X.J ICE AND ITS WORK. 153 



which ice may be actually formed at the bottom of a 

 stream, and remain there for some time. This forma- 

 tion of groimd-ice is occasionally seen in parts of the 

 Thames. 



Dr. Plot, the first keeper of the Ashmolean Museum at 

 Oxford, published in the year 1677 a famous work on the 

 Natural History of Oxfordshire, in which he refers to the 

 freezing of the Thames in the following words : — " I find 

 it the joint agreement of all the watermen hereabout that 

 I have yet talked with that tlie congelation of our river is 

 always begun at the bottom, which, however surprising it 

 may seem to the reader, is neither unintelligible nor ridi- 

 culous. They all consent that they frequently meet the ice 

 vieers (for so they call the cakes of ice thus coming from 

 the bottom) in their very rise, and sometimes in the under 

 side including stones and gravel." 



To explain the formation of such ground-ice, it has been 

 suggested that the action of the running stream mechanically 

 mixes the cold surface-water with the warmer water below 

 until the temperature becomes uniform throughout ; and 

 when the air is very cold the whole mass may thus be 

 reduced to the freezing-point. The formation of ice will 

 then be determined at the bottom, in consequence of the 

 greater tranquillity of the water and the contact of cold 

 stones and other objects which have become chilled by free 

 radiation. This ground-ice is generally found in little masses 

 clinging to stones and. weeds; and, when the temperature 

 rises after sunrise, the loose bodies are lifted to the surface by 

 the ice, just as if buoyed up with corks. The ice then floats 

 down the river, bearing its little freight of gravel, which is 

 dropped on the bed of the river when the ice is broken up 

 or melted. The Rev. J. C. Clutterbuck, who has paid 

 great attention to the study of the Thames, tells us that he 



