June 4, 190S] 



NATURE 



101 



five months the temperature of the water remains 

 ahnost stationary. During this time a tremendous 

 struggle goes on between water and ice, growing more 

 severe as the air temperature falls further from the 

 freezing point. The outward calm of the ice-bound 

 river gives no indication of the contest beneath, but 

 it is only during the annual spring break-up that the 

 volume of existing ice is realised. At that time the 

 river frees itself from its icy burden in a few days by 

 a mighty shove, which is viewed by thousands along 

 the river shore at a safe distance from the relentless 

 piling of great ice-blocks. 



In all the quieter parts of the river, surface-ice forms 

 into a sheet, and protects the water from excessive 

 loss of heat. Wherever the water flows too swiftly for 

 the surface sheet to form, the comparatively warm 

 water is exposed to the winter weather with all its 

 severity. Ice-crystals are produced all along its 

 surface, and are carried under by the currents, to be 

 whirled about for miles, until finally swept under the 

 barrier-ice at the beginning of quiet water. There 

 they rise and become attached to the under-side of the 

 surface sheet, building downwards immense hanging 

 dams, which form as effective a barrier to the flow of 

 the river as so much rock. Winter floods and ice 

 shoves are the result of this packing of the ice, and, in 

 one part of the St. Lawrence, the damming is so com- 

 plete as to change the course of the river every year. 

 The natives call this fine ice " frazil," meaning cinder- 

 ice, and this term has now come into general use in 

 Canada. 



During the ice survey of the river by the Montreal 

 Flood Commission in 1886, it was revealed that the 

 packing of the frazil extended to the bottom in many 

 places, giving an unusiial amount of solidity to the 

 surface sheet. In one case, a depth of 80 feet of 

 solidly packed frazil was measured. Hanging dams 

 of the ice are observed to a greater or less e.\tent for 

 a distance of twelve miles below the barrier-ice at the 

 foot of the Lachine Rapids, and the magnitude of the 

 ice accumulation may be realised when it is stated that 

 the winter level of so mighty a river as the St. Law- 

 rence, which is two miles wide at Montreal, is twice 

 as high as in summer. 



A careful study of the winter river temperature by 

 means of very delicate instruments has revealed the 

 fact that the formation and growth of the ice is an 

 accompaniment of a minute temperature depression 

 in the water of the order of a few thousandths of a 

 degree. When the temperature equilibrium of water 

 and ice is upset by this minute amount, fresh ice- 

 crystals are formed, and, being supercooled, adhere 

 to anything in their path which is likewise super- 

 cooled. In this way large quantities of anchor-ice are 

 formed on the stones and boulders over which the ice- 

 laden water sweeps. The crystals themselves stick 

 together and form frozen masses. When carried 

 through the rack or screen at the intake of a power- 

 house they freeze to it, and rapidly choke the free 

 water-way. They stick to the turbines, and glue the 

 wheels fast in a short time. When the temperature 

 equilibrium of the river is restored, the ice no longer 

 adheres, and a rise of a few thousandths of a degree 

 above the freezing point changes the ice to a mass 

 of a soft and spongy consistency, capable of passing 

 easily through the most delicate machinery. 



It may be said that the whole condition which deter- 

 mines the rapid formation of ice in its harmful 

 adhesive condition hinges on this temperature balance 

 in the water. Since this important fact has been recog- 

 nised, effective means have been devised for the judi- 

 cious application of heat about the vulnerable parts 

 of a power-house during such time as supercooling 

 exists. There is no need to melt the ice or to warm 



NO. 2014, VOL. 78] 



the total volume of water flowing, so long as the 

 machinery itself is prevented from falling in tempera- 

 ture with the water. The ice is as effective as water 

 in producing a head. What the engineer has to 

 guard against is that the ice does not stick in its 

 passage through the turbines. 



Fig. I shows the interior of a penstock, or wheel- 

 pit, after it has been completely blocked with frazil 

 ice. At the time of this photograph the stop logs had 

 been introduced at the rack, the water removed, and 

 more than one-half the frazil-ice shovelled out. This 

 condition is a result of the slight supercooling of the 

 machinery by the water. Where artificial heat is used, 

 conditions like this no longer occur. 



The greatest factor for preventing this minute super- 

 cooling in the water is the absorption of the sun's 

 radiant heat. During the sunny hours of the day no 

 ice troubles are ever experienced, no matter how low 

 the air temperature may be. Nocturnal radiation, on 

 the other hand, is one of the most effective agencies 

 in supercooling the water and objects immersed in 

 it. Anchor-ice is formed by this means in large quan- 

 tities, and it has been known to grow on the river 

 bottom before the temperature of the water itself had 

 reached the freezing point. During cold, clear nights 



Fig. I. — Interior of a penstock or wheel pit after the water has been 

 removed, showing the accumulaticn of adhesive frazil ice. 



anchor-ice forms in large quantities. When the air 

 is cold enough to produce supercooling in the water, 

 frazil crystals adhere readily to the anchor-ice and 

 assist in building it up. On cloudy nights, anchor-ice 

 does not usually form, unless the supercooling is great 

 enough to bring the bottom of the river into a super- 

 cooled condition. .A bridge is found to protect the 

 river bottom from anchor-ice, and even in the severest 

 weather the anchor-ice is always less thick under such 

 a covering. 



Anchor-ice is never found to grow under surface-ice. 

 ^^'hen produced previous to a surface sheet, which in 

 some places does not form until the severest weather, 

 the masses are detached by the natural heat of the 

 earth, and rise to the under-side of the sheet. This 

 has been observed extensively by M. Wladiniiiof. who 

 has found in such cases an exact correspondence be- 

 tween the line of attached masses under the surface- 

 ice and the river bottom. 



Farquharson observed, in the small Scotch streams, 

 that overhanging' weeds protect the bottom from the 

 frost, just as a tree will protect the ground from the 

 dew or hoar-frost deposited at night. 



