2. Description of a Sliding Scale for Facilitating the Use of the Moist-bulb Hygrometer. By James Dalmahoy, Esq.

The instrument described in the paper is made of German silver, and is about a foot in length, and of an inch in breadth; along the middle of it there is a groove for a slider. On the right edge of the groove is engraved a scale of inches, and on the left the degrees of temperature from 0° to 85° Fahrenheit, each being placed exactly opposite that point of the scale of inches which measures the corresponding tension of vapour. engraved a scale of equal parts, each edge, and having the same zero, is a vernier, applicable to the scale of inches. The lines on these scales are ten times larger than those which the symbols in the dew-point formula represent, but their numerical designations are not changed.

On the left edge of the slider is of an inch; on the right

The instrument is to be used as follows:- -Find on the slider the number which expresses the difference between the indications of the dry and moist bulb thermometers, and bring it opposite the number on the left scale, denoting the temperature of the moist-bulb; then zero on the slider will indicate, on the left scale, the temperature of the dew-point; and on the right scale, the corresponding force of vapour.

The paper concludes by shewing that the ordinary hygrometric formula, which suggested the idea of the sliding scale, indicates also a geometrical construction for finding the temperature and tension of vapour at the dew-point, which, however, would not be practically applied with convenience.

3. Account of Experiments to Measure the Direct Force of the Waves of the Atlantic and German Oceans. Thomas Stevenson. Communicated by David Stevenson, By Esq.

The author has attempted to supply a great desideratum in the practice of marine engineering, by instituting a series of experiments to ascertain what force the waves exert against opposing barriers. For this purpose he suggests, in some peculiar situations, the use of columns of water or of air, by which the force of each wave can be ascertained; in the one case by the rise of the water-column, or in the other by a pressure-gauge, shewing the same result in atmospheres by compression. But in all the observations as yet made he has used an instrument which may be termed a 'self-registering

66

Marine Dynamometer." This dynamometer is contained in an iron cylinder, which is fixed to the rock where the experiments are to be made. The instrument consists of a plate or disc attached to a powerful spring, which is lengthened by the action of the waves. In graduating the instrument, the pressure required to lengthen the spring, a given quantity is ascertained by loading the disc with weights, so that when the quantity that the spring has yielded by the action of the sea is known, the pressure due to the area exposed is known also. The discs employed varied from 3 to 9 inches in diameter, and the resistance of the springs from about 10 lb. to about 50 lb. for every inch of elongation. With a view to check the results, three instruments, of very different powers of springs, were besides placed each other on an exposed rock, for a space of about six months, and the results were found to be remarkably concordant As the action of a wave may be supposed to combine the effects of a sudden impact with a subsequent continuous pressure, an objection might be urged against estimating these effects statically; and the author has accordingly made some remarks relative to this subject, which it is not, however, necessary here to state.

The results obtained are, up to this date, 260 in number, and these embrace a continuous register of the agitations of the Atlantic (as ascertained at the Skerryvore Rocks, Argyllshire,) for the last 22 months; together with a later train of similar observations, on the German Ocean, made at the Bell Rock Lighthouse. lowing is a digest of the results obtained :

Atlantic Ocean.

:

The fol

Average of results for 5 summer months, during the years 1843 and 1844, is 611 lb. per square foot.

Average of results for 6 winter months during the same years is 2086 lb. per square foot, or thrice as great as in the summer months.

Greatest result yet obtained at Skerryvore, being on the 20th December 1844, is 4335 lb. per square foot.

German Ocean.

Greatest result yet obtained at the Bell Rock, being on the 9th October 1844, is 3013 lb. per square foot.

The greatest effect of the sea, which has been observed, is, therefore, that of the Atlantic, which is equal to about 2 tons per square foot.

There are also a few observations (made in April and June 1842) upon the Irish Sea, on the coast of Kirkcudbright, but the weather

was unfavourable for such observations; the highest result was 840 lb. on a square foot. The communication concludes with an account of several instances of the effects of the waves in the elevation of spray, and in the transportation of heavy masses of rock. The greatest observed elevation of spray was at the Bell Rock Lighthouse, on the 20th November 1827, during a calm with a ground swell. On this occasion the spray was projected to the height of 106 feet, which shews the existence, on the large scale, of a pressure of about 3 tons. The pressure which projected this column of spray exceeds, therefore, the greatest result obtained by the Marine Dynamometer. The largest stone that is mentioned in the paper, as having been moved by the sea, is 42 tons weight. This stone, which is on the shores of one of the Hebrides, was seen to move under the influence of each wave.

4. A Verbal Communication in regard to Chevalier's Experiments on the Decomposition of certain Salts of Lead by Charcoal. By Dr Traill.

The following Donations were announced:

Journal of the Royal Asiatic Society of Bengal. Nos. 144, 145.— By the Society.

On the Nature of the Nervous Agency. By James Stark, M.D., F.R.S.E. By the Author.

Researches on the Brain, Spinal Cord, and Ganglia, with Remarks

on the Mode by which a continued flow of Nervous Agency is excited in, and transmitted from, these organs. By James Stark, M.D., F.R.S.E.-By the Author.

Philosophical Transactions of the Royal Society of London for 1844. Part ii.

Proceedings of the Royal Society of London. No. 59. By the Royal Society.

Magnetical and Meteorological Observations made at the Royal Observatory, Greenwich, in the year 1842, under the direction of George Biddell Airy, Esq., M.A., Astronomer-Royal. By the Royal Society.

Outlines of Chemistry for the use of Students. By William Gregory, M.D., Professor of Chemistry in the University of Edinburgh. By the Author.

Monday, 3d February 1845.

Sir T. M. BRISBANE, Bart., President, in the Chair.

The following Communications were read :—

1. On a Peculiar Modification of the Doubly Refracting Structure of Topaz. By Sir D. Brewster, K.H.

While examining, in polarised light, some of the crystals which he had discovered in Topaz, the author observed certain optical phenomena, depending on a peculiarity of structure. This peculiarity is manifested either in the depolarisation of light, when it gives rise to four quadrants of light, separated by the radii of a black rectangular cross similar to the central portion, or the tints of the first order in the uniaxal system of polarised rings, or in the unequal refraction of common light, which gives rise to the mirage of a luminous point, in the form of concentric circles surrounding the centre of force. In every case there was found a quadrangular cavity in the centre of the intersection of the cross, generally dark and opaque, but in one case having a luminous spot in the centre. These cavities are from the 3000 to the 400 of an inch in diameter.

These cavities are quite distinct from all those formerly described by the author; and from the phenomena above described, he concludes that the contents of each cavity have exerted an elastic force on the surrounding mineral while in a plastic state. In some cases fissures are seen proceeding from the central cavities, but these are supposed to have been produced after the mineral had become indurated, and had already been subjected, in the plastic state, to the pressure or force above indicated.

These cavities never accompany the cavities with two fluids, but occur in specimens containing numerous embedded crystals, differing little from Topaz in refracting power.

Since the mineral must have been plastic when it yielded to the pressure here noticed, it cannot have been formed by the aggregation of molecules having the primary form of the crystal.

These considerations, along with others connected with the crystals, which occur in the cavities of Topaz, have led the author to adopt the idea of a new and peculiar kind of crystallization, to which he will soon direct attention.

2. Extracts from Letters to the General Secretary, on the Analogy of the Structure of some Volcanic Rocks with that of Glaciers. By C. Darwin, Esq., F.R.S. Specimens were exhibited. With Observations on the same sub

ject, made by Professor Forbes.

"I take the liberty of addressing you, knowing how much you are interested on the subject of your discovery of the veined structure of glacier ice. I have a specimen (from Mr Stokes's collection) of Mexican obsidian, which, judging from your description, must resemble, to a considerable degree, the zoned ice. It is zoned with quite straight parallel lines, like an agate; and these zones, as far as I can see under the microscope, appear entirely due to the greater or lesser number of excessively minute, flattened air cavities. not avoid suspecting that in this case, and in many others, in which I canlava of the trachytic series (generally of very imperfect fluidity) are laminated, that the structure is due to the stretching of the mass or stream during its movement, as in the ice-streams of glaciers. * * * If the subject of the lamination of volcanic rocks should interest you, I would venture to ask you to refer to p. 65–72 of volume of Geological Observations on Volcanic Islands.' small

my ,* I there

* The laminated, volcanic rocks of Ascension, consist, as described by Mr Darwin, of excessively thin, quite parallel layers of minute crystals of quartz (determined by Professor Miller) and diopside; of atoms of an oxide of iron, and of an amorphous, black angitic mineral; and, lastly, of a more or less pure feldspathic stone, with perfect crystals of feldspar placed lengthways. The following is a portion of the passage referred to:

-“Several causes appear capable of producing zones of different tension in masses semiliquified by heat. In a fragment of devitrified glass I have observed layers of spherulites, which appeared, from the manner in which they were abruptly bent, to have been produced by the simple contraction of the mass in the vessel, in which it cooled. In certain dykes on Mount Etna, described by M. Elie de Beaumont, as bordered by alternating bands of scoriaceous and compact rock, one is led to suppose that the stretching movement of the surrounding strata, which originally produced the fissures, continued, whilst the injected rock remained fluid. Guided, however, by Professor Forbes's clear description of the zoned structure of glacier ice, far the most probable explanation of the laminated structure of these feldspathic rocks appears to be, that they have been stretched, whilst slowly flowing onwards in a pasty condition, in precisely the same manner, as Professor Forbes believes, that the ice of moving glaciers is stretched and fissured. In both cases, the zones may be compared to those in the finest agates; in both, they extend in the direction in which the mass has flowed, and those exposed on the surface are generally vertical. In the ice, the porous laminæ are rendered

VOL. II.

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