10 Geological Work of Frost and Fire. 



parts. Such, remarks, however, indicate his omissions, and we 

 always deem it an ungracious tiring when any one renders a 

 decided service to science, to descant upon what he has left 

 undone, instead of recognizing with becoming thankfulness 

 what he has performed. 



In an early part of Mr. Campbell's work some clever 

 experiments are recommended as exhibiting to the eye the 

 nature of the air and water currents which affect our globe. 

 He recommends that a common aquarium, or oblong fish tank, 

 shall be half filled with clear water., and placed in the sun. At 

 one end a few lumps of rough ice are to be floated, and a black 

 stone sunk at the other. When the water has settled, milk is 

 to be poured gently on the ice at the rate of an ounce to each 

 gallon of water in the tank. The sun's heat is absorbed by 

 the black stone, and communicated to the adjacent water, pro- 

 ducing an ascending current, while at the other end of the 

 tank a descending current results from the cooling action of 

 the ice. The milk, which if carefully poured in, mixes very 

 slowly with the water, forms clouds whose movements indicate 

 the currents and the amount of force by which they are im- 

 pelled. " Cloud forms," says Mr. Campbell, " are copied with 

 marvellous fidelity in this water toy, and, because the move- 

 ment is very slow, they are easily seen and copied. But the 

 tank gives a section of air as well as water. The miniature 

 sea has an atmosphere, and the same forces work both engines. 

 Let a bit of smouldering paper, tinder, rope, touchwood, or any 

 such light combustible fall on the ice-raft, and cover the tank 

 with a sheet of glass to keep in the smoke." By this means 

 imitations of all sorts of natural clouds may be produced. 



The same tank may be made to illustrate the movements of 

 water about to freeze, when the apparatus is placed in a cold 

 atmosphere. At one corner Mr. Campbell hangs a small ther- 

 mometer, just dipping into the water ; at the opposite corner 

 he places another thermometer, with its bulb reaching the 

 bottom, and capable of being elevated or depressed without 

 making much disturbance. On some ice, floating near the 

 centre of the water surface, he paints some lamp-black, 

 sinks a globular block-tin bottle, filled with boiling water 

 and corked, near the thermometer which touches the bottom, 

 and covers one side of the tank with a screen of thin paper. 

 When the ice begins to melt, the lamp-black begins to 

 move. "If it is warmed by the sun, a dark revolving 

 column sinks slowly down. But beneath the ice are layers 

 which contain intricate patterns of curved lines of black, 

 which bend and move slowly, but keep near the ice." 

 "When a water-bottle, filled with hot water, coloured with 

 lamp-black, is sunk through an ice dome without the stopper, 



