RESUMEN
Measurements of the movement and deformation of tectonic plates are needed for many research areas in geodynamics, but observations with adequate accuracy and frequency of measurement are not feasible if classical geodetic methods are used. Long-baseline microwave interferometry and laser ranging to Earth satellites are among the new techniques that have been developed within the past decade to make the required measurements. Fixed and mobile stations using both these methods have been constructed in several countries and are now being used in an internationally coordinated research program. Baseline length accuracy better than 2 to 3 centimeters (1 standard deviation) is expected within the next 5 years.
RESUMEN
Precursors to normtial seismic waves of the PKPPKP type in the distance range of 55 degrees to 75 degrees are ascribed to reflection of this phase from within the earth's upper mantle. The new observations confirm the existence of a sharply defined transition zone, probably worldwide in extent, at a depth of approximately 650 kilometers. These data are shown to be a useful tool for the study of upper mantle structure on a global basis.
RESUMEN
A large number of cultures of the bacterium E. coli have been grown in weak alternating magnetic fields of square waveform, at frequencies of 50 Hz and 16.66 Hz. Control cultures were simultaneously grown under ambient conditions identical except for the almost complete absence of any magnetic field. The mean generation time (MGT) for a culture subjected to alternating magnetic fields is significantly reduced by comparison with that for the control cultures. Application of the F-ratio test indicates a probability of less than one in two million that the effects observed are due to chance. A marked threshold effect is observed, along with strong indications of periodicity in the graph of MGT against magnetic field strength. Within the limits of experimental error, these effects correspond to integral changes in the number of magnetic flux quanta linking an individual bacterial cell during the process of division.