them has been given by Professor Hennessy in a Report, printed in the volume of the Reports of the Association for 1861*.

If these experiments should lead to an available means of communication between two ships in company at sea, or between a ship and the coast during foggy weather, an important purpose would be accomplished.

At first the attention of the Committee was directed to repeating M. Colladon's experiments, substituting for the bell he employed cylindrical bars of steel, from 6 to 8 feet in length, and from 1 inch to 1 inch in diameter; these were supported on, or suspended from, their nodal points, and struck with hammers of different weights at one of their ends, so as to excite them longitudinally. These experiments were made in the large water-trough of the Polytechnic Institution, and subsequently in the ornamental waters of the Regent's Park: the available distance in the former case was about seventy yards, in the latter about half a mile. Employing Colladon's ear-trumpet, the sounds were very distinctly heard, and even at the short distance in the Polytechnic, the sounds through the air were separated from those heard through the water by a distinct interval. The character of the sound was, however, very different in the two cases; that transmitted through the water being more abrupt, though in both cases they were mere blows or impulses, as the method of excitation was not intended to produce continuous musical sounds. Though the sounds were not of a character produced by any musical instrument, yet a pitch could be recognized in them in the same manner as a pitch can be perceived in blows made at different parts of a table. By selecting two bars of different lengths, the sounds produced by each might be combined in the different orders of succession, which constitute the telegraphic alphabet. We did not extend these experiments further on account of the expense which would be incurred by the purchase of a sufficient number of bars to enable us to ascertain the best dimensions for the effective production of the required sounds, and also in the expectation that we might, by the operation of some members of the Association, obtain the temporary loan of such materials.

Professor Hennessy, who resides on the sea-side, near Dublin, is willing to undertake such further experiments as would be required for testing the application of sound-signals in extensive spaces out at sea. On this point Dr. Gladstone has already made a few experiments at Eastbourne. He and his children had taken two boats when there was considerable movement on the surface, and the sounds were produced from one boat while they were listened for from the other. Musical sounds appeared to be immediately stopped, while a blow struck end ways on an iron bar could be heard at a great distance.

Sounds produced in the air did not seem to penetrate through the water; but the sound of breaking waves on the shingle of the shore was distinctly heard through the water. This noise heard through the water resembled a series of sharp ticks, and could be easily distinguished at a considerable distance. The detection of this kind of sound is manifestly interesting with reference to the guidance of vessels approaching a coast during the prevalence of a fog. Such noises, though extinguished in their passage through air during a fog, would still be transmitted through water, so as possibly to act at certain parts of the coast as a natural fog-signal.

The attention of the Committee has been specially directed to the production of musical sounds under water. The instruments which appeared to be most available for this purpose were Cagniard de la Tour's Syren, and pipes or whistles, in which the vibrations were caused by currents of water in masses

Thirty-first Report, p. 173.

of the same liquid. When limited volumes of water were employed, powerful sounds were obtained in both cases; but in large reservoirs we met with an unexpected difficulty, for we found that musical sounds, which could be heard through considerable distances in air, became totally extinguished at very short distances from the point of origin in water. Even when sounds were produced with considerable intensity in a confined vessel, as a pail or tub, when the vessel was plunged in a large reservoir the sound communicated to the air became remarkably deadened, and the intensity was more diminished as the instrument was placed at a greater distance beneath the surface of the water. The rapid extinction of musical sounds in water renders it almost hopeless to employ them for communicating signals in that medium. We must therefore, if this investigation is to be continued, revert to experiments similar to those of M. Colladon, and confine ourselves to the transmission of shocks or impulses communicated to bars or pieces of metal of various forms and dimensions.

Report of the Lunar Committee for Mapping the Surface of the Moon. By W. R. BIRT, at the request of the Committee, consisting of JAMES GLAISHER, F.R.S., Lord ROSSE, F.R.S., Sir JOHN HERSCHEL, Bart., F.R.S., Professor PHILLIPS, F.R.S., WARREN DE LA RUE, F.R.S., Rev. W. R. DAWES, F.R.S., Rev. T. W. WEBB, F.R.A.S., J. N. LOCKYER, F.R.A.S., H. S. ELLIS, F.R.A.S., Herr SCHMIDT, and W. R. BIRT, F.R.A.S.

In the Report which I had the honour to present to the Members of the British Association for the Advancement of Science at Birmingham, the steps taken by the Committee appointed at Bath for ensuring as full and accurate a Register of Lunar Objects as could be obtained, were described in detail with the Forms issued by the Committee for obtaining this object, to which were added a few notices of the more remarkable features of the Lunar Surface which had presented themselves in the course of observation.

It was in the first instance proposed to construct an outline Map of the Moon's Surface four times the area of that of Beer and Mädler, or 75 inches in diameter; every object entered in the Register to be inserted on the Map: the outline of a Map of this size was exhibited to the Section.

In the Resolution reappointing the Committee, the object expressly mentioned, is that "of making further progress in mapping the surface of the Moon ;" and while the Committee has not lost sight of the objects contemplated in its original appointment, it has, in consequence of some remarks of the President of the Association, Professor Phillips, when the Report was read, mainly directed its efforts to the construction of an accurate outline Map of 100 inches in diameter.

In noticing the progress made in this department of its labours, it may be well to glance at the materials at present available for the purpose. These are, well-determined positions of the First Order, and existing Photographs. In Appendix I. will be found the rectangular coordinates of all the positions of the first order determined up to the time of Beer and Mädler. I am not aware that any have been determined since, unless Herr Schmidt, of Athens, may have added to the number, but his measures are as yet unpublisned.

Of existing photographs, I am aware of only one that can be employed. in connexion with positions of the first order for obtaining an approximation to the true places of unmeasured points; but this, which was taken by Warren De La Rue, Esq., on October 4, 1865, at 9h 0m 4, G. M. T., represents the disk at an epoch so near that of mean libration, which occurs only once in three years, that the abscissa X may be measured on it without appreciable error, and the ordinates Y require but a small correction. I now proceed to notice the work done under these heads.

My first step was to project orthographically on one sheet of paper a quadrant of the moon's disk of 50 inches radius. The quadrant chosen as the most convenient was the fourth. After laying down the meridians and parallels, I inserted in this quadrant all the points of the first order, amounting only to twenty-three (see Appendix I.). It is greatly to be regretted that these points are so few, and that the triangles of which they form the points are so large, as in employing photographs taken at any other epoch than that of mean libration, or, indeed, by using any method, so far as I am aware, except determinations of the first order, libration enters so extensively, that even at short distances from these points the results of measures become very uncertain; and as the formula for computing the existing libration at any given epoch are only available for determining the selenographical longitude and latitude of the centre of the apparent disk at that epoch, and do not assist in the determination of the position of any other point except by the aid of direct measures and the computation of certain angles, it is the more important to augment the number of positions of the first order. With this view, I have prepared from Lehrmann's work, ‘Topographie der Sichtbaren Mondoberfläche,' compared with Beer and Mädler's Der Mond,' a modification of the forms adopted by those selenographers for this purpose according to the method of Encke, which necessarily includes the computation of the libration. (See Appendix II.)

The twenty-three points of the first order in Quadrant IV. were carefully laid down by direct measurement from the equator and first meridian, and checked in every case by measuring from point to point the sides of the triangles formed by them, and given by Beer and Mädler in Der Mond,' pp. 80-82. Taking the moon's semidiameter equal unity, the greatest error (a solitary instance) amounted to 0008, which is much less than errors arising from contraction &c. of the paper employed. In addition to points of the first order, several of the second order have been inserted.

The coincidence of the equator of the photograph of October 4, 1865, enlarged to 10 inches in diameter, with that of the moon was next ascertained. At mean libration the moon's equator is projected on the disk as a straight line, and if the photograph be taken at the exact instant of mean libration, the moon's equator will coincide with a straight line across the disk equally distant from each pole. The appearance of the moon when full differs, as is well known, from that which is presented at the various phases; many prominent objects quite disappear, and it is not so easy to pick out those that can be seen as when they are near the terminator. I was able, however, to ascertain, with some degree of precision, the following points:-South of the Equator-Messier, Theophilus, Dollond, Albategnius, Herschel, and Gassendi; North of the Equator-Picard, Dionysius, Linné Aristillus, Pico, and Kepler. Appendix IV. contains the measures of these points from the apparent equator on the photograph, and the comparison of them with the abscissæ of the same points given by Beer and Mädler. The mean difference for those south of the equator is 0019, and for those north of the equator

⚫0052, moon's semidiameter = 1.0. It is consequently assumed that measures for latitude south of the equator will not involve any great error in transferring them to the larger scale of 100 inches. The libration in latitude =—0° 20′ ; in longitude — 0° 40′ ±.

The measures of the above-named points, from the apparent central meridian for longitude, were, as might be expected from the greater amount of libration in longitude, not so accordant with the ordinates as those for latitude were with the abscissæ; the mean of the most accordant differences west of the meridian amounts to as much as 0195, while east=·007. A correction of 020 has consequently been applied to all measures west of the meridian. Under these circumstances, it is considered that as close an approximation as a combination of direct measurement with measures on photographs taken at or near the epoch of mean libration will afford, has been obtained for the basis of the Map. Still, for obvious reasons, it would be well to augment positions of the first order, especially as outlines laid down from a photograph taken at full moon differ materially from those furnished by a photograph taken at an earlier or later phase.

Among the forms issued by the Committee last year was one (Form No. 2) for aiding in the formation of a catalogue of lunar objects by symbolizing them (see Report, 1865, p. 288), by means of which each area of 5° of latitude and 5° of longitude is distinguished by a distinct symbol, IV Aa, IV AB, &c., for example. Every object discernible on the photograph of October 4, 1865, between 0° and 15° of longitude and 0° and 100 of latitude, has been carefully measured, and inserted on the projection of Quadrant IV. above-mentioned, the areas included being IV Aa, IV A3, IV AY, IV A, IV A”, and IVAo. The angular points bounding the portion of the surface thus measured are as follows:-The centre of the moon's visible disk in mean libration, and the region between Ptolemæus and Albategnius on the east, and Dollond and Theon Senior on the west. This region forms part of the mountainous district between the Sinus Medii and Mare Tranquillitatis, and is characterized by great diversity and irregularity of surface. As the direction in which the light is received from the moon when full is nearly the same-but reversed- -as that in which it falls upon the surface from the sun, it is clear we have on the photograph of the full moon the "ground markings" on the visible disk destitute of all hypsometrical affections, and the light and shade indicate the reflective power of the surface only. There are a variety of degrees of reflective power; but from an attentive consideration of the photograph, they may generally be regarded as five, from the bright white surface surrounding many craters to the dark surfaces of the Maria. A tracing of the markings thus laid down has been executed, in which a conventional mode has been introduced for distinguishing the variety of tints. This tracing is very useful for comparing the features of the full moon with those observed at earlier and later phases, and some interesting results have been obtained, to which allusion will be made presently.

If the features of the full moon only were laid down on a map, the student would be utterly unable to recognize any of the minute details which are scen near the terminator. In the absence of a photograph taken sufficiently near the epoch of mean libration for the coordinates of each object as seen near the terminator to be measured, that it may at once be transferred to its proper place on the Map (opportunities for obtaining such a photograph will not occur until 1868), the mode that presents itself for dealing with the more minute details is to measure on another photograph such details from the nearest point of the first order, having identified as nearly as possible the

corresponding prominent features in the two photographs. As before remarked, the photograph of October 4, 1865, is that of full moon; the one employed for the smaller and more striking detail is that taken by Warren De La Rue, Esq., on February 22, 1858, enlarged on glass to 8.75 inches in diameter. On neither of the six arcas above mentioned does a point of the first order occur, and the nearest to arcas IV A, IV A is the central mountain of Albategnius in IV A. From the very nature of the apparent changes effected in the visible disk by libration, it is clear that such measures as those just alluded to cannot possess much claim to accuracy when referred to the mean projection, except when made in the immediate neighbourhood of a point of the first order, or near a point that has been well identified; all other positions can only be considered as approximate; indeed, when the lines. measured approach the tenth part of the moon's semidiameter in length they are quite useless; still, with a proper amount of care the approximation capable of being attained may be sufficiently close for all the purposes of a map, especially if all well-determined positions be distinctly indicated. The reader may easily convince himself of the difficulty of combining portions of photographs taken at differing intervals from the epoch of mean libration by simply making enlarged tracings of such portions and superposing the one on the other; he will soon see they will not fit; added to this is the effect produced by variation of distance; two photographs taken at mean libration will not quite agree, the features of that taken at perigee will manifestly be larger than if it were taken at apogee.

Notwithstanding these difficulties, numerous objects have been inserted on Areas IV Aa, IV AB, IV A, and IV A" from the photograph of February 22, 1858, and other sources, and a drawing made of the area IVA. This has been enlarged to a scale of 400 inches moon's diameter*. Each object, as it is inserted in the map and drawing, is entered in Form No. 3. (See Report, 1865, p. 296.) Appendix III. contains a catalogue of these objects. The numbers in each of the above-named areas are as follows:-IVAa, 88, IV Aß, 21, IV A, 114, IV A, 25.

There are a few points of interest which attach to the features thus inserted. It is well known that Tycho is the centre of the most magnificent system of rays or lucid bands on the moon's surface, and that this system is seen to the greatest advantage at the time of full moon; accordingly, the photograph of October 4, 1865, furnishes the best means for depicting under that aspect, the rays emanating from Tycho. Three of these rays cross the areas above-mentioned; the two eastern rays cross the arcas IV Aa, IV A3, and the west ray crosses the areas IV A3, IV A". These rays will be referred to in Appendix III., which contains an abbreviated catalogue of the objects already mapped and inserted in the drawings; nevertheless it may be proper to mention here that all three are coincident with ranges of high land, as seen in the photograph of February 22, 1858, which in some places are much broken, and in others rise into rocky eminences. The middle of these rays passes along the east border of Albategnius, and the western along the west border. The west border of Ptolemæus forms part of the eastern ray.

Another feature bearing remotely on the above-named areas is the existence of two "ray-centres" in the neighbourhood of Furnerius. These raycentres are depicted by Hevelius in his Selenographia, figs. O and P, similar as Beer and Mädler remark to two pairs of crab-claws, the rays going north

It was exhibited at the Meeting.