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unless special precaution were taken against it, as pointed out by Professor W. Thomson*. To avoid this source of error, the galvanometer circuit is broken between 3 and 4, figs. 7 and 8, at K,, and should only be closed after the battery circuit has been completed at K and equilibrium established throughout all the conductors.

Before passing to a detailed description of the apparatus as actually constructed, some remarks are required as to the means of making temporary connexions. All connexions which require to be altered may be the means of introducing errors, inasmuch as the points of contact are very apt to offer a sensible but uncertain resistance. In measuring small resistances, the resistance at the common binding-screws is found to create very considerable errors. Binding-screws have therefore to be avoided at all points where an uncertain resistance could cause error. Mercury-cups, made as follows, have been found in practice very suitable for temporary connexions, and have been adopted in the apparatus. The bottom of each cup is a stout copper plate, with its surface well amalgamated, forming one of the two terminals to be joined. A stout copper wire, 4 inch in diameter, with a flat end well amalgamated, forms the other terminal. When the amalgamation is good, and care is taken that the wire shall rest on the plate, this form of connexion offers no sensible resistance. The amalgamated wire is easily kept bright and clean by being dipped from time to time in a solution of chloride of mercury and wiped. The copper plate should also be removed from the cup, cleaned, and re-amalgamated occasionally. All permanent connexions should be soldered.

The apparatus itself, as actually constructed, will now be described (figs. 1 to 6). It consists of a wooden board, about 12 in. x 7 in., containing the mercury-cups, the adjusting wire, WX, the key, K, and the terminals to which the battery and galvanometer are connected. The letters in the figures 1 to 6 correspond exactly to those used in the diagrams 7 and 8; and the apparent complexity of the connexions can thus be easily disentangled. cc, aa, are two pairs of mercury-cups, into which the terminal wires on the bobbin, C, A, dip. This bobbin contains the two coils, C and A, forming the arms of the balance. rr, and ss, are mercury-cups, into which the terminals of the standard and coil to be adjusted are placed. These mercurycups are so connected with the four cups, d, d, f,f, that when d is connected with d, and ƒ with f,, by a couple of wires in a small square of wood, D, then A, C, S, and R are connected as in fig. 7; but when D is turned round, so as to connect d with f, and d, with f,, A, C, R, and S are connected as in fig. 8. D is called the commutator. The same end might be effected without a commutator by simply interchanging R and S; but it is frequently inconvenient to do this. All these connexions are made by short stout copper bars, dotted in fig. 2. The wire WX, the sliding brass piece H, carrying a spring for the contact at U (fig. 4), and the scale E, by which the position of H is observed, will be readily understood from the drawing. The sliding piece, H, is connected with the proper points by the helix of copper wire, h, and the screw, I. GG, and BB, are common binding-screws, to which the wires from the galvanometer and battery are attached. K is the key, by depressing which, first, the battery is thrown into circuit, and then the galvanoIt consists of three brass springs, 1, 2, 3 (fig. 5), each insulated one from the other, and connected by three screws, 1, 2, 3 (fig. 2), with the necessary points of the arrangement. A fourth terminal, 4 (figs. 2 and 6),

meter.

* Vide Phil. Mag. August 1862.

is immediately under the free end of the springs, and is armed with a small platinum knob or contact-piece. The three springs are also all armed with platinum contact-pieces, all in a line one above the other (fig. 6). When the finger-piece, T, is pressed down, 1 and 2 are first joined, and then 3 and 4; 3 is insulated from 2 by the vulcanite, Q. All the connexions permanently made, under the board, are shown in fig. 2. Those which have no sensible resistance are stout copper bars, and form the bottoms of the mercury-cups those of which the resistance is immaterial are made of wire, insulated by gutta percha, and are simply shown as dotted irregular lines in fig. 2; they will be found, on comparison, to correspond with the thin lines on fig. 7. It will also be found that all those parts shown by thick lines in the diagram are made by thick bars or rods and mercury-cups.

2

Three sets of arms, CA, C, A,, C2 A,, are provided; the shortest pair is first used, and U adjusted by the slide, H, till the galvanometer does not deflect when T is pressed down. The commutator, D, is then turned round, and U adjusted afresh. The coil, R, is then altered according to the two positions of U, and this process repeated, using the second and third pair of arms as required, until the desired approximation between R and S has been obtained. An astatic galvanometer, with a very long coil, will, for most purposes, give the best results; and one or two elements will be found a sufficient battery. The construction of R and S recommended, and the precautions to ensure perfect equality of temperature, will form part of next year's Report.

The apparatus, although specially designed for the production of equal coils, is applicable to ordinary measurements of resistances by comparison with a set of resistance-coils; for this purpose the terminals of the resistance-coils should be put in the place of the standard S, and any conductor of which the resistance is to be measured in the place of R. If a comparison by equality is to be made, the wire WX can be used as already described; it is, however, frequently desirable to make a comparison with one arm tenfold or a hundredfold greater than the other, by which means measurements of resistances can be made ten or a hundred times greater or smaller than could be done if equality alone between R and S were measured; for this purpose the three pairs, A C, A, C,, A, C,, are made exactly decimal multiples one of the other, and then, by taking A and C,, or A and C,, &c., in the cups aa, and cc,, the required decimal ratio is obtained. The resistance of the wire WX would, however, falsify this ratio, and it is eliminated by a simple copper rod, which is placed for the purpose between the two cups e e1, and maintains the whole wire WX at sensibly one potential. The commutator also is useless in measurements of this kind, and should be left untouched in the position shown in fig. 1.

The apparatus exhibited was manufactured for the Committee by Messrs. Elliott Brothers, of London, and gives excellent results.

Preliminary Report of the Committee for Investigating the Chemical and Mineralogical Composition of the Granites of Donegal, and the Minerals associated with them.

IN accordance with the resolution of the General Committee at the Manchester Meeting, the Committee, consisting of Sir R. Griffith, the Rev. Prof. Haughton, and Mr. Scott, proceeded to investigate "the chemical and mine

ralogical composition of the granites of Donegal, and the minerals associated with them." In furtherance of this object, Mr. Haughton and Mr. Scott repaired, last Easter, to the northern part of the county, as they had visited the S.W. portion of the district in the summer of 1861. They were accompanied on their tour by Mr. Jukes, Local Director of the Geological Survey of Ireland, who gave them the valuable benefit of his experience and assistance throughout the tour. The exploration commenced at Moville, on the E. shore of Innishowen, whence a section was carried along the N. coast of that peninsula nearly as far as Malin Head. This section exhibited a great thickness of primary rocks, consisting of quartzite and mica-slate, accompanied by several beds of limestone, and a number of beds of igneous rocks, which appeared to be contemporaneous with the sedimentary rocks. These are best exhibited at a place called the Mintiaghs or Bar of Inch, where there are several alternations of quartz-rock and syenite exhibited in an escarpment of several hundred feet in height. This locality is situated about five miles N. of Buncrana. From Buncrana, the granite of Urrismenagh, near Dunaff Head, was visited.

From Milford an excursion was made to the extremity of the promontory of Fanad, lying between Lough Swilly and Sheep Haven, in order to visit the granite of this district. This patch of granite is not a continuation of that which traverses the country in a N.E. and S.W. direction, as it lies to the N. of that axis and exhibits a slight difference in composition from the granite of the central axis. From Milford the route lay to Dunfanaghy; and a section was made across the northern end of the granitic axis of the county at Glen, in which its gneissose character was very strongly exhibited. This was marked in a most decisive manner between Lackagh Bridge and Creeshlagh, where the rock might be observed changing from gneiss, by almost insensible gradations, on the one hand into granite, and on the other into hornblende slate and crystalline syenite. The latter is most highly crystalline at Horn Head, where it contains large quantities of titanic iron. On the return-journey from Dunfanaghy to Letterkenny, it was determined to make two sections across the granite; so that Mr. Haughton and Mr. Scott took the road from Creeshlagh through the Gap of Barnesbeg, while Mr. Jukes took that by Owencarrow Bridge, about four miles higher up the valley.

It having now been found necessary to compare the facts observed with those which were to be observed in other countries, Sir R. Griffith repaired to Scotland in the month of July. Mr. Haughton traversed the centre of Scotland, and paid a visit to Sweden, Finland, and Russia. Both these gentlemen discovered facts strongly confirming the views propounded at the Manchester Meeting, of the similarity of the geological structure of Donegal to that of the Scandinavian peninsula and of Scotland. For this latter fact the Committee had been prepared by the examination of a series of specimens of Scotch granites which had been furnished to them by Sir R. I. Murchison, in accordance with his kind promise made at the last Meeting.

While these tours were in progress, Mr. Scott repaired, for the third time, to Donegal, and spent the month of July in the re-examination of several points connected with the geology of the southern district. He visited the granite of Barnesmore, near the town of Donegal, which is essentially nongneissose, and is penetrated by numerous pitchstone dykes, some of which are amygdaloidal. Numerous minerals were discovered here, which were in some cases new to the district. In the neighbourhood of Glenties, a considerable quantity of andalusite was found in the mica-slate—a mineral which is replaced near Barnesmore by kyanite, and in the Rosses, near Dungloe, by a white variety of kyanite.

From Dungloe, as head-quarters, the structure of Crohy Head was carefully examined, and also the island of Arranmore, which differs materially in its structure from the mainland of Ireland, from which it is only distant three miles. The southern portion of this island is nearly entirely composed of white granite, penetrated by numerous dykes of syenite and of felspathic porphyry. The strike of these rocks is nearly E. and W., while that of the flaggy quartz-rocks on the northern shore of the island approaches N. and S. During the course of this tour, two more sections were made across the granite of the main axis, exhibiting the same facts which had been observed before, viz. numerous beds of limestone and of altered slate lying in the granite, stratified nearly conformably with it. These were observed in the centre of Glenveagh, close to Ballaghgeeha Gap, on the pass through the Poisoned Glen from Dunlewy. At Glenleheen, where the same occurrence of non-granitic rocks had been observed in the previous year, four beds of limestone and several beds of slate were discovered. Almost all these beds of limestone contained garnet, idocrase, and epidote in quantity; and at Glenleheen itself, scapolite, a mineral whose occurrence in the British Islands has escaped the notice of modern English mineralogists, was discovered. Inasmuch as the specimens brought home by the members of the Committee from their several tours are very numerous, it is not possible for them to present their complete report at this Meeting. They hope to embody in it some valuable information relating to the granitic rocks of Canada, which Dr. T. Sterry Hunt has kindly offered to supply to them. They have to express their thanks to him and to Mr. Harte, C.E., county surveyor of the western district of the county, who, with the Rev. Frederick Corfield, has afforded them most efficient assistance. They have succeeded in procuring some of the granite of Rockall, through the kindness of the officers of H.M.S. Porcupine, who furnished it to Mr. Harte, and will include its analysis in their paper.

On the Vertical Movements of the Atmosphere considered in connexion with Storms and Changes of Weather. By HENRY HENNESSY, F.R.S., M.R.I.A., &c., Professor of Natural Philosophy in the Catholic University of Ireland.

THE labours of the Committee, consisting of Admiral FitzRoy, Mr. Glaisher, and myself, who were appointed, at Manchester, for the purpose of studying the vertical disturbances of the atmosphere with the aid of instruments, have, for the present, been restricted to the work of a single observer. This has arisen from the circumstance that the money-grant appropriated to the Committee has sufficed only to defray the cost of erecting a single instrument. As this instrument is likely to afford opportunities for observing the vertical motions of the atmosphere more completely than has been hitherto possible, it is to be hoped that similar apparatus will before long be in the hands of the other members of the Committee. The fact that all the preliminary work has thus necessarily devolved on the writer of the present Report will sufficiently account also for its provisional nature.

Hitherto the only kind of atmospherical currents which have formed the subjects of definite observation by instruments are those whose existence is manifested by the movements of ordinary wind-vanes and anemometers. But as these instruments indicate horizontal movements exclusively, ordinary

winds as well as storms are almost always conceived as currents flowing in perfect parallelism to the earth's surface. It is true that no physical theory of the motions of the atmosphere can be attempted without some considerations which involve the necessity of vertical and oblique motions among the masses of air, as well as horizontal motions; but while direct comparisons of the latter among themselves have continued for many years to be made in different parts of the world, we possess scarcely any such data relative to non-horizontal movements as would enable us to make them subjects of exact inquiry.

The only writer who, as far as I am aware, has hitherto endeavoured to deduce any well-defined results from observation relative to the vertical movements of the atmosphere is M. Fournet, and his studies were almost exclusively directed to the elucidation of the phenomena of some remarkable local winds that frequently prevail among the Alps and in the valley of the Rhone*. A local phenomenon in Ireland + induced me to study the vertical motions of the air in a more general way than was necessary for the explanation of this phenomenon itself; and my first step was an attempt at devising a vane capable of showing the existence and direction of non-horizontal currents. This was a non-registering instrument, and the results obtained were therefore somewhat unconnected; but they seemed to establish some important relations between vertical currents and other atmospherical disturbances. Among these, I may be permitted to notice the phenomena which preceded the disastrous gale of February 9, 1861. For many days, at the close of January and beginning of February, the weather was remarkably fine, and no vertical currents were observed; but on the 7th very distinct evidences of vertical disturbance came under my notice, while the air had as yet no remarkable horizontal motion. On the 8th, at 2 P.M., my attention was called to the vane by its shifting round through N. towards N.E., with decided and frequent downward plunges of the disk exposed to the vertical action of the air. It appeared as if showers of cold air were descending; for the thermometer showed at the same time a rapidly falling temperature. While vertical convection had become already highly developed, the horizontal motion of the air was not as yet greater than that of an ordinary brisk breeze.

Next day, during the storm, although the disk of the vane was in constant oscillation from the undulatory motion which my observations had already shown to be a necessary accompaniment of all high winds passing over terrestrial obstacles, no marked prevalence of upward or downward motions could be observed corresponding to the plunges of the disk noticed on the preceding day. The mercury in the barometer had been falling with great regularity during four days before that on which I had noticed the first decided indications of vertical disturbance. On that and the next day, as well as on the very day of the storm, the barometric column was rising, while the temperature was steadily falling. Here the rise in the barometer was accompanied by north-easterly winds, and the air at the earth's surface was thus rapidly mingled with cooler masses descending from above, as shown by the vane; so that the increased pressure was due to the increased density of the entire aërial column above the barometer.

*See Annales de Chimie et de Physique, tome lxxiv. p. 337; and a résumé of his results in a note to M. Martin's translation of Kaemtz's Meteorologie, p. 35.

+ Proceedings of the Royal Irish Academy, vol. iv. p. 279.

Atlantis, vol. iii. p. 166; Phil. Mag. for May 1860; and Proceedings R. I. A. for May 1861, p. 232.

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