256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 57 



tage over the photographic that it can operate for 24 hours of the 

 day regardless of sunlight, moonlight, or other sky illuminations. It 

 is interesting that one of the daylight streams, according to the results 

 of Clegg, Hughes, and Lovell (1947) turns out to be a recurrence of 

 the Taurid meteor stream contributed by Encke's comet. In a sense, 

 the writer predicted the existence of this stream (Whipple, 1939), 

 although in 1939 he had no premonition that radio techniques might 

 eventually be developed to observe it. It seemed quite likely, how- 

 ever, that bright fireballs from the other intersection of the Taurid 

 stream with the earth's orbit might be seen emanating from the 

 general direction of the sun. 



Davies of Manchester has recently developed a most remarkable 

 method for using radio techniques, to measure not only the velocity 

 of a meteor, but also its trajectory and spatial orbit. Davies' method 

 depends upon simultaneous observations from three stations, and pro- 

 vides meteor velocities and orbits for particles several times smaller 

 than those visible to the eye. He finds that these smaller bodies move 

 in orbits that are smaller and more nearly circular than those of the 

 larger photographic meteors. The explanation of this observation 

 will bring us around, full circle, to the problems of the micrometeorites. 



Van de Hulst (1947) and Allen (1947) have demonstrated that 

 micrometeorites are sufficiently numerous near the plane of the earth's 

 orbit to scatter most of the sunlight seen in the zodiacal light, the 

 twilight glow along the zodiac near sunrise or sunset. They find also 

 that along the line of sight near to the sun these small particles diffract 

 the sunlight and scatter it sufficiently to form an appreciable fraction 

 of the solar corona (see pi. 6). The corona, of course, consists also 

 of sunlight scattered by electrons as well as extremely strong bright 

 lines from the million-degree gases in the sun's huge extended atmos- 

 phere. From his calculation of the scattering and diffracting power 

 of the micrometeorites near the plane of the earth's orbit in space, 

 van de Hulst estimates that some 10,000 tons of this fine dust 

 should fall on the earth per day. He also concludes that most of the 

 dust particles are smaller than 0.03 cm. (0.01 inch) in diameter. This 

 estimate of the total fall on the earth is more than 1,000 times greater 

 than Watson's ( 1941 ) earlier estimate based upon the inf all of larger 

 pieces of meteoritic material. Some direct substantiation of van de 

 Hulst's conclusion, however, is given by the fact that noises of meteoric 

 impact on high altitude rockets have been recorded by Bohn and 

 Nadig (1950), of Temple University, and by Berg and Meredith 

 (1956) , of the Naval Research Laboratory. 



Pettersson and Rotschi (1950, 1952) find also that deep-sea oozes 

 contain appreciable quantities of nickel which may possibly derive 

 from this interplanetary dust. 



