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991.
The violation of mirror symmetry in the weak force provides a powerful tool to study the internal structure of the proton. Experimental results have been obtained that address the role of strange quarks in generating nuclear magnetism. The measurement reported here provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. We also report evidence for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies in atomic systems such as cesium.  相似文献   
992.
The Pioneer Saturn vector helium magnetometer has detected a bow shock and magnetopause at Saturn and has provided an accurate characterization of the planetary field. The equatorial surface field is 0.20 gauss, a factor of 3 to 5 times smaller than anticipated on the basis of attempted scalings from Earth and Jupiter. The tilt angle between the magnetic dipole axis and Saturn's rotation axis is < 1 degrees , a surprisingly small value. Spherical harmonic analysis of the measurements shows that the ratio of quadrupole to dipole moments is < 10 percent, indicating that the field is more uniform than those of the Earth or Jupiter and consistent with Saturn having a relatively small core. The field in the outer magnetosphere shows systematic departures from the dipole field, principally a compression of the field near noon and an equatorial orientation associated with a current sheet near dawn. A hydromagnetic wake resulting from the interaction of Titan with the rotating magnetosphere appears to have been observed.  相似文献   
993.
994.
Calculations show that, at a mean altitude of 1000 miles, radiation pressure can displace the orbit of the 100-foot Echo balloon at rates up to 3.7 miles per day, the orbit of the 12-foot Beacon satellite at 0.7 mile per day. For certain resonant conditions this effect accumulates, drastically affecting the satellite's lifetime.  相似文献   
995.
Studies of food remains from the Preceramic monumental site of E1 Paraíso, Peru (1800 to 1500 B.C.), have shed new light on a debate regarding the relative importance of seafood versus terrestrial resources and the role of cultigens in subsistence economies during the early development of Peruvian civilization. Fish was the primary animal food at the site whereas plant foods consisted of a mixture of cultivated resources (squashes, beans, peppers, and jicama) with an additional reliance on fruits (guava, lucuma, and pacae). Wild plants, especially the roots of sedges and cat-tail, also may have accounted for a substantial part of the diet. Cotton was a chief crop, used in making fishing tackle and the textiles that served as clothing and items of high value and status. As an example of the beginnings of civilization, El Paraíso is a case in which impressive architecture was built on a relatively simple subsistence economy and energy was expended in the production of resources useful in local and regional exchange systems.  相似文献   
996.
997.
Natural populations of guppies were subjected to an episode of directional selection that mimicked natural processes. The resulting rate of evolution of age and size at maturity was similar to rates typically obtained for traits subjected to artificial selection in laboratory settings and up to seven orders of magnitude greater than rates inferred from the paleontological record. Male traits evolved more rapidly than female traits largely because males had more genetic variation upon which natural selection could act. These results are considered in light of the ongoing debate about the importance of natural selection versus other processes in the paleontological record of evolution.  相似文献   
998.
Hemoglobin Sphakiá: a delta-chain variant of hemoglobin A2 from Crete   总被引:2,自引:0,他引:2  
A new variant of the normal minor component Hb A(2) has been detected in a family that lives in Sphakiá, Crete. Chemical studies of this abnormal hemoglobin, designated Hb A(2) delta Sphakiá, indicates a substitution of the histidyl residue number two of the delta-chain by an arginyl residue.  相似文献   
999.
Rinderpest is a highly contagious ruminant viral disease manifested by a rapid course and greater than 90% mortality. Infectious vaccinia virus recombinants were constructed that express either the hemagglutinin or the fusion gene of rinderpest virus. All cattle vaccinated with either recombinant or with the combined recombinants produced neutralizing antibodies against rinderpest virus and were protected against the disease when challenged with more than 1000 times the lethal dose of the virus.  相似文献   
1000.
13-Methylhentriacontane has been identified in the feces and larvae of the corn earworm, Heliothis zea (Boddie), as the major constituent that triggers the short-range host-seeking response of the parasite Microplitis croceipes (Cresson). This chemical, the first found that mediates the complex host-parasite relation, could upgrade present efforts to use parasites for insect control. Bioassay of closely related compounds indicated that the structural requirements for activity are remarkably specific.  相似文献   
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