071 the principal Geysirs of Jcclatid. 397 



was noted ; the thermometer, inclosed in a copper or iron case, 

 the lid of which was hermetically closed by means of a screw 

 and red-lead cement, was let down into the spring ; after 

 remaining there thirty minutes, it was raised and compared 

 again with the standard ; the mercury stopped at a certain 

 division of the stem, and the quantity of mercury which had 

 escaped from the stem during the observation, converted into 

 centigrade degrees and added to the first temperature indi- 

 cated by the standard, gave the temperature of the spring. 

 The thermometers employed by M. Bunsen were constructed 

 upon the same principle; only that, instead of a capsule sol- 

 dered to the upper part of the stem, the capillary opening of 

 the latter was closed by a small point of iron, kept vertical by 

 means of a spring, which the mercury could raise, so as to fall 

 over into the case. 



In each of the five experiments which we made at the 

 Great Geysir, we employed five thermometers arranged on a 

 single line, and separated by nearly equal intervals. 



It is known that the Great Geysir has formed by successive 

 incrustations a very regular basin, having the shape of a trun- 

 cated cone, the exterior slopes of which have a mean inclina- 

 tion of 8°, whose summit is occupied by a cup or vase*, pre- 

 senting only on the edges some small clefts through which is 

 carried the overflow of the water, when its level rises. The 

 mean inclination of the sides of this cup is 13°. 



In the centre of the basin is a circular well, which appears 

 perfectly regular as far as the eye can discern. Between two 

 consecutive eruptions, this well and the cup which surrounds it 

 are completely filled with water, a very small portion only of 

 which overflows after the upheavings, which take place about 

 every hour and half: but immediately after the great erup- 

 tions, the cup is completely empty, and in the well the water 

 descends below the bottom of the cup from one metre to 

 2"*50, according as the projected column had attained a 

 greater or less height ; the former level is not restored until 

 after six or seven hours. 



The following are the principal dimensions which we have 

 found for the different parts of the Geysir : — 



Diameter of the basin or cup, on the edges, from ,„ 



north to south 16'00 



Diameter of the basin or cup, in a perpendicular 



direction 18-00 



Diameter of the central well 3*00 



• SirW. J. Hooker thus describes it: — "To compare great things with 

 small, the shape of this bai^iii resembles that of a saucer with a circular hole 

 in its niidtllc." Tour in Iceland, p. 1)7. — Tii. 



