Mon 23 18 6 Greatest Easterly elonga- 4th Oc. D. 11 18

tion of Mercury, 27° 4′

1st Oc. D.

8 59'5

14 37

THE PLANETS FOR JULY.

Mercury sets after the Sun during the month, and may therefore be observed in the evening. The planet reaches its greatest easterly elongation on the 24th.

Venus is a brilliant object in the evenings, setting about a quarter-past ten o'clock at the beginning of July, and about an hour earlier at the end of the month.

Decl.+ 19 81/1
13 44
Illuminated portion of the disc of Venus, o'793.

Mars rises about a quarter before one o'clock in the morning at the beginning of the month, and shortly before midnight at the end of July. Ist R.A. 2 45 50 Decl. + 14 57 Diameter 6"-6

Illuminated portion of the disc of Mars, 0.893.

Jupiter is in opposition to the Sun on the 20th of July, and consequently is as favourably situated for observation as his low altitude will admit of. Ist R.A. 20 7 49 Decl. 20 41 Diameter 44" 4

15th

4

44"-8

Saturn is visible in the south-west in the evenings, setting about one o'clock in the morning at the beginning, and about a quarter before eleven in the evening at the end of the month.

1st R.A. 14 16 54 Decl. II 5 Diameter 16" 2

14 16 43

II 8

15th 15"-6 Dimensions of ring:-Outer major axis, 39"; outer minor axis, 12".

THE MINOR PLANETS.

The following minor planets arrive at opposition this month:

NEW MINOR PLANET.-The 87th of the group was discovered on the 16th ult. by Mr. Poyson at Madras. Its position was, May 17, R.A. 16h. 14m. 32s.; Dec. 17° 29'. Mr. Poyson has named it Sylvia. Its magnitude is about 112.

THE VELOCITY OF LIGHT.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In your last number Mr. Reddie suggests several objections to the commonly received theory that light travels with a certain definite velocity. As it is confessedly a matter of the utmost importance in astronomical science, that we should be possessed of a correct theory on this subject, it may not be amiss to examine some of the questions he has raised. I would however, in the first place beg to remind Mr. Reddie that the theory to which he objects is not the mere product of the 'internal consciousness' of astronomers, but owes its origin to certain actually observed and well ascertained facts connected with eclipses of Jupiter's Satellites, for which no other explanation, as far as I am aware, has ever been offered. His first step therefore in any future letter on the subject should be to shew how the phenomena alluded to can consistently be explained otherwise than by the theory which he impugns.

The doctrine of the finite velocity of light which Mr. Reddie calls in question is stated in the following passage which he gives as a quotation.

"

"A star of the twelfth magnitude presents itself to our eye as it was 4,000 years ago, so that supposing 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. To this he objects, that if such be the case "it must be because a ray of its light continues to exist after the star itself has been annihilated; but if so," he adds, "there appears to be no reason why this ray should ever disappear.

How this conclusion can be drawn I cannot understand. Let me offer a simple illustration. If stones be thrown in quick succession into the same place in a large pond, the result will be a continuous series of undulatory rings on the surface travelling towards a spectator in any direction, whom we will suppose seated in a boat at a distance from the margin of the pond. This continuous series of undulations is strictly analogous to the intermitted light which travels to us from an existing star or luminous object at a distance. Now suppose the shower of stones suddenly to cease, what will be the result? From that instant no new rings will be formed, but the undulations previously set in motion will not therefore be arrested in their course, but will continue to travel on just as before. But when the last of them has reached and passed the spectator on its way towards the bank, the surface of the water in his immediate neighbourhood will resume its normal state of rest. And so in the supposed case, until the last undulation of the ray of light that started on its journey towards us 3,000 years ago has reached us, we can be conscious of no change or cessation at its origin. But when the last undulation has reached and passed us, a state of rest succeeds, and darkness is the instantaneous result.

But Mr. Reddie objects, 2ndly, "In this 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." Here again, on what grounds so strange a conclusion is to be drawn, I cannot conceive. Why the last undulation of a ray of light, that started from a star before the latter was suddenly annihilated, should appear to bring the star with it, more than any of the preceding ones, is left to conjecture. Can Mr. Reddie offer any analogy for such a supposition? The last sign of a telegraphic message comes no otherwise than the first, or any of the intermediate ones; it does not bring the battery with it!

The paragraphs numbered 3, 4, and 5, I must confess myself unable clearly to understand.

As for objection 6th, I would remark that the cases adduced are not analogous, unless indeed the stars were no more than a few thousand miles distant from us. When the stars set, they disappear instantaneously, because. their light is cut off by a screen, which we may consider quite close to our eyes, (as the distance of the visible horizon from us cannot exceed a few miles, a distance absolutely inappreciable when considered in connexion with the velocity with which light travels.) But if a star be annihilated, it is equivalent to its light being cut off by a screen close to its origin, in which case, as I have

VOL. IV.

pointed out, we must wait till the last of the undulations previously set in motion has reached us, before darkness supervenes.

As for 7; Mr. Reddie does not, I presume, mean to deny that new stars are continually being discovered which have not been previously noted: the companion of Antares affords a recent instance. But if the stars were created many millions of ages ago (and the inspired record tells us that the heavens were created in the beginning,) this objection falls to the ground altogether. With respect to paragraph 9; Mr. Reddie may be assured that the dark object which he supposes suddenly called into existence and placed in the firmament would be and remain utterly invisible to us, being projected upon a dark background; 'the subdued gray light' of which he speaks existing only in our own atmosphere, and consequently between us and the dark object.

According to his idea we ought to see the dark side of the moon clearly projected against the bright sky during the day any time between new and full moon, when conveniently placed; but the facts are opposed to this. Whenever we see the dark part of the moon at all, it is when it is not absolutely dark but partially illuminated by earthlight.

I fear to make my letter too long. But I will just add that the seeming contradiction in asserting that light, except under certain circumstances, is invisible, arises like many other mistakes from incorrect definition of the thing of which we speak. Light should be more properly defined as "that agent which makes things visible," but experience teaches us that it is not itself visible otherwise than when reflected or refracted directly to our eyes. Can any instance be quoted in support of the opposite doctrine? I remain, Sir, Yours faithfully,

Alton, Hants: May, 1866.

ALFRED W. DEEY.

REVIEW.

L'Espace Cèleste et la Nature Tropical: par Eмм. Lias.

Paris, 8vo. 1866.

This is one of those fully illustrated pictorial Books which are constantly issuing from the French Press. Our author appears to have entered the Paris observatory in 1854, at the same time as Monsieur Leverrier, and after four years passed in that institution he was, in 1858, detached from the observatory to take charge of a scientific expedition to Brazil. One of the subjects discussed in the work before us is the distance of the Sun from the Earth, and the results deduced from observations made in the month of July 1860 upon the Planet Mars, when in opposition, are well worthy of attention. "In July 1860 Mars was in opposition at his least distance from the Earth, and for seventeen years previously the planet had not been as favourably placed at this Epoch, the Southern Declination was 27°, and it passed near the Zenith at Rio Janeiro. For the determination of the Parallax, I measured at night to the East, and in the morning to the West, with the aid of my repeating Theodolite, the difference between Mars and a neighbouring star; this operation repeated four times morning and night, gave me, by a method well known to astronomers, the Parallax of the planet at these instants, and from that I deduce 8" 76 as the amount of Solar Parallax, instead of 8" 576, the value given by the Transit of Venus in 1769.’

In a foot note on page 35 he says, "It is very curious to see that formerly when the Parallax of 8" 576 was adopted, all the other determinations were found to be in accordance: now-a-days it is found that the determinations of the last two years give 8" 95 or 8"-96! The distance of the Earth from the Sun does not change like the fashions!"

The total Eclipse of the Sun, Sept. 7th, 1858, and that of the 18th July, 1860 are fully described, as well as the more recent annular one of the 25th April 1865, when the red protuberances were not seen.

The Chapter on the discovery of Neptune is very interesting in many ways; some of the merit is awarded to Eugene Bouvard, the nephew of the Calculator of the Tables of Uranus, whose conclusions from the observed irregularity of the orbit of Uranus, previously to the discovery of Neptune, he thus sums up, page 488: "Seek near the ecliptic about 328 degrees of heliocentric

Longitude, when that point of the heavens is in opposition, (in which case the geocentric Longitudes are equal to the heliocentric longitudes,) and in extending your researches five degrees in advance and five degrees behind this point, you will find a Planet, whose distance from the Sun is between 28 and 32 times the radius of the terrestrial orbit. " The calculations of Adams are prominently brought forward, as are also the observations of Challis made at the Cambridge Observatory.

Monsieur Lias is the astronomer who was able to negative the observation of the French Doctor, (Lescarbault) who imagined on the 26th March, 1859, he had seen a small planet on the Sun's disk: an extract from the author's notebook made upon that very day and hour in South America, shews that the "Régions de Soleil sont très uniformes d'intensité peu de pointille."

If our space would permit, we should like to go through many more subjects so carefully elaborated; the velocity of Light from Foucault's experiments; the spectra of the Stars; the Solar Spots; the Zodiacal light; the Comets of Donati and 1860, and the divided Comet of Biela; but we must forbear and recommend our friends to read for themselves what we think to be one of the most valuable books on Astronomy of the present day. The Meteorological and Terrestrial Chapters are full of information, and the numerous illustrations are beautifully engraved; and we must not forget to add that there is a well written introduction by M. Babinet of the French Institute.

INSTRUMENTS, &c. FOR SALE.

These Notices, which are restricted to two lines each, are inserted free of charge to subscribers: applications respecting them to be made to the Editor, with a stamped envelope for reply, without which no answer can be sent.-For Advertisements with more complete details a small charge will be made.-N. B.-It is particularly requested that we may be informed when instruments are disposed of, in order that the notice may be withdrawn, and fruitless applications prevented.

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