56 



discoloured indicated, moreover, that they must have come from a com- 

 paratively tranquil portion of the river bed — a condition which is found 

 precisely at the place where they were observed. Mud can deposit in 

 the Dodder only for a short distance at the backs of weirs and dams ; 

 at all other portions of its middle and upper course, the bed is coated 

 with gravel or rock. As the rising of the spongy-looking fragments of 

 ice took place about the middle of the stream, and as the pieces were 

 afterwards rapidly carried over the weir, I made no attempt to obtain 

 a specimen. It was, however, easy to see that their structure 

 approached as closely to the rough pieces with projecting crystals which 

 I had broken from the edge of the stream under water, as it deviated 

 from that of the slabs of ice belonging to the surface. 



The true cause of the formation of ground ice could not be more 

 clearly illustrated than by the phenomena here adduced. The con- 

 ditions which promote freezing, cited in the order of their relative im- 

 portance, are — (1) low temperature; (2) stillness of the liquid; and (3) 

 contact of rough solid substances. On the surface of a pond or lake, 

 when the temperature falls below 32° F., the first two and most im- 

 portant conditions are both perfectly fulfilled. In a deep lake the two 

 last conditions can in general alone prevail near the bottom, while the 

 first and indispensable condition will not be sufficiently intense. In 

 the middle of rapidly running water the first condition may exist ; but, 

 as it must then be alone, we never see ice formed in the axis of a 

 swiftly flowing stream. Ice may be found in a shallow and rapid river 

 along the banks, and on stones at the bottom, because in these positions 

 the velocity of the cold current becomes sufficiently reduced to allow 

 of the operation of condition (2), while the growth of ice crystals is 

 directly promoted by the existence of condition (3), at the points of 

 contact between the river and its bed. Condition (3) is in general 

 most likely to exist in perfection in short rivers descending from an 

 elevated source into the plains ; and, as such rivers are always shallow 

 except immediately after rain or the melting of snow, the water flowing 

 in their beds will usually be very fully exposed to any cooling influences 

 which may result from the weather. 



Let us more closely examine what takes place in a small river, such 

 as the Dodder, when the temperature falls considerably below the freez- 

 ing point of still water. This stream has a rapid fall, and its longitu- 

 dinal section presents a series of great and small inequalities which 

 essentially promote the thorough refrigeration of water flowing 

 over them. The following numbers will make this more clearly 

 understood : 



