1853.) OF THE ROYAL INSTITUTION. 335 
Sir Geo. Mackenzie it is well known was the first to introduce the 
idea of a subterranean cavern to account for the phenomena of the 
Geiser. 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 Geiser 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 shewn 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 thirteen hundred 
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 Geiser. — 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 cease. The.spring however continues to deposit its silica 
and forms a daug or cistern. Some of these in Iceland are of a 
depth of thirty or forty 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 incrusta- 
tions on the cistern walls; while at the bottom is observed the mouth 
of the once mighty Geiser. There are in Iceland traces of vast, 
but now extinct, Geiser operations. Mounds are observed whose 
shafts are filled with rubbish, the water having forced a way under- 
neath and retired to other scenes of action. We have in fact the 
Geiser in its youth, manhood, old age, and death, here presented 
to us:—Zin its youth as a simple thermal spring, in its manhood 
as the eruptive spring, in its old age as the tranquil Jaug, while 
its death is recorded by the ruined shaft and mound which testify 
the fact of its once active existence. 
Next to the Great Geiser the Strokkur is the most famous 
eruptive spring of Iceland. The depth of its tube is forty-four feet. 
It is not, however, cylindrical like that of the Geiser, but funnel- 
shaped. At the mouth it is eight feet in diameter, but it diminishes 
gradually, until near the centre the diameter is only ten 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 Geiser. Its action was illustrated experimentally in the 
lecture, by stopping the galvanized 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. 
ea! 
