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Devonport.-Time-balls are dropped on Mount Wise at 16 and at 1h 5m Greenwich mean time.

Greenwich.-A time-ball is dropped daily at 1 P.M.; a ball is automatically dropped at Deal. Time-balls are proposed for Portsmouth and Start-Point. Hourly currents are sent from the observatory to the Post-office, from which they are distributed to subscribers. Currents are sent automatically at 10 A.M. and 1 P.M., aud widely distributed; and almost daily signals are sent directly to about 600 places, including railway termini, and from these places they are repeated to the lines radiating therefrom. The system is practically complete.

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VISIBILITY OF STARS IN THE DAY-TIME. Dr. Rudolf Wolf communicates to the Philosophical Society of Zurich (20th year, page 179) a note on the visibility of stars in the day-time from the bottom of deep wells. It appears that Mr. F. Carpentier, of Zurich, tried the experiment of looking for stars in the day-time from the bottom of a deep well (ninety feet), and that he was successful. It was done when the experimenter was but a lad, and although there is no doubt as to the fact, the names of the stars seen and the particulars of the observation can not be recovered.

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MEASURE OF TIME BY HOUR-GLASSES. Dr. Wolf, of Zurich, baving occasion to use an hour-glass which was intended to mark half-hours, endeavored to determine the probable error of a measure made by means of this instrument. The ending of the flow of sand could be observed quite exactly, i. e., to within one second of time; the mean of sixty trials gave him for the time of flow 28m 29.95 +5.78. The probable error of a single observation was no less than 43.9 seconds. It thus appears that this instrument, which was extensively used by the ancients, is not a trustworthy one for exact determinations.

B. TERRESTRIAL PHYSICS AND METEOROLOGY.

MECHANICAL THEORY OF OCEAN CURRENTS.

The application of mathematics and the laws of mechanics to the theoretical investigations of the currents of the atmosphere and the ocean, with which subject American students have become so familiar through the labors of Professor Ferrel, has of late years been taken up quite energetically in Europe ; and to the labors of Colding, Peslin, Everett, Thomson, Hann, and others, we now have to add the somewhat elementary memoir of Professor Blazek on the elements of a mechanical theory of the ocean currents, which communication is, he implies, introductory to a more extended memoir that may be expected from him. He considers that every particle of water is actuated by two forces—that of gravity toward the centre of the earth, and the centrifugal force due to the daily rotation of the earth. The centrifugal force in and of itself can produce no constant current, since it is in obedience to it that the earth owes its permanent ellipsoidal shape; but if on such an ellipsoid the temperature in general diminishes toward the equator and toward the two poles, there results a tendency toward a new distribution of the mass of the water, which therefore demands a new figure of equilibrium. The dense, cold waters of the two polar seas, therefore, flow toward the equator, while the lighter warm waters flow from the equator toward either pole. Less water flows from regions of low temperature toward the regions of high temperature, and the northern hemisphere sends to the equator less cold water than the southern hemisphere. Consequently, as he demonstrates, the centres of all the closed portions of the equatorial currents in all oceans are found between the 35th and 30th parallel of latitude. The centrifugal force drives the cold water toward the equator and the warm water toward the polar regions, and in consequence of the earth's rotation these latitudinal currents are turned aside-those from the poles to the west, and those from the equator to the east.-Sitzb. K. Böhm. Gesell., 1874, 195.

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THE TEMPERATURE OF THE SEA, Observations on the temperature of the sea have been made at Falmouth, England, continually since the autumn of 1871. It was at first proposed to make the observations about the time of high water in Falmouth harbor, it being thought that, owing to the great extent and depth of the harbor, the influence of the shore would not be felt when the water had freshly come in from the open sea, But it soon became apparent that results obtained in the harbor and near the shore were worthless. Experiments proved that differences of several degrees of temperature existed within very small limits of space; and Mr. Dymond, by whom the observations had been made, states that he has reason to think that observations on the temperature of the sea are, in general, of little or no value unless they are made from a vessel considerably outside of low-water mark. It was therefore determined, within a few months after the observations were begun, to make them in the open sea, at distances varying from half a mile to three or more miles from the nearest land. During the three years—1872, 3, and 4-observations were made on 747 days; and the mean of all these readings is 53.23°, while the mean of the 36 highest and 36 lowest monthly readings is 53.25° The monthly range of the sea temperatures varied from 2° in March to 6.5° in July. The importance of making similar investigations at other stations where sea temperatures are systematically observed is forcibly suggested by this result of Mr. Dymond's studies, and it is evidently highly desirable that the light-ships stationed along our coasts should be utilized, whenever possible, for the purpose of observing sea temperatures. — Forty-second Annual Rep. Royal Cornwall Pol. Society, p. 105.

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ORIGIN OF VOLCANIC ASHES. Professor Von Roth communicates to the Berlin Academy of Sciences an investigation into the origin of the volcanic ashes that fell in Scandinavia on the 29th and 30th of March. He shows that these were the product of the great volcanic outbreak in the eastern half of Iceland at Vatua, as well as in the Dynqu Mountains.-Monatsber. Berlin Acad. der Wiss., 1875, 256.

ABNORMAL DEFLECTIONS OF THE PLUMB-LINE. Lientenant F. V. Greene, Engineer Corps, U.S. A., has recently discussed the station-errors of the 41 astronomical stations established on the 49th parallel of latitude by the joint commission of English and American engineer officers, for the purpose of marking the boundary-line between the United States and the British possessions. The 41 stations were all observed with the zenith telescope, and of them 19 were observed by the English alone, and the average probable error of the final latitude of a station was £0.088"; 17 were observed by the Americans alone, and the probable error of a station established by them was +0.059" (76 observations); and 5 were jointly observed by the two parties, their independent results differing by 0.28",0.27",0.07",0.29", and 0.31" respectively. The various stations were connected geodetically by the method of tangents and offsets; and the difference between the astronomical and geodetic determinations was assumed to be due to abnormal variations in the local directions of gravity, and was called "stationerror."

Sufficiently accurate topographical work was done to enable 200-feet contours to be laid down on the maps. From these contours Lieutenant Greene has calculated A (see table), which is the deflection of the plumb-line produced by all masses above the ground within a radius of ten miles, and B, which is the deflection which would be produced by masses between ten and sixty miles distant. This has been done with reference to each of the 41 stations--a work of no little difficulty-and the results are given in the accompanying table, D is the observed deflection, and D-(A+B) is the outstanding residual and unexplained deflection. These quantities for 37 stations in the Ordnance Survey of Great Britain were D=2.05", and D-(A+B)=1.35". From this it appears that for 29 stations the abnormal phenomena are partially accounted for and in the right direction; for 12 stations the computed deflection is in the wrong direction. The quantity D=2.15" which is to be accounted for is about the same in magnitude as the mean error in latitude (2.64") found by Bessel from a discussion of all the great arcs previously measured, and goes to show that the mean heterogeneity of large tracts of the earth's surface is about the same in every part of the earth itself:

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D.
A.*

B. D–A+B).
Stations.
Station Errors.

Computed Computed Deflec-
Deflections.

Unexplained
Mean Parallel=0.

tions.

Deflections. 1 to 10 miles, 10 to 60 miles.

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-2.01 -1.60 -.52 -2.66 -1.75

-.16 + .70 + .23

-2.31 +1.52 +2.33 +3.18 +3.50 +2.95 +2.22 +1.40

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-.78 + .06

.30 -1.91 -2,23 -.50

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.23
.27
.46
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+2.857
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+2.03
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21. 22. 23. 24. 25.. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.. 36.... 37.... 38....

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+4.56 +3.31 +1.14 +.76

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41

1.442"

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..... 43.97" 43.98"

36.08" 16.82" Means.. 2.146"

The stations at which deflections, A, are not computed were situated in the midst of plains, where the deflection, A, would not amount to 0.4", the uncertainty of the determination of D.

† Carried to 80 miles.

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