402 M. Descloiseaux's Physical and Geological Observations 



texision is able to overcome the resistance of the column of 

 water which fills the basin, and to project it into the air. 



A thermometer placed at four metres above the bottom of 

 the central well during an eruption which took place July 14, 

 1846, at 3'^ 15"" in the morning, and which raised the column 

 of water to 49'""37> according to a measurement taken by M. 

 Waltershausen, indicated a temperature of 124°'24, which 

 corresponds perfectly to what would have been found at the 

 same point in the third and fourth experiments. 



In the intervals of the great eruptions it often happens that 

 the column of water is raised to one metre, or 1™*50 above the 

 usual level, and it is easy to conceive that it must be thus ; for, 

 if the vapour formed in the subterranean reservoir is not con- 

 densed entirely at its contact with the water of the sinuous 

 channel of which I have spoken, and if it is accompanied with a 

 sufficient quantity of atmospheric air, or of other gases, it must 

 in the end divide the liquid colunui, and escape at the sur- 

 face, producing those subterranean detonations and those jets 

 which all travellers have remarked. A fact worthy of atten- 

 tion is, that these detonations appear to be produced periodi- 

 cally with sufficient regularity; whereas the great eruptions 

 only take place at quite unequal intervals, as is shown by 

 the following table of a series of observations which we con- 

 tinued for several days. This difference might be explained 

 by admitting that the production of vapour is not perfectly 

 regular. In the following table, the three columns indicate 

 the date of the observations, the hours when the detonations 

 or jets took place, and those when great eruptions were pro- 

 duced : — 



• The great eruptions are denoted by an *■ 



