Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/575

Rh (fig.2). If the tube ab be filled with and ed at two points, first at a and then at b, the following succession of changes is produced. The water at a beginning to, the superincumbent column is consequently raised, and the stratum of which was on the point of ing at b being raised to d is there subjected to a diminished ; a sudden evolution of  accordingly takes place at d, and the superincumbent  is violently ejected. Received in the basin c, the -cooled sinks back into the tube, and the  of the whole column is consequently lowered; but the under strata of  are naturally those which are least affected by the cooling process; the ing begins again at A, and the same succession of events is the result (see R. Bunsen, “Physikalische Beobachtungen über die hauptsächlichsten Gisire Islands,” in Poggendorff’s Annalen der Physik and Chemie, vol. lxxii., ; and J. Müller, “Ueber Bunsen’s Geysertheorie,” ibid., vol. lxxix., ). The principal difference between the ificial and the geyser-tube is that in the latter the effect is not necessarily produced by two distinct sources of  like the two s of the al apparatus, but by the continual influx of  from the bottom of the shaft and the differences between the  of the different parts of the column owing to the different s of the superincumbent. This may be thus illustrated:—AB is the column of water; on the right side the figures represent approximately the  calculated according to the ordinary laws, and the figures on the left the actual  of the same places. Both gradually increase as we descend, but the relation between the two is very different at different heights. At the top the is still 39° from its, and even at the bottom it is 19°; but at D the deficiency is only 4°. If, then, the stratum at D be suddenly lifted as high as C, it will be 2° above the there, and will consequently expend those 2° in the formation of steam. Any capable of depositing material by the  of its  may in course of  transform itself into a geyser, a tube being gradually built up as the level of the basin is raised. And every geyser continuing to deposit material is preparing its own destruction; for as soon as the tube becomes deep enough to contain a column of  sufficiently  to prevent the lower strata attaining their boiling points, the whole mechanism is deranged. In geyser districts it is easy to find busy with the construction of the tube; warm pools, or laugs, as the ers call them, on the top of  mounds, with the mouth of the shaft still open in the middle; and dry basins from which the  has receded with their shafts now choked with rubbish. Geysers exist at the in many regions, as in the,, and ; but the three localities where they attain their highest development are , , and  in the. The very by which we call them indicates the  priority of the  group. It is an old word—geysir, equivalent to gusher or rager—from the  geysa, itself a derivative of gjosa, to gush. In it is the proper  of the Great Geyser, and not an appellative—the general  hver, a, making the nearest approach to the an sense of the  (see Cleasby and Vigfusson, Icelandic English Dictionary, s.v.).

The geysers are situated about 50 N.W. of, in a broad  of  formation, at the foot of a range of s from 300 to 400  in height. Within a circuit of about two, upwards of one hundred may be counted, varying greatly both in character and dimensions. The Great Geyser in its calm periods appears as a pool 72  in  and 4  in depth, occupying a basin on the summit of a mound of  ; and in the centre of the basin is a shaft, about 9  in diameter and 70  in depth, lined with the same  material. The clear -green flows over the eastern rim of the basin in little runnels. On the surface it has a of from 76° to 89°, or from 168° to 188°  Within the shaft there is of course a continual shifting both of the average  of the column and of the relative s of the several strata. The results of the observations of Bunsen and Descloizeaux in were as follows (cf. Poggendorff’s Annalen, loc. cit., and Comptes Rendus, vol.xxiii.):—About three s after a great eruption on  6th, the  6  from the bottom of the shaft was 121·6°; at 9·50, 121·1°; at 16·30 , 109° (?); and at 19·70 , 95° (?). About nine s after a great eruption on 6th, at about 0·3  from the bottom, it was 123°; at 4·8  it was 122·7°; at 9·6, 113°; at 14·4 , 85·8°; at 19·2 , 82·6°. On the 7th, there having been no eruption since the previous fore, the at the bottom was 127·5°; at 5  from the bottom, 123°; at 9, 120.4°; at 14·75 , 106·4°; and at 19 , 55°. About three s after a small eruption, which took place at forty s past three o’clock in the afternoon of the 7th, the at the bottom was 126·5°; at 6·85  up it was 121·8°; at 14·75, 110°; and at 19 , 55°. Thus, continues Bunsen, it is evident that the of the column diminishes from the bottom upwards; that, leaving out of view small irregularities, the  in all parts of the column is found to be steadily on the increase in proportion to the time that has elapsed since the previous eruption; that even a few s before the great eruption the  at no point of the  column reached the  corresponding to the   at that part; and finally, that the  about half-way up the shaft made the nearest approach to the appropriate, and that this approach was closer in proportion as an eruption was at hand. Observations made by Mr Robert Walker in  remarkably conﬁrm those of Professor Bunsen (see Proceedings of Roy. Soc. of Edinburgh, vol.viii. p.514). The Great Geyser has varied very much in the nature and frequency of its eruptions since it began to be observed. In and, e.g., according to Hooker and Mackenzie, its columns were 100 or 90  high, and rose at intervals of 30 s, while, according to Henderson, in  the intervals were of 6 , and the  from 80 to 150. About 100 from the Great Geyser is the Strokkr or, which was first described by Stanlay in. The shaft in this case is about 44 deep, and, instead of being, is -shaped, having a width of