55 



In order to make the connexion between the results recorded in this 

 Table and the formation of the ground ice more clearly manifest, I 

 append a graphical representation, in which the dotted broken line re- 

 presents the march of mean temperature, and the undotted line the 

 march of minimum temperature. The two cold periods are well denned 

 by the rising and falling of the curves above or below the line of frost. 

 A remarkable feature in the first period is the sudden great rise of 

 temperature from the 3rd of January to the 6th; whence resulted a 

 sudden thaw, which had an important influence in bringing very sig- 

 nificant phenomena distinctly under observation. 



Towards the close of the first week in January, I frequently walked 

 on the right bank of the Dodder between Eathgar and Rathfarnham 

 bridges. The greater partof this portion of the streamremained unfrozen ; 

 and wherever the current was extremely rapid, the ice was restricted 

 to a thin edging along the banks. On breaking a portion of this edg- 

 ing where there was a swift current, I found rough pieces of ice, with 

 long needle-shaped crystals, jutting beneath the water. Water was 

 flowing over some of these pieces, but they were easily hooked up with 

 a stick, On a weir situated farther up stream I noticed many icicles 

 attached to the stones over which the water was dashing. Still more 

 decisive proofs of the existence of true ground ice under the stream 

 were furnished soon after the commencement of the thaw. 



On the morning of Sunday, the 6th, when the thaw was fully 

 developed, I took a position a little above one of the weirs, and watched 

 the breaking up and removal of the ice which overspread the river at 

 this point. After a short interval I noticed, in addition to the smooth 

 angular and uniformly thick slabs resulting from the breaking of the 

 surface ice, several rough spongy pieces, more or less discoloured by 

 mud, and having in some instances sand or small gravel attached to 

 them. I could not at first discover whence these singular-looking 

 pieces of ice had come ; but after another short interval I saw similar 

 fragments rise in succession to the surface of the water from below. 

 This occurrence was repeated more than once, and it attracted the 

 attention of other observers. I recently verified my impression of the 

 facts by asking gentlemen* who were present as to what they had seen, 

 and their reply completely accords with what I relate. Whenever a 

 large sheet of surface ice was burst by the rapidly rising waters of the 

 stream, rough lumps of the spongy ice were generally disclosed beneath. 

 These could not have arrived at their position by drifting down the 

 river ; for the drifted fragments were heaped over the upper edge of the 

 yet unbroken sheets of ice. The rough pieces must have floated up 

 from the bed to the under side of the surface ice, and they were dis- 

 closed to view on the removal of the latter from the position it had held 

 anterior to the thaw. The mud with which most of these fragments were 



* Among these I may especially refer to Messrs. Joseph and John Hanley, both of 

 whom reside close to the right bank of the Dodder. 



