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crucible to about 100° C., and gently stirred until the zinc is completely dissolved in the mercury. The amalgam is liquid while warm, and must be poured into the cell before it becomes solid on cooling.

The vessel containing the element consists of two vertical tubes. These, as shown in the figure, are closed below and open above into a common neck, which can be closed by a ground stopper of glass. The two tubes should be 2 cm. in diameter and 3 cm. in length. The neck should be at least 15 cm. in diameter and 2 cm. long. A short length of platinum wire is sealed through the bottom of each tube.

The end of the wire in one tube is covered by a small quantity of pure mercury, that in the other tube by the zinc-mercury amalgam.

Above the mercury a layer about 1 cm. thick of the mercurous sulphate paste is placed; above this, and also above the amalgam, a layer, also about 1 cm. in thickness, of zinc-sulphate crystals, and the vessel is filled up with the saturated zinc sulphate solution.

The zinc-sulphate crystals are obtained by evaporating at a temperature of less than 30° C. some of the zinc-sulphate solution prepared as in 4 of the specification.

The stopper is then inserted, leaving a small air bubble above the liquid, and sealed on the outside with shellac dissolved in alcohol.

The ends of the platinum wires outside the cell form the two poles, and should be connected to suitable terminals.

The Application of Photography to the Elucidation of Meteorological Phenomena. Fourth Report of the Committee, consisting of Mr. G. J. SYMONS (Chairman), Professor R. MELDOLA, Mr. J. HOPKINSON, and Mr. A. W. CLAYDEN (Secretary). (Drawn up by the Secretary.)

IN presenting their report on the work of the last year your Committee have but little to say on the subject of the representation of clouds and lightning by photography. They consider that their collection is nearly complete so far as the different varieties of cloud form are concerned, and it is only likely to be increased slowly and at long intervals by photographs of scarce forms of clouds or by particularly interesting series. During the year the Secretary has secured many new negatives; but since the collection already includes satisfactory examples of the same types, it has not been thought desirable to add more duplicates, and the offers of co-operation from other photographers have not been fulfilled. With regard to photographs of lightning also the collection has not been increased, for your Committee have not been made aware of any such photographs which show any features not already familiar, and no opportunity has occurred for the Secretary to make any observations for the further elucidation of the known phenomena.

Your Committee propose to invite the Royal Meteorological Society to take charge of such photographs from their collection as are not likely to be required for further investigation.

The attention of the Committee has been drawn to another application of photography which seems to open up a possibility of very valuable work; this is in the measurement of cloud altitudes. This is a question which has become more important since the acceptance by the Munich

Congress of the system of cloud nomenclature devised by Hildebrandsson and Abercromby, and it is remarkable that so few actual measurements have been carried out.

So far as your Committee are aware, the only measurements of the kind which have been systematically organised, at least in this country, are those which were begun some years ago at Kew.

Now it is not only important to have more observations, but it is especially desirable to have them from other places than the vicinity of London for comparison, and in the residence of the Secretary at Exeter such an opportunity is presented.

In the course of experiments on methods of cloud photography it has been found easy to secure well-defined images of clouds even when the sun is in the middle of the field of view. If, then, two such photographs are taken simultaneously by a pair of cameras at some distance apart, there will be a displacement of the image relatively to that of the sun. The amount of this displacement will depend upon a number of things, but it will be increased by adding to the focal length of the lens and by increasing the distance between the two cameras. By knowing these values and the altitude and azimuth of the sun, the distance of the cloud and its height above the ground may be calculated without difficulty.

The azimuth and altitude of the sun at the time of exposure may be ascertained by direct observation, or it may be found by calculation. from the known time at which exposure was made. There seems to be a

manifest advantage in thus using the sun as a fixed point of reference, since it provides a means whereby any error in the observation of altitude and azimuth may be effectively checked.

Your Committee have therefore prepared a pair of cameras so constructed that they may be easily directed towards the sun. They are provided with lenses of 18 inches focus covering a plate of whole plate size, thereby giving a large displacement and allowing room for a displacement of several inches. The lenses are provided with adjustable shutters, which can be simultaneously freed by an electrical attachment. They are placed on stands, which serve as cupboards for them when not in use.

At present for purely trial purposes they are placed in the Secretary's garden at a distance of 35 yards, yet even that short distance gives a displacement of half an inch with clouds 3,780 feet distant. This, of course, is too small for very accurate measurement, and would be far smaller with high-level clouds, the determination of the altitudes of which is most important.

The intention of your Committee is to place them on a plot of level ground by the side of the London and South-Western Railway near Exeter. There is available a strip of waste ground, just over a quarter of a mile in length, commanding an uninterrupted view of the sun from sunrise until nearly sunset. The ground is level, and the cameras can be placed due east and west, thereby greatly simplifying the reduction of the observations. The directors of the London and South-Western Railway have kindly consented to allow the ground to be used under conditions which seem to your Committee quite satisfactory, but which involve the payment of a nominal rent of 11. per annum; and the cameras would have been placed in position by the present time had it not been necessary to get another meeting of the Committee to sanction the agreement. The method is easy to apply, and promises to yield results at least as accurate as any which have yet been tried; so your Committee ask for reappointment, with a grant of 101.

Earth Tremors.-Report of the Committee, consisting of Mr. G. J. SYMONS, Mr. C. DAVISON (Secretary), Sir F. J. BRAMWELL, Professor G. H. DARWIN, Professor J. A. EWING, Dr. ISAAC ROBERTS, Mr. THOMAS GRAY, Sir JOHN EVANS, Professors J. PRESTWICH, E. HULL, G. A. LEBOUR, R. MELDOLA, and J. W. JUDD, Mr. M. WALTON BROWN, Mr. J. GLAISHER, Professor C. G. KNOTT, Professor J. H. POYNTING, Mr. HORACE DARWIN, and Dr. R. COPELAND (drawn up by the Secretary), appointed for the Investigation of Earth Tremors in this Country.

APPENDIX

I. Account of Observations made with the Horizontal Pendulum at Nicolaiew.
By Professor S. KORTAZZI

II. The Bifilar Pendulum at the Royal Observatory, Edinburgh. By Professor

R. COPELAND

Mr. H. Darwin's Bifilar Pendulum.

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THE preliminary trial of the bifilar pendulum last year led to the discovery of one or two possible sources of error, chiefly resulting from alterations in the distribution of temperature near the instrument. In order to eliminate these as far as possible, Mr. Darwin has made several changes in the latest form of the pendulum.1

When the gas-jet was kept burning for some time, it was found that the expansion of the tube produced an apparent tilting to the east, i.e., away from the source of heat. As soon as the flow of heat through the instrument became nearly steady, a far more considerable movement of the mirror in the opposite direction became evident, which was perhaps due to the action of convection currents in the surrounding oil.

The expansion of the tube is greatest on the side towards the gas-jet. Its disturbing effect is therefore a maximum when the gas-jet is in a plane at right angles to that in which the silver wire lies. In the new instrument the mirror is held in a frame so that the plane of the mirror is perpendicular to that of the silver wire, and the principal effect of the expansion is merely an inappreciable change in the sensitiveness of the pendulum. At the same time we should expect that this method of mounting the mirror would diminish the disturbing action of convection currents, as the surface exposed to them lies chiefly in a plane at right angles to that in which the movements of the ground are being measured.

In order to avoid any straining of the tube the lever used in determining the angular value of the scale divisions is prolonged above the tilting-screw. To this upper portion is attached a movable weight, which can be adjusted so that the centre of gravity of the lever coincides with the axis of the tilting-screw. The lever is moved by a rocking-arm worked from a distance by a pair of pneumatic bellows.

The instrument rests on three foot-screws, two of which are in a line parallel to the plane of the silver wire. A tangent-screw is connected with these two, so that one can be raised and the other depressed by an equal amount, and so enable the sensitiveness to be varied. A second tangent-screw is attached to the third foot-screw, or 'back-leg,' by means

For the account of these improvements I am indebted to notes supplied by Mr. Darwin. See also a paper, 'Bifilar Pendulum for Measuring Earth-tilts,' Nature, vol. 1. 1894, pp. 246–249.

1894.

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of which the pendulum can be tilted in the plane perpendicular to that of the silver wire, and the spot of light readjusted to the centre of the scale or photographic paper. Both screws are worked from a distance by long wooden rods.

A bifilar pendulum, with the changes above described, was erected early this year at the Royal Observatory, Edinburgh. The instrument is also further protected by a cover from heat effects. Dr. Copeland, Astronomer Royal for Scotland, informs me that, with these arrangements, it is not at all affected by momentary changes of temperature.

The Greek Earthquake Pulsations of April 1894.

On April 20 a severe earthquake took place in north-east Greece, causing much damage in several towns and villages. Soon after the news of its occurrence was published I made frequent observations with the bifilar pendulum at Birmingham, and was fortunate enough to watch the greater part of the remarkable series of pulsations proceeding from the second great disturbance, that of April 27. An account of these movements is given in its proper place below.

A few weeks later I received from Dr. von Rebeur-Paschwitz a list of the records of the same pulsations made by the horizontal pendulum at Nicolaiew. As these gave a somewhat greater velocity for the pulsations, it seemed possible that conclusions of some interest might result from an endeavour to trace the pulsations as they spread outwards from their origin. I accordingly wrote to the directors of the leading magnetic and geodynamic observatories on the Continent and in this country, and I am indebted to their courtesy for much information, a summary of which is given below. Additional details relating to the Italian observatories have been extracted from the valuable 'Bollettino Meteorico' (Supplementi 104 and 105) of the Ufficio Centrale di Meteorologia e Geodinamica' of Rome.

The total number of shocks belonging to this earthquake series must amount to several hundred. The strongest were those, already mentioned, on April 20 and 27. Both were felt over the whole of Greece. The epicentral areas seem to have been situated in the eparchy of Locris, and probably not far distant from its capital, Atalante. In the estimates of the velocity which follow I have supposed the earthquake pulsations to start from this town, the position of which is 38° 39′ N. lat., 23° 0′ E. long., and about 98 kilometres from Athens. For convenience the recorded times have all been reduced to Greenwich mean time.

Athens (Dr D. Eginitis), 37° 58′ 20′′ N., 23° 43′ 48′′ E. The earthquakes were registered by Brassart seismoscopes. These are well regulated, so that the times may be regarded as very exact. April 20, 5h. 17m. 5s. P.M., duration 4 seconds; followed by a second shock at 5h. 17m. 35s. P.M., duration 7 seconds. April 27, 7h. 46m. 11s. P.M., a very strong shock, duration 12 seconds.

Catania (Professor A. Ricco), 37° 28' N., 15° 4' E. April 20, 5h. 23m. 8s. P.M. April 27, 7h. 47m. 19s. P.M. The photographic record of the normal tromometer shows six series of decreasing oscillations, lasting for about 18 minutes.

Benevento (Boll. Meteor.'), 41° 8' N., 14° 45′ E. April 20, 5h. 19m. P.M., a very distinct trace indicated by the Cecchi seismograph. April 27, 1 The observatory is situated at a short distance from Catania, but I have been unable to find its exact position. In several cases, the positions of the Italian observatories are only approximate.

7h. 45m. P.M.

On both occasions the tromometer oscillated so much that

it was not possible to determine the amplitude.

Mineo (Boll. Meteor.'), 37° 15′ N., 14° 42′ E. April 20, 5h. 26m. (some seconds), P.M. April 27, 7h. 53m. P.M.

Portici (Boll. Meteor.'), 40° 50′ N., 14° 19′ E. April 27, 7h. 51m. 9s. P.M., movement indicated by a Brassart seismograph.

Velletri ('Boll. Meteor.'), 41° 41' N., 12° 47' E. April 20, 5h. 26m. P.M. Rocca di Papa (Dr. A. Cancani and Boll. Meteor.'), 41° 54' N., 12° 29′ E. April 20, 5h. 20m. P.M., the beginning of the pulsations indicated by the tromometro avvisatore.' The Brassart seismograph displaced at 5h. 22m. ±20s. April 27, 7h. 45m. P.M., the arrival of the pulsations announced by the tromometro avvisatore.' The great seismograph (7 metres in length and 100 kilogrammes in mass) shows the beginning of small earthquakes in the S.E.-N. W. component at 7h. 47m. 30s. At about 7h. 49m. 30s. the large oscillations in the S.E.-N.W. component began, and at 7h. 49m. 49s. in the N.E.-S.W. component. These large oscillations had a period of 7.2 seconds, and present a principal maximum in the N.E.-S.W. component at 7h. 50m. 40s., that of the other component not being well defined. This great undulatory movement ceased at 7h. 57m. 20s. in the N.E.-S.W. component, and at about 8h. 2m. 20s. in the S.E.-N.W. component.

Rome (Professor Tacchini and Boll. Meteor.'), 41° 54′ N., 12° 29′ E. April 20, beginning of the movement about 5h. 20m. 20s. P.M. in the N.W.-S.E. component, about 5h. 22m. Os. in the N.E.-S.W. component. The movement gradually increased until the following maxima were presented: 5h. 25m. 35s., 5h. 26m. Os., 5h. 26m. 55s. (principal maximum), 5h. 28m. 20s., 5h. 29m. Os., after which the traces irregularly and slowly decreased, the end of the movement taking place at about 5h. 33m. 15s. in the N.W.-S.E. component, and about 5h. 35m. 10s. in the N.E.-S.W. component. April 27, the beginning of the movement in both components at about 7h. 47m. 50s. P.M.; a series of maxima, first increasing and then decreasing, at 7h. 50m. 55s., 7h. 51m. 40s. (principal maximum), 7h. 52m. 10s., 7h. 52m. 25s., 7h. 53m. Os., 7h. 53m. 45s, 7h. 55m. 55s., and 7h. 57m. 10s.; the end of the movement in both components may be taken at about 8h. 6m. 20s., but not improbably it was prolonged still further.

Siena (Boll. Meteor.'), 43° 19' N., 11° 20′ E. April 20, 5h. 23m. 40s. (about 10s.) P.M., beginning of the movement in the N.N.E.-S.S.W. component, as registered by the Vicentini seismograph; the oscillations gradually increased in amplitude until they attained the following maxima: between 5h. 25m. 40s. and 5h. 26m. 40s. (two principal maxima), at 5h. 26m. 58s., 5h. 28m. 4s., and 5h. 28m. 40s.; the oscillations then slowly disappeared, the total duration being about fifteen minutes. During the first seven minutes the average period of the oscillations in the E.S.E.-W.N.W. component was about five seconds, and in the other, during the first ten minutes, about four seconds. At about 5h. 49m. 40s. there was a group of fourteen small oscillations, lasting for one minute. April 27, about 7h. 47m. 40s. P.M., beginning of the oscillations, which increased suddenly in amplitude; the first maximum at 7h. 51m., after which there were four others, the principal maximum being at 7h. 53m. 6s. During an interval of 552 seconds sixty-five oscillations were counted in the N.N.E.-S.S.W. component, and sixty-one in the E.S.E.-W.N.W. component, giving an average period of eight and a half seconds for each complete oscillation.

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