified and repeated in various forms ever since. The light of a star of the first magnitude has been calculated to take from three to twelve years to reach the earth, stars of the second magnitude twenty years, and so on, up to stars of the twelfth magnitude, whose light would take 4,000 years to reach our globe, upon this hypothesis.

And it has been said, "a star of the twelfth magnitude presents itself to our eye as it was 4,000 years ago: so that, suppose such a star to have been annihilated 3,000 years back, it would still be visible on the earth's surface for 1,000 years to come; or suppose a star of the same magnitude had been created at the time the Israelites left Egypt, it will not be perceptible on the earth for nearly 700 years from this date.' Now, sir, I venture to reject all this, as incredible and philosophically absurd, for the following reasons:-I. If a star might appear as still in its place in the heavens for 1,000 years, or even a single year, after it had ceased to exist, it must be because a ray of its light continues to exist after the star itself has been annihilated; but, if so, there appears to be no reason why this ray should ever disappear; whereas 2. If it does, it must be because the ray, which originally reached from the star to the earth, becomes shortened at its most distant end; in which case the star could no longer appear as at its original distance, but would seem to approach the earth as if falling, till the ray became wholly extinguished at the surface of the earth; which is absurd, because 3. In that case, all the stars ought to appear as thus close to the surface of the earth, which their rays touch, if it be these supposed rays that enable us to see the stars, and if we do not see them at their distant places where they are;-or else we ought to see the lines of their rays; for 4. If these rays of light are capable of an independent existence after their cause has vanished, we must discard the notion that there

are precisely two rays from every star directed to the retina of our individual eyes, and which appear to us, (seen endwise) as points, or in short as the stars themselves, the so-called "rays" being merely rays of vision. For 5. If the rays of light are not merely rays of vision, they must not only proceed from the stars to each individual eye, but be diffused all round in space, like the light of day; but even then, we must remember, that the light of day, which is diffused in this manner, is quite distinct from the appearance of the sun itself, whose brilliant body the eye beholds distinct from the diffused light.

But, 6. When the sun sets, or before it rises, while the diffused light is seen as twilight, the body of the sun is only seen when above the horizon, or when its image is refracted so as to appear as if above the horizon, and the moment it sinks beneath this point, its appearance vanishes though its diffused light remains. And so also the stars rise and set every night, and do not continue visible after they are obscured by the earth coming between us and them; and still less would they continue visible after being "extinguished altogether" by ceasing to exist.

7. Were the theory true, and if the stars are set in space at enormously varying distances, then the most distant stars ought to have been continually, as it were popping into visibility in the sky, as their light for the first time reached the earth,-a phenomenon which has never been witnessed.

8. With reference to the notion that the light of a star of the twelfth magnitude (meaning its visible appearance, or vision, in the heavens) would not disappear for 1,000 years to come, had the star ceased to exist 3,000 years ago; this is really tantamount to saying that darkness and light travel with like velocity; which is no doubt perfectly true in meaning, though the language is objectionable. For this is merely another way of saying that the blank space ("or darkness"), caused by the star being blotted out, would reach our eyes,-or would "travel"-with the same velocity as the light from the star was supposed to do.

I might go on with many other objections to the theory; in which as already hinted-it would appear the velocity of light as a diffused substance, and the velocity of vision, have been confounded. But,

9. Suppose a large dark globe and brilliant meteor of equal size, were together to start into being, side by side, at an immense distance from the earth, say in the twilight when a dark and bright object would equally contrast with the subdued grey light of the atmosphere,-I venture to say they would be seen simultaneously, by those whose eyes were directed to the place of their appearance, and both "in the twinkling of an eye,"-and, I am inclined to add, irrespective of their distance.

10. Not to make this letter too long, I shall conclude by saying that the theory of "the velocity of light," or of light "travelling" at a certain rate, seems to be a natural off-shoot of the corpuscular theory of Newton, now abandoned. It scarcely has a meaning upon the "undulatory hypothesis," as I shall hereafter be prepared to show, and generally I venture to object, that the quickness with which we see a distant object (whether dark or light) ought not to be confounded with the "velocity of light," or the "travelling" of any material or ethereal particles, or of "images" of the thing seen, from it to us. Yours faithfully, Hammersmith: March 9. 1866. J. REDDIE.

On the possibility and advantages of establishing an Observatory at the North Pole.

An interesting paper was lately read before the Royal Geographical Society by Mr. W. E. Hickson, on the Climate of the North Pole and Circumpolar Exploration. The author points out that the climate at the Pole is most probably warmer than in some other parts of the Arctic Circle, as at no time of the year is the Pole either the nearest or farthest point from the sun, and that in consequence of its position the mean temperature at the Pole must be more equable than that of England or any other country, and therefore not nearly so trying to the human constitution as where great and sudden changes

prevail. Greater cold at the Pole than elsewhere may only be expected if that point is occupied by a lofty range of mountains, the elevation of which would render it a region of glaciers; but the fact is, that on all sides on which the North Pole has been approached, there have been found seas. The great impediment to arctic navigation is not the ice at the pole, but that drifting from it, and blocking up all approaches; and the problem is, how to avoid being caught in this drift. Now by way of Spitzbergen a vessel may sail before the middle of August as high as 82° N., almost without touching a piece of ice, and should the presence of ice then baffle further progress, the homeward way could be pursued without danger. There would be no need of wintering in this case, for Spitzbergen may be reached in a fortnight from London by steam, and with an open sea, a voyage to the Pole and back would be an affair of only six weeks, for a Cunard steamer.*

An Admiralty vessel would be better employed in making this experiment, than in merely cruising about, as at present, to prevent the machinery rusting. August would be the month for starting, and early in October the return to the Thames would be accomplished, possibly with the news toat there exists no practical obstacle to the erection of an observatory under Polaris, upon which depends the solution of many scientific problems of the highest interest, and the final correction of all our latitudes and longitudes.

As to the scientific objects to be attained by reaching the Pole, the first is that of acquiring a knowledge of the exact figure of the earth. The English mile and the geographical mile do not agree; the number of knots, or nautical miles in a degree being different at different parts of the earth's surface, which adds great risk to the ordinary dangers of navigation. Astronomers and geodetic surveyors are at issue as to the rules to be laid down for the guidance of mariners on this subject, but the data most required for determining the true figure of the earth are measured lengths of degrees from and round one or other of the Poles and along the Equator.

Another question of equal importance belongs to the compass; that of the exact direction of the four cardinal points: but where is the true north? We cannot obtain its bearings and distance from any known headland, within a limit of error of four or ten miles, for the exact latitude and longitude of most places visited in the Arctic regions is doubtful. There is a difference of twenty eight miles in the position of Cape Constitution, as given by American and English hydrographers. The almost horizontal path of the heavenly bodies in high latitudes interferes with the simpler astronomical observations. At the Pole, the midsummer sun has no culmination; noon may be mistaken for midnight, and time can only be determined approximately by Chronometers. These and many other considerations make it highly desirable that, if the Pole be found accessible and its climate not too unfavourable, arrangements should be made for establishing a permanent station there, so that the work of polar identification may be performed in such a manner, that no future question may arise about the place of the Pole of our day. Different nations might combine to raise there a monument as enduring as the pyramid of Gizeh, from which could be read in the changed aspects of the heavens the history of the earth's past and future: for it is principally from data to be obtained at the Pole that we have to expect an answer to the question geologists have long asked of astronomers, "What is the cause of those changes of climates we find recorded in the earth's crust?" There must once have been a time, when animal and vegitable life abounded in the arctic regions, and recent indications point to the return of such a period; the change depending on the cause of the displacement of the plane of the Ecliptic. Two thousand years ago the obliquity was 23° 51' 52"; it is now 23° 27 26"; at which rate the plane of the Ecliptic and that of the Equator would coincide in about 100,000 years. The change of seasons. depends on the obliquity of the Ecliptic to the Equator, and geological evidence supports the conclusion, that the angle between the two planes must have been at one time, much wider than at present. With an Ecliptic at * Coal strata have been discovered in the Parry Islands; the work of excavating coal in underground galleries could be as easily carried on in the Arctic Regions as at Newcastle. Coaling stations at intermediate points may be used as places of refuge, or for provisions, and with regard to labour, why not send our convicts north, where their labour could be utilised, as our colonies refuse to receive them?

right angles, the sun would be in the zenith at the North Pole at midsummer, and remaining there during the whole twenty four hours, would produce an intensity of heat far greater than that now experienced in the torrid zone. In winter the sun would continue 90° below the horizon: storms and convulsions of great violence would be the necessary result of these changes.

The diminution of obliquity gradually leads to an opposite state of things: universal calm, and freedom from physical disturbances; perpetual summer from the equator to the British Isles, and further north, perpetual spring. Winter would be unknown, and the only winds would be trade winds, blowing constantly from the same quarter.

But not to dwell on such eventualities, the point is that we want more information than we at present possess of the causes that govern the present inclination of the earth's orbit and axis; and to obtain it, we must establish astronomical and geodetic stations at such extreme points of the earth's circumference as would settle the question of the earth's figure, and of its exact angular position in all apparent perturbations of the heavenly bodies.

THE PLANETS FOR MAY.

Mercury will be a morning star during the month of May, rising about half an hour before the Sun: the planet is at its greatest westerly elongation on the 12th.

Venus will be visible in the evenings, setting about nine o'clock at the beginning of the month, and about an hour later at the end: on the 30th, she will be one degree north of Uranus.