158 



REPORT 1875, 



velocity due to a height of some 2 or 3 metres ; but as soon as such a cleft 

 is totally opened, the water runs down all around the perimeter of the gallery 

 without showing signs of pressure. Springs from the bottom (which are by 

 no means rare) have never shown signs of pressure. 



The temperature of springs is higher when they are first tapped than at any 

 subsequent period. The springs at the distance of from 780 to 820 metres 

 from the Italian portal fell, in the first fourteen days, from 10°*52 to 9°-75 C, 

 and those at the distance of from 1495 to 1500 metres from the Swiss portal, 

 which had a temperature of 17°'l when tapped in November 1874, have now 

 fallen to 16°"2, after the lapse of ten months. 



The temperature of springs, even when first tapped, is lower than that of 

 the surrounding rock. The average amount of this difference for the first 

 2200 metres from the Italian portal was 3°'14 C. In the first 1200 metres 

 it was generally greater, and in the remaining 1000 metres always less than 

 this average value. At 2180 metres it was reduced to -77 of a degree. 

 As these diflferences constitute one of the most noteworthy results of Dr. 

 StapfTs observations, they are here presented in tabular form. The degrees 

 are Centigrade. 



Distance from 



Italian portal, 



in metres. 



Temperature of 



rock, deduced 



from observations 



m air. 



loo ii'5o 



200 13'43 



300 i5'oo 



400 i3'i3 



500 ii-oj 



600 'i'S3 



700 13'03 



800 i4'o8 



900 i4'84 



1000 iS'o4 



iioo lyiS 



1200 i6'97 



1300 i7"38 



1400 i7'63 



1500 lS'2I 



1600 2o'45 



1700 20-84 



1800 20'64 



1900 2171 



2000 2i'38 



2100 3o"9S 



2180 21'27 



Mean 



3'i4 



The strongest springs are those in the first 1300 metres from the Italian 

 portal. Their temperature varies, to the extent of a few tenths of a degree, 

 with the quantity, as dependent on rainfall, being lowest when the quantity 

 is greatest. 



The conformation of the ground and the course of the tunnel are such that 

 at equal distances from the two portals the Italian portion is the more distant 

 from the surface. It is not, however, upon the whole, warmer than the 

 Swiss portion; but for distances (from the portals) intermediate between 

 200 metres and 1400 metres is decidedly colder — an eflTect, probably, of the 

 abundant infiltration of cold water. 



