The general distribution of prominences, including both genera, follows that of faculæ much more closely than that of spots. From Father Secchi's and Professor Respighi's observations, 1869-71, were derived the first clear ideas on the subject, which have been supplemented and modified by the later researches of Professors Tacchini and Riccò at Rome and Palermo. The results are somewhat complicated, but may be stated broadly as follows. The district of greatest prominence-frequency covers and overlaps by several degrees that of the greatest spot-frequency. That is to say, it extends to about 40° north and south of the equator.[617] There is a visible tendency to a second pair of maxima nearer the poles. The poles themselves, as well as the equator, are regions of minimum occurrence. Distribution in time is governed by the spot-cycle, but the maximum lasts longer for prominences than for spots. The structure of the chromosphere was investigated in 1869 and subsequent years by Professor Respighi, director of the Capitoline Observatory, as well as by Spörer, and Brédikhine of the Moscow Observatory. They found this supposed solar envelope to be of the same eruptive nature as the vast protrusions from it, and to be made up of a congeries of minute flames[618] set close together like blades of grass. "The appearance," Professor Young writes,[619] "which probably indicates a fact, is as if countless jets of heated gas were issuing through vents and spiracles over the whole surface, thus clothing it with flame which heaves and tosses like the blaze of a conflagration." The summits of these filaments of fire are commonly inclined, as if by a wind sweeping over them, when the sun's activity is near its height, but erect during his phase of tranquillity. Spörer, in 1871, inferred the influence of permanent polar currents,[620] but Tacchini showed in 1876 that the deflections upon which this inference was based ceased to be visible as the spot-minimum drew near.[621] Another peculiarity of the chromosphere, denoting the remoteness of its character from that of a true atmosphere,[622] is the irregularity of its distribution over the sun's surface. There are no signs of its bulging out at the equator, as the laws of fluid equilibrium in a rotating mass would require; but there are some that the fluctuations in its depth are connected with the phases of solar agitation. At times of minimum it seems to accumulate and concentrate its activity at the poles; while maxima probably bring a more equable general distribution, with local depressions at the base of great prominences and above spots.