From June 12 to 15, 1958, a large, moderately bright prominence was observed at the southwest limb of the sun with the 60-foot tower telescope on Mount Wilson. On June 14 it had the appearance of an eruptive prominence, so a series of 15 photographs were made during the day to keep track of its activity. On June 15 it lay between -41° and -63° latitude and was slowly rising. By 19:25 UT it became evident that the prominence was going to erupt, so a continuous sequence of photographs five

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... apart was begun at 19 :55 UT. Sixty photographs were then made of the eruptive stages with the 13-foot spectroheliograph at a scale of 2 inches to the solar diameter. Since this prominence erupted at the west limb, there was a good chance that one could find the position of its base by an examination of Ha disk spectroheliograms taken on several days prior to the eruption. Instead of one continuous filament, only several small clumps were found in the correct region (Plate la). The patch marked "C" was probably the western terminus, and "B" and "A" were the eastern termini. Table I gives the positions for several days of the probable termini and the three most likely positions on the 15th. The true position of the western terminus (C) was probably that given by the last measure (6/11), since it was then rather well developed. On the ninth and tenth it was a very small patch and could easily have been high above the surface, introducing an unknown projection factor. It is evident that the bases of this prominence were about 35° and 58° apart in longitude and 8° and 10° apart in latitude. The sharpness of the inverted "V" shape of the arch was obviously due to the presentation angle of the "ribbon/' the observer being only some 10° to 15° away from looking at it endwise. If the arch was reasonably symmetrical, we can conclude that its top was over = -45° and À = +70° to +85°, or quite close 307