Birth order and its sequelae.

dimensions of the grains. The actual charge may be calculated if one has a measure of the capacitance of the grain. Only if the charge is strongly negative does the size of the dust grain enter significantly into the calculation of the equilibrium potential. 9. M. J. S. Belton, thesis, Univ. of California, Berkeley (1964). 10. H. Hinteregger, Ann. Rept. Mass. Inst. Technol. Conf. Phys. Electron. 14th (1954), pp. 41-49; J. Geophys. Res. 66, 2367 (1961). 11. C. W. Snyder and M. Neugebauer, in Space Res. Proc. Intern. Space Sci. Symp. 4th (1964), p. 89. 12. E. N. Parker, Interplanetary Dynamical Process (Interscience, New York, 1963). 13. A. Hewish and J. D. Wyndham, Monthly Notices Roy. Astron. Soc. 126, 469 (1963). 14. WX. Bernstein, in Space Physics, D. F. Le Galley and A. Rosen, Eds. (Wiley, New York, 1964). 15. Radio observations of sources through the interplanetary gas by A. Hewish and J. D. Wyndham (see 13) can be interpreted in terms of filamentary structures with a characteristic thickness of 103 to 10' km, extending out from the sun to at least 60 solar radii. E. N. Parker (see 12) has suggested that these structures may exist as far from the sun as the earth's orbit. For particles moving with a characteristic velocity of 10 km/sec, the characteristic time for passage through such a filament is --10:3 seconds. 16. M. F. Ingham, Space Sci. Rev. 1, 576 (1963). 17. H. Elsasser, Planetary Space Sci. 11, 1015 (1963). 18. R. H. Giese, Space Sci. Rev. 1, 589 (1963). 19. J. L. Weinberg, Ainn. Astrophys. 27, 718 (1964). 20. M. Harwit, Mem. Soc. Roy. Sci. Liege 9, 506 (1960); A. W. Peterson, Astrophys. J. 138, 1218 (1963). 21. J. Over, Koninkl. Ned. Akad. Wetenschap. Proc. 51B, 74 (1958). 22. 0. Struve, Astrophys. J. 98, 129 (1943). 23. F. G. Fessenkov, Ann. Astrophys. 22, 820 (1959). 24. R. E. Briggs, Astron. J. 67, 710 (1962). 25. L. G. Jacchia and F. L. Whipple, Smithsonian Contrib. Astrophys. 4, No. 4 (1960). 26. K. P. Chopra, Rev. Mod. Phys. 33, 153 (1961). 27. Secular acceleration is acceleration that is directly proportional to some power (not necessarily constant) of the time. The term is used further to imply that the acceleration is not a periodic function of time. 28. J. Ring, D. Clarke, J. F. James, M. Daihler, J. E. Mack, Nature 202, 167 (1964). 29. W. M. Alexander, Science 138, 1098 (1962). 30. , C. W. McCracken, J. L. Bohn, ibid. 149, 1240 (1965). 31. It is not clear at the moment how important the "sweepout" effect (see 4) of the convecting interplanetary magnetic field is. I neglect magnetic effects in this part of the discussion. 32. There are a few exceptions. The most notable of these is Comet Humason (1961c), which displayed a tail as far from the sun as 5 astronomical units. The tail, which was almost certainly composed of ionized gas, did not have the usual appearance of type-I tails. 33. A good exposition of current thought about the dynamics of type-I comet tails is given by L. Bierman and R. List in The Moon, Meteorites and Comets, B. M. Middlehurst and G. P. Kuiper, Eds. (Univ. of Chicago Press, Chicago, 1963), chap. 18. 34. D. Osterbrock, Astrophys. J. 128, 95 (1958). 35. S. K. Vseksviatsky, Russ. Astron. J. (English Transl.) 36, 503 (1959). 36. J. C. Brandt, Icarus 1, 1 (1962). 37. M. J. S. Belton, Astron. J., in press. 38. , paper presented at the 13th Astrophysical Symposium, Liege, 1965. 39. This article is contribution No. 120 of the Kitt Peak National Observatory. The observatory is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. Part of the work reported here was performed at the University of California and was supported by project NR 046-796 under grant Nonr (G)-00051-62.