On the Gey sirs of Iceland. 391 



The resistances of wires are to each other directl}' as their 

 lengths, and inversely as the products of their sections and 

 conductibiHties. If the wire of the galvanometer helix be as 

 above, 1000 inches in length and yi^dth of an inch in dia- 

 meter, then eight inches of wire d, being of platinum, one- 

 sixteenth of an inch in diameter, and having, suppose, one-fifth 

 of the conducting power of copper, will equal in resistance 

 about an inch and a quarter of the helix wire ; therefore since 



r=1000 and the greatest value of tiy=l;[, the quantity ■ 



cannot in this instrument be of any importance in the deter- 

 mination of resistances, and may always be safely neglected. 

 For the accurate measurement, with an instrument having 

 finer wires, of resistances offered to weaker forces, the fraction 

 may be useful. 



LX. Physical ajid Geological Observations on the prificipal 

 Geysirs of Iceland. By A. Descloizeaux*. 



IN Iceland the name o^ Gey sir is generally given to a jet- 

 ting thermal spring, and the names of hver (cauldron) and 

 laug (baths) are applied to springs simply bubbling up, or 

 wholly tranquil, contained in basins almost always regular, 

 circular or elliptic. 



The jetting springs most celebrated on account of their 

 volume, the beauty of their eruptions, and the importance of 

 the deposits which they have formed and which they still form 

 at the present day, are those known by the names of the Great 

 Geysir and Strokkur. Although most of the travellers who 

 have visited Iceland have described the eruptions of these 

 two springs which they witnessed, and of which some have 

 published pretty accurate representations, I will here relate 

 the manner in which this phasnomenon takes place. 



In the Great Geysir, whose waters, as I shall observe here- 

 after, have formed for themselves a very regular basin in the 

 form of a truncated cone hollowed at the summit, the erup- 

 tions are preceded by subterraneous detonations, which have 

 always been justly compared to a distant sound of artillery, 

 and which powerfully shake the base and lateral parts of the 

 cone. After each detonation, the column of water which 

 occupies the central cliannel is upraised, in the form of a 

 hemisphere, some metres f above the surface of the basin; 

 then all becomes calm again. 



These detonations and these upliftings of the liquid mass 

 are produced pretty regularly every two hours, as we have 



♦ Tiansliited froin the ./««. <lc C/iim. ct de Pltys. for April 184/. — The 

 Icelandic orthography has been preserved in this memoir. 



+ The metre is equivalent to about .'J feet 3^; inches. — Tii. 



