第29章 LETTER VII(11)

We had now been keeping watch for three days over the Geysir,in languid expectation of the eruption which was to set us free.All the morning of the fourth day I had been playing chess with Sigurdr;Fitzgerald was photographing,Wilson was in the act of announcing luncheon,when a cry from the guides made us start to our feet,and with one common impulse rush towards the basin.The usual subterranean thunders had already commenced.A violent agitation was disturbing the centre of the pool.Suddenly a dome of water lifted itself up to the height of eight or ten feet,--then burst,and fell;immediately after which a shining liquid column,or rather a sheaf of columns wreathed in robes of vapour,sprung into the air,and in a succession of jerking leaps,each higher than the last,flung their silver crests against the sky.For a few minutes the fountain held its own,then all at once appeared to lose its ascending energy.The unstable waters faltered,drooped,fell,"like a broken purpose,"back upon themselves,and were immediately sucked down into the recesses of their pipe.

The spectacle was certainly magnificent;but no deion can give any idea of its most striking features.The enormous wealth of water,its vitality,its hidden power,--the illimitable breadth of sunlit vapour,rolling out in exhaustless profusion,--all combined to make one feel the stupendous energy of nature's slightest movements.

And yet I do not believe the exhibition was so fine as some that have been seen:from the first burst upwards to the moment the last jet retreated into the pipe,was no more than a space of seven or eight minutes,and at no moment did the crown of the column reach higher than sixty or seventy feet above the surface of the basin.

Now,early travellers talk of three hundred feet,which must,of course,be fabulous;but many trustworthy persons have judged the eruptions at two hundred feet,while well-authenticated accounts--when the elevation of the jet has been actually measured--make it to have attained a height of upwards of one hundred feet.

With regard to the internal machinery by which these waterworks are set in motion,I will only say that the most received theory seems to be that which supposes the existence of a chamber in the heated earth,almost,but not quite,filled with water,and communicating with the upper air by means of a pipe,whose lower orifice,instead of being in the roof,is at the side of the cavern,and BELOW the surface of the subterranean pond.The water kept by the surrounding furnaces at boiling point,generates of course a continuous supply of steam,for which some vent must be obtained;as it cannot escape by the funnel,the lower mouth of which is under water,it squeezes itself up within the arching roof,until at last,compressed beyond all endurance,it strains against the rock,and pushing down the intervening waters with its broad,strong back,forces them below the level of the funnel,and dispersing part,and driving part before it,rushes forth in triumph to the upper air.The fountains,therefore,that we see mounting to the sky during an eruption,are nothing but the superincumbent mass of waters in the pipe driven up in confusion before the steam at the moment it obtains its liberation.[Footnote:

Professor Bunsen has lately announced a chemical theory,which I believe has been received with favour by the scientific world.He points to the fact that water,after being long subjected to heat,loses much of the air contained in it,has the cohesion of its molecules much increased,and requires a higher temperature to bring it to boil;at which moment the production of vapour becomes so great,and so instantaneous,as to cause explosion.

The bursting of furnace boilers is often attributable to this cause.Now,the water at the bottom of the well of the Great Geysir is found to be of constantly increasing temperature up to the moment of an eruption,when on one occasion it was as high as 261degrees.Fahrenheit.

Professor Bunsen's idea is,that on reaching some unknown point above that temperature,ebullition takes place,vapour is suddenly generated in enormous quantity,and an eruption of the superior column of water is the consequence.]

The accompanying sketch may perhaps help you to understand my meaning.

[Figure:fig-p074.gif]

The last gulp of water had disappeared down the funnel.

We were standing at the bottom of the now empty basin,gazing into each other's faces with joyous astonishment,when suddenly we perceived a horseman come frantically galloping round the base of the neighbouring hill towards us.The state of the case was only too evident.He had seen the masses of vapour rising round the fountain,and guessing "what was UP",had strained every nerve to arrive in time.As there was no mutual friend present to introduce us to each other,--of course under ordinary circumstances I should have wrapped myself in that reserve which is the birthright of every Briton,and pretended never even to have noticed his arrival;but the sight we had just seen had quite upset my nerves,--and I confess,with shame,that I so far compromised myself,as to inaugurate a conversation with the stranger.In extenuation of my conduct,I must be allowed to add,that the newcomer was not a fellow-countryman,but of the French tongue,and of the naval profession.