One of the methods of sea-crossing leveling is how to get the difference in the height of sea level from the measurements of the sea level at the two points on both sides of a strait. This method is named the oceanographic leveling. In this case the dominant factors which possibly function over the observation of the sea level are oceanic current, wind and the atmospheric pressure difference on both sides of the strait. As the observation of the oceanic currents and the wind in and over a strait s a hard and laborious work, the calculation of the difference of sea level between the two points on the sides of a strait is often made out with the induced electric potential which occurs when water is running in place of the oceanic current speed, and with the atmospheric pressure gradient in place of the wind velocity. Here the author has figured out the regression equation (15) to explain the relations between the values of the difference of sea level and of the induced electric potential and the values of atmospheric pressure gradient. Then, applying the above theory to the Tsugaru Strait, the author tried to figure out the difference of sea level between the two points, Asamushi and Hakodate, on both sides of the strait, with the data from October 1959 to March 1960, and we get the result that the sea level is higher at Asamushi by 167 mm than at Hakodate.

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Of the tidal stations in the world, as is present in Fig. 1 and Table II, were picked out 36 stations where they have made annual observations continuously for more than 40 years, in principle. With the data from those chosen tidal ststions the secular variation and long-periodic variation of the sea level have been investigated. As for the method of the investigation, to get the secular variation of the sea level, the changes in the annual mean sea level were applied in a linear equation, and then, to get the periodicity of the long-periodic variation, it was calculated that the serial correlation' coefficient of the annual mean sea level subtracted the linear variation on sea level from the observed values. The annual rate of the rise in the sea level and the serial correlation coefficient for each tidal station are given in Table III and Fig. 3 respectively. (1) At least in the northern hemisphere, the sea level has been in a rising tendency for these scores of years, and the annual quantity is 1 milliliter. (2) No common period of variation for all the stations could not be found, and the pre-eminence of the 19-yearly periodicity by the shifting of the moon's ascending node could not be explained neither. (3) The amplitude of the 19-yearly tide seems to be scarcely different from the theo retical value by the equilibrium law. (4) In the data of the tidal stations where observations have been made for longer periods, the 10-11 yearly periodicity can be found pre-eminent, and it is quite noticeable that this is almost corresponding to the periodicity of the sun spots.

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In geodesy, it is often necessary to compute a value x when a value of y = f(χ) is given. Until now, a methode using the polynomial expansion of an inverse function has been widely used, but this method is much time consuming. In this report, Newton-Raphsons method, which is becoming popular in numerical method, is applied to the above-mentioned problem. Using this method, the computation can be made more easily and automatically. The following two examples were solved; 1) the camputation of latitude when the meridian arc from the equator is given, 2) the computation of the latitude of the ellipsoid when the latitude of the corresponding sphere is given in the Gauss' double conformal projection.

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The Geographical Survey Institute has established the net work of magnetic stations in Japan and the data obtained so far enable us not only to constract detailed magnetic charts for clarifying the distribution of local magnetic anomalies but also to discuss locally anoma lous geomagnetic secular variations which are becoming noticeable in connection with other geophysical phenomena, especially the occurrence of earthquake. Concerning the accuracy of epoch reduction of magnetic survey, traditional methods using the record of only one reference magnetic observatory are becoming insufficient for much precise discussion of removing the effect of transient magnetic fields from the observed values at field stations, because the old reduction methods usually neglect the geographical difference of any transient fields. In this paper a new method is proposed, in which the observed value at a field station is reduced to average night values on the presumed quiet day instead of the daily mean value, because the latter can not be regarded to have no theoretical foundation as the datum line of the transient field. At first, the night values on the actual quiet days satisfying several proposed conditions are read from the records at all available magnetic observato ries in Japan, namely, Kakioka, Kanozan, Shimosato, Memanbetsu and Kanoya. Next, these night values are expressed as quadratic function of time, by which the presumed average night values of quiet day M_n(t) can be computed on arbitrary day at these observa tories. Using these values, the geographical distribution of the deviation of actual value from the quiet night value; δM_n(t) = M(t)-M_n(t), which may be regarded the magnitude of transient field, can be obtained at the time of observation in field survey, so that the value of δM_n(t) at the magnetic field station may be determined by the geographical inter polation. Then the average night values of quiet days can be computed at any station, consequently we obtaine the magnetic datum line at the station as a regression line of the distribution of quiet night values. This method is superior against the old one both on the theoretical consistency and the accuracy of removing the transient field. The method was applied to the analysis of the 2nd Order Magnetic Survey in the Tohoku District where the observations were first made in 1954 and also repeated in 19601961. Remarka ble local anomalies of the secular variation are found especially in the northern half and along the coast line of Tohoku District.

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An aeromagnetic survey covering the area between latitudes 33°20' and 36°00'N and longitudes 136° and 139°15'E has been conducted by the Geographical Survey Institute in February 1968, as one of the serial surveys over the whole Japan and her vicinity. The instrument used is a proton precession magnetometer, by which the value of the total magnetic force is recorded continually on an analog recorder with the measuring accuracy of±1 γ. The measurements have been made every 6 seconds along N-S flight courses having altitude of 3, 000 m. The spacing between successive courses is about 10 km, the total measured range amounting to 7000 km. After correcting the altitude reduction and the effect of magnetization of the aircraft to the observed values, the epoch reduction has been made to the epoch of 1965.0 by using the continuous record of the total force at the Kanozan Geodetic Observatory. The isodynamic curves of the total force are drawn in the interval of 100 γ and the isanomalic chart is constructed by taking the difference of the reduced values and the computed values from the quadratic formula of the total force in terms of latitude and longitude. As the result, the following aspects of magnetic anomaly are found. 1) In general, large magnetic anomalies are found over the Fossa Magna, the Izu peninsula and her southern sea area, while over the western land and coastal area from the Fossa Magna and the Suruga bay anomalies are very small or approximately zero. 2) Three couples of magnetic lineation patterns are found over the Izu peninsula and the southern sea area. Each couple of the lineation pattern has the trend of about east-west direction and the distances between southern high and northern low in the patterns are about 7, 12 and 15 km respectively, while the distance between successive patterns is about 50 km. These lineation patterns seem to be connected in their east extention to the similar patterns which N. Fujita found previously over the southern Kanto District. It seems likely that these linear anomalies are caused by the intrusion of basaltic material along the second order tectonic lines or ridges which are parallel to the east part of the Median Tectonic Line.

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The level-line that runs along the coast of Boso Peninsula was surveyed four times at the year of 1898, 1923-4924, 1930, 1965 respectively. Therefor we can compile the three charts showing the vertical deformation of the crust during the three different periods. From the results of these charts and considering the distribution of Bouguer gravity anomalies in this Peninsula together with other author's seismological, geomorphological and geological discussions, the following conclusions are obtained; (1) Under the region of Chiba City, down-buckling of the crust is observed and its thickening is calculated as about 12 km from the gravity anomalies. This amount is equal to the down of the upper surface of the distribution of deep earthquakes that are frequently occured at the upper mantle in this region. (2) The rate of the secular inclination of the Peninsula toward the centher of the down-buckling of the crust is about 0.01" 0.03" per year. This is the same order as geomorphological conclusion about the tectonics in this Peninsula. (3) Vertical movement of the B. M. 3880 that lies at the most southern part of the level-line in Peninsula shows that it was upheaved at the occurence of the great earthquake of 1923 and is gradually sinking after earthquake. (4) The region bounded by the Hota-Kamogawa line and the Enmyoji line shows the gradual subsidence relative to the surrounding region, and also the relatively negative gravity anomalies. A graben-like crustal structure is presumed from gravity anomalies analysis. We can recognize some active blocks in this region.

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A gravity survey in Ooshima Island was carried out by the Geographical Survey Insti tute in September 1967 for detecting the secular variation of gravity in connection with the volcanic activity. The remeasurements by means of the LaCoste and Romberg gravitimeter No. 29 have been made at about thirty stations, among which 4 stations were previously occupied two or three times from 1950 and other stations were first occupid in 1963. Assuming that the gravity value at the station of Ooshima Meteorological Observatory has been constant during the period from 1950 to 1967, the maximum gravity decrease of about 0.9 mgals has been found between this period at Gozinka tea house near the crater, and the distribution of gravity decrease seems to become smaller in proportion to the distance from the crater. This result might indicate the sinking of magmatic substance under the volcano in recent years. Considering the observed gravity variations and other results of geophysical observations, it may be concluded that after the great explosions of 19501951, volcanic activity at Mt. Miharayama, Ooshima Island, has been declining.

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