15. Standard form for plotting the apparent paths of flight 395 16. Standard sectors for designating flight trends 398 17. The meaning of symbols used in the direction formula 399 18. Form used to compute zenith distance and azimuth of the moon 400 19. Plotting sector boundaries on diagrammatic plots 402 20. Form to compute sector densities 403 21. Determination of the angle [alpha] 404 22. Facsimile of form summarizing sector densities 405 23. Determination of net trend density 406 24. Nightly station density curve at Progreso, Yucatán 407 25. Positions of the cone of observation at Tampico, Tamps 411 26. Average hourly station densities in spring of 1948 414 27. Hourly station densities plotted as a percentage of peak 415 28. Incidence of maximum peak at the various hours of the night in 1948 416 29. Various types of density-time curves 418 30. Density-time curves on various nights at Baton Rouge 422 31. Directional components in the flight at Tampico, Tamps 428 32. Hourly station density curve at Tampico, Tamps 429 33. The nightly net trend of migrations at three stations in 1948 431 34. Stations at which telescopic observations were made in 1948 437 35. Positions of the cone of observation at Progreso, Yucatán 443 36. Hourly station density curve at Progreso, Yucatán 444 37. Sector density representation on two nights at Rosedale, Miss. 451 38. Over-all sector vectors at major stations in spring of 1948 455 39. Over-all net trend of flight directions shown in Figure 38 456 40. Comparison of flight trends and surface weather conditions on April 22-23, 1948 460 41. Winds aloft at 10:00 P. M. on April 22 (CST) 461 42. Comparison of flight trends and surface weather conditions on April 23-24, 1948 462 43. Winds aloft at 10:00 P. M. on April 23 (CST) 463 44. Comparison of flight trends and surface weather conditions on April 24-25, 1948 464 45. Winds aloft at 10:00 P. M. on April 24 (CST) 465 46. Comparison of flight trends and surface weather conditions on May 21-22, 1948 466 47. Winds aloft at 10:00 P. M. on May 21 (CST) 467 INTRODUCTION