trace, we gave our whole attention to the appearance of the beads.

My previous opinion on this subject was very decided. I had carefully studied the annular eclipse at Edinburgh, with an excellent reflector carrying a power of about 100; had seen the well-defined angular projections of the moon cut the sun's edge, and followed their disappearance to the last without perceiving the slightest distortion of the solar limb, or of any of the spots which were successively eclipsed. At another time, with the same instrument, I observed an occultation of Saturn. The end of the ring, which then shewed almost as a line, was seen gradually to shorten, and, when the moon's edge came up to the disc of the planet, the sharp angular contour of a mountain was plainly seen and observed by several of our party, while the planet retained its form unchanged. For these reasons, while preparing the notice of the eclipse of 1847, which was almost central here, I drew the attention of our observers to the ruggedness of the lunar contour as sure to produce the appearance of a string of brilliants, about the time of each of the internal contacts. In all matters of difficult observation, one's pre-formed ideas go for a great deal; for this reason I have stated mine. On examining the edge of the sun's image made on the screen, there was found a very extensive trembling, subject to sudden fluctuations, as thin hazes and fleecy clouds passed before us the depth of the undulations varied from 5" to 8" or 10". This undulation prevented any accurate measurement of the altitude of the lunar ridges, although it left their existence sufficiently apparent; the cusps also were rendered indistinct. Had we not been acquainted with the excellent performance of the instrument, we might have complained of great spherical aberration, as the large and palpable undulations were accompanied by a minute undertremor which gave to the outline a permanent indistinctness analogous to that arising from a bad adjustment of the focus.

Towards 3h 15m Greenwich time, the cusps shot out and retracted considerably with a leech-like motion, and at 3h 33m

separate roundish beads began to appear; these became more frequent as the angle of intersection grew less.

On attending closely to that part of the moon's edge, which was about to pass off the sun's disc, whenever we found an indentation and followed it up to the cusp, the hollow was seen to separate from the rest of the illuminated surface, and to assume a roundish form, which fluctuated with the undulations. Carefully observed, it seemed that the indistinct roundness of the form was due to the retention, by the retina, of the rapidly succeeding impressions.

At the incoming cusp again, whenever a bead made its appearance and was followed until it united with the rest of the illuminated surface, that portion of the moon's limb which had come on with it was found to be deeply indented. The last of this class of phenomena which we noticed was so distinct, as of itself to be decisive. A large and well separated bead was seen coming on, and its distance from some irregularities on the moon's edge was estimated. After the bead had amalgamated, that part of the moon's edge which had the same distance from these irregularities was found to be marked by two prominences whose height above the general surface might be 3" or 4", and whose summits were at the distance of 8", with a deep hollow of about 6" between them.

On the whole, the impression on my mind was confirmatory of my previous opinion, that these detached portions of light result simply from the ruggedness of the moon's edge. had not previously had an opportunity of witnessing the appearance under such favourable circumstances. What I have now seen has convinced me that the distortions arising from the unequally heated state of the atmosphere, or from the imperfections of the telescope, have been taken for something else.

The decision of this question is not without important results in practical astronomy, for the instant of occultation of a star must be subject to an uncertainty depending on the configuration of that part of the moon's limb on which it falls; and this uncertainty must pervade all determinations

of the longitude by this means. During the eclipse there was seen a range of elevation measuring in length about 150", and of which the altitude was some 6" above the general outline. Now the moon's mean motion is roughly one second of are in two seconds of time, and the occultation of a star by this ridge would therefore have preceded by no less than 12′′, the computed occultation; and even by a longer time if the appulse were oblique. A very small change of latitude also will bring the observer opposite to a different part of one of these irregularities.

With a

This unevenness of outline also opens up a discussion as to what is to be considered as the moon's diameter. power of only 50 the bright edge is seen as an irregular line not merely indented and studded with prominences, but irregular in the large way, that is, deviating greatly from the circular form. Photographic portraits of the moon in various states of her libration would be of immense value in enabling astronomers to compute exactly an occultation. On our globe the ocean spreads over the greater part of the surface, and, by its unity, determines the general form; but on the moon there is nothing analogous. Even those spots which appear with a low power as extensive plains, are found, when more narrowly examined, to be undulated and even studded with small volcanic mountains. What then is to be regarded as the moon's diameter, and from what starting point are we to reckon the heights of the lunar hills?

To return to our eclipse; the rapid formation of heavy clouds warned us of the propriety of determining our local time, for which purpose the altitude of the sun's lower limb was taken with Adie's circle, face east and face west, and then the end of the eclipse was waited for; a few seconds before it happened the sun's disc was completely concealed by a cloud, and thus we had neither the beginning nor the end from which to compute our longitude.

On the evening of the 29th we determined the latitude from the observed altitudes of a Polaris and of Vega to be north 44° 36′ 17". The chronometer was also compared at the same time with the local siderial time, and thence, by help of observations made here before our departure and

after our return, the longitude was found to be 18m 04s east of my present residence.

As I had hitherto made no very careful observations to determine my longitude here, not anticipating any protracted stay, I resolved to postpone sending my report until an opportunity should have occurred of verifying it. For this purpose the occultation of 74 Aquarii was observed on the 9th September, and at the same time the right ascension of the meridian was taken by help of the altitudes of a Polaris, Arcturus, and Vega. This plan I was forced to by the want of a distant night-signal for my meridian. On making the computations strictly without any approximate operations there came out lat. 41° 03′ 06′′, lon. 1h 55m 51s, which thus gives for the longitude of the quarantine station at Sebastople 2h 13m 55s.

As we had not been fortunate enough to observe either of the contacts so as thence to deduce directly the longitude of Sebastople, we were thrown upon our other resources, viz., the angular distances of the cusps, and the breadth of the illuminated part. Seeing, however, that with the sextant we can hardly come within 15", especially when we use darkening glasses, and that this would give an uncertainty of 30s in longitude; and seeing also that towards the end of the eclipse the cusps approach each other very rapidly, it followed that our most favourable observation was the last made by Mr Jelesnoff. From it, and from the observations on the altitude of the sun's lower limb, the longitude was found to be 2h 14m 08, only 13s more than that found by the chrono

meter.

All of these results indicate a greater longitude for Constantinople than that usually given in the books, for which reason I have deferred the transmission of this report, awaiting the opportunity of another occultation. Having prepared the preliminary calculations for that of B.A.C. 6607, I have just been disappointed by the otherwise welcome appearance of rain-clouds. The next occultation visible here is that of Ceti on the 11th, for which, considering the chance of broken weather, it is scarcely worth while to wait.

In conclusion, I have to regret that an unaccountable delay

in the transmission of the "Suggestions" should have prevented me from making arrangements in time for the packet, and that, notwithstanding the energetic assistance which was accorded me by His Excellency Baron Titoff, and by the officers of the Perseus, we were unable to attain the principal object of our mission. There is, however, this consolation, that, as the Russian astronomers had made complete arrangements, no loss to science has accrued from our misadventure.

CONSTANTINOPLE, October 2, 1851.

On the Scratching of Rocks by the passage over them of Sharp Detrital Matter. By Lieut.-Colonel PORTLOCK, F.R.S., President of the Geological Society of Dublin.

As the old opinion regarding the effect of the passage of detritus over the surface of rocks in forming the scratchings, &c., so generally observed, is again brought forward by our friend the present President of the Geological Society of Dublin, we doubt not our readers will be interested by the following details, as given in Colonel Portlock's Address,* which has just reached us.

"In my communication on Bantry Bay I brought under your notice one of those examples of the scratching of rocks, by the passage over them of sharp detritic matter, the surface having been previously worn smooth and partially polished by a similar action. The example, which I described from my own personal observation, was taken from one of those bluff clay banks so common in Bantry Bay, the interior portion of which-meaning that part within Whiddy Island— appears to have been formed by denudation of a comparatively recent date, or subsequent to the formation of posttertiary deposits. The wear of this great mass of gravelly clay studded with boulders, or, in other words, of boulderclay, is still continuing, and must continue, so long as any

* Vide Journal of the Geological Society of Dublin, vol. v., Part 1st, 1851.