1009 



GEYSERS. 



GIANT. 



1010 



the interval of two eruptions the temperature of the water in the 

 tube towards the centre and bottom gradually increases. Bunsen 

 succeeded in determining its temperature a few minutes before a greal 

 eruption took place; and these observations furnished to his clear 

 intellect the key of the entire enigma. A little below the centre the 

 water was within two degrees of its boiling point, that is, within two 

 degrees of the point at which water boils under a pressure equal to 

 that of an atmosphere, plus the pressure of the superincumbenl 

 column of water. The actual temperature at 30 feet above the 

 bottom was 122 centigrade, its boiling point here is 124. We have 

 just alluded to the detonations and the lifting of the geyser column 

 by the entrance of steam from beneath. These detonations and the 

 accompanying elevation of the column are, as before stated, heard and 

 observed at various intervals before an eruption. During these 

 intervals the temperature of the water is gradually rising. Let us see 

 what must take place when its temperature is near the boiling point. 

 Imagine the section of water at 30 feet above the bottom to be raised 

 six feet by the generation of a mass of vapour below. The 

 liquid spreads out in the basin, overflows its rim, and thus the 

 elevated section has six feet less of water pressure upon it ; its boiling 

 point under this diminished pressure is 121 ; hence in its new 

 position its actual temperature (122) is a degree above the boiling 

 point. This excess is at once applied to the generation of steam : 

 the column is lifted higher, and its pressure further lessened ; more 

 (team is developed underneath ; and thug, after a few convulsive 

 efforts, the water is ejected with immense velocity, and we have the 

 geyser eruption in all its grandeur. By its contact with the atmo- 

 sphere the water is cooled, falls back into the basin, sinks into the 

 tube through which it gradually rises again, and finally fills the basin. 

 The detonations are heard at intervals, and ebullitions observed ; but 

 not until the temperature of the water in the tube has once more 

 nearly attained its boiling point is the lifting of the column able to 

 produce an eruption. 



" In the regularly-formed tube the water nowhere quite attains the 

 boiling point. In the canals which feed the tube, the steam which 

 causes the detonation and lifting of the column must therefore be 

 formed. These canals are in fact nothing more than the irregular 

 continuation of the tube itself. The tube is therefore the sole and 

 sufficient cause of the eruptions. Its sufficiency was experimentally 

 shown during the lecture. A tube of galvanised iron six feet long 

 was surmounted by a basin ; a fire was placed underneath and one 

 near its centre to imitate the lateral heating of the geyser tube. At 

 intervals of five or six minutes throughout the lecture eruptions took 

 place ; the water was discharged into the atmosphere, fell back into 

 the basin, filled the tube, became heated again, and was discharged as 

 before. 



" Sir George Mackenzie, it is well known, was the first to introduce the 

 idea of a subterranean cavern to account for the phenomena of the 

 Geyser. His hypothesis met with general acceptance, and was even 

 adopted undoubtingly by some of those who accompanied Bunsen to 

 Iceland. It is unnecessary to introduce the solid objections which 

 might be urged against this hypothesis, for the tube being proved 

 sufficient, the hypothetical cavern disappears with the necessity which 

 gave it birth. 



" From the central portions of the geyser tube downwards, the water 

 has stored up an amount of heat capable, when liberated, of exerting 

 an immense mechanical force. By an easy calculation it might be 

 shown that the heat thus stored up could generate, under ordinary 

 atmospheric pressure, a column of steam having a section equal to 

 that of the tube and a height of nearly 1300 yards. This enormous 

 force is brought into action by the lifting of the column and the 

 lessening of the pressure described above. 



" A moment's reflection will suggest to us that there must be a limit 

 to the operations of the Geyser. When the tube has reached such an 

 altitude that the water in the depths below, owing to the increased 

 pressure, cannot attain its boiling point, the eruptions of necessity 

 cense. The spring however continues to deposit its silica and forms a 

 ' laug,' or cistern. Some of these in Iceland are of a depth of 30 or 

 40 feet. Their beauty is indescribable ; over the surface a light vapour 

 curls, in the depths the water is of the purest azure, and tints with 

 its own hue the fantastic incrustations on the cistern walls ; while at 

 the bottom is observed the mouth of the once mighty Geyser. There 

 tuee in Iceland traces of vast, but now extinct, geyser operations. 

 Mounds are observed whose shafts are filled with rubbish, the water 

 having forced a way underneath and retired to other scenes of action. 

 We have in fact the Geyser in its youth, manhood, old age, and death, 

 here presented to us : in its youth as a simple thermal spring, in its 

 manhood as the eruptive spring, in its old age as the tranquil laug, 

 while its death is recorded by the ruined shaft and mound, which 

 testify the fact of its once active existence. 



" Next to the Great Geyser the Strokkur is the most famous eruptive 

 spring of Iceland. The depth of its tube is 44 feet. It is not how- 

 ever cylindrical like that of the Geyser, but funnel-shaped. At the 

 mouth it is 8 feet in diameter, but it diminishes gradually, until near 

 the centre the diameter is only 10 inches. By casting stones and 

 peat into the tube and thus stopping it, eruptions can be forced which 

 in point of height often exceed those of the Great Geyser. Its action 

 WM illustrated experimentally in the lecture, by stopping the gal- 

 NAT. BUT. DIV. VOL. II. 



vanised iron tube before alluded to loosely with a cork. After some 

 time the cork was forced up and the pent-up heat converting itself 

 suddenly into steam, the water was ejected to a considerable height 

 thus demonstrating that in this case the tube alone is the sufficient 

 cause of the phenomenon." (' Proceedings of Royal Institution.') 



The results of the researches of Professor Bunsen on the Geyaers 

 of Iceland seem to throw great and unexpected light on the pheno- 

 mena of volcanoes. Sir Charles Lyell closes his account of Bunsen's 

 researches with the following remarks : 



" In speculating therefore on the mechanism of an ordinary volcanic 

 eruption, we may suppose that large subterranean cavities exist at 

 the depth of some miles below the surface of the earth, in which 

 melted lava accumulates, and when water containing the usual 

 mixture of air penetrates into these, the steam thus generated may 

 press upon the lava and force it up the duct of a volcano, in the same 

 manner as a column of water is driven up the pipe of a Geyser. In 

 other cases we may suppose a continuous column of liquid lava, 

 mixed with red-hot water (for water may exist in that state, as Pro- 

 fessor Bunsen reminds us, under pressure), and this column may have 

 a temperature regularly increasing downwards. A disturbance of 

 equilibrium may first bring on an eruption near the surface, by the 

 expansion and conversion into gas of entangled water and other con- 

 stituents of what we call lava, so as to occasion a diminution of 

 pressure. More steam would then be liberated, carrying up with it 

 jets of melted rock, which being hurled up into the air may fall in 

 showers of ashes on the surrounding country, and at length, by the 

 arrival of lava and water more and more heated at the orifice of the 

 duct or the crater of the volcano, expansive power may be acquired 

 sufficient to expel a massive current of lava. After the eruption has 

 ceased a period of tranquillity succeeds, during which fresh accessions 

 of heat are communicated from below, and additional masses of rock 

 fused by degrees, while at the same time atmospheric or sea-water is 

 descending from the surface. At length the conditions required for a 

 new outburst are obtained, and another cycle of similar changes is 

 renewed." ('Principles of Geology,' p. 558.) 



GIANT, a man of great or unnatural stature. In the following 

 article we shall mention those deviations from nature which sometimes 

 occur in the proportions of the human form, both as regards the arrest 

 or excess of development, and thus include the description of Dwarfs, 

 or men who are unnaturally small, and beneath the usual size, as well 

 as of Giants, who exceed the ordinary proportions of the human race. 

 In considering this subject we will first allude to the question, 

 whether the size of man generally was ever different from what it is 

 at the present time. Secondly, we will inquire if it is probable that 

 races or nations of giants or pygmies ever existed. Lastly, we will 

 state the ordinary proportions of the human frame, and enumerate 

 a few examples of men who have much exceeded or have fallen far 

 short of the common standard, and inquire whether these peculiarities 

 of stature can be accounted for in any satisfactory manner. 



It is a very common opinion, that in the earlier ages of the world 

 men in general possessed superior physical properties, and were of a 

 greater size than they are at present ; and this notion of diminished 

 stature and strength seems to have been just as prevalent in ancient 

 times as at present. Pliny observes of the human height (vii. 16), 

 that " the whole race of mankind is daily becoming smaller ; " a most 

 alarming prospect if it had been true. Homer more than once makes 

 a very disparaging comparison between his own degenerate contempo- 

 raries and the heroes of the Trojan war. But all the facts and circum- 

 stances which can be brought forward on this subject tend to convince 

 us that the human form has not degenerated, and that men of the 

 present age are of the same stature as in the beginning of the world. 

 In the first place, though we read both in sacred and profane history 

 of giants, yet they were at the time when they lived esteemed as 

 wonders, and far above the ordinary proportions of mankind. All the 

 remains of the human body (as bones, and particularly the teeth), 

 which have been found unchanged in the most ancient urns and 

 burial-places, demonstrate this point clearly. The oldest coffin in the 

 world is that found in the great pyramid of Egypt, and Mr. Greaves 

 observes that this sarcophagus hardly exceeds the size of our ordinary 

 coffins, being scarcely six feet and a half long. From looking also at 

 the height of mummies which have been brought to this country, we 

 must conclude that the people who inhabited Egypt two or three 

 thousand years ago were not superior in size to the present inhabit- 

 ants of that country. Lastly, all the facts which we can collect from 

 ancient works of art, from armour, as helmets and breastplates, or 

 from buildings designed for the abode and accommodation of men, 

 concur in strengthening the proofs against any decay in nature. That 

 man has not degenerated in stature in consequence of the effects of 

 civilisation is clear ; because the inhabitants of savage countries, as 

 the natives of America, Africa, Australia, or the South Sea Islands, 

 do not exceed us in size. It has been supposed that before the Deluge 

 men might have been of a larger form than they are at present, as 

 ;hey are said to have lived to a much greater age ; but this is a mere 

 assumption, unsupported by any evidence whatever. 



When investigating the subject, 'whether any peculiar races of men 

 lave ever existed who have greatly varied iu size from the ordinary 

 proportions of our form, we need not allude to the fabulous stories of 

 .he giants and pygmies of antiquity, the former of whom are said to 



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