The growth should be carefully noted under different conditions, attention being given to the atmospheric influences and other general effects, and the regulation of the motion, based on ascertained results at the locality where applied. As the ice thickens, its growth is slower at the same, or even a lower, temperature than that which at first made ice very rapidly. The earth at the bottom and sides of the ice field radiate heat into the water. The heat rays of the sun pass through the ice, if it is clear, into the water below, with very little effect upon the ice itself. The ice, being a poor conductor of heat, is, under these conditions, an obstacle to its own growth. It shuts in the water from contact with the cooler air, prevents agitation of its surface by passing breezes, and retards the escape of air and heat. On running streams, these conditions are much modified. In passing over shallows or rapids, where the current is swift, the water remains open and exposed to the air. At these points in its course it parts with its accumulated air and heat very rapidly, a thin vapor or mist being often perceptible in the air at such places, owing to the rapid radiation. The tumbling and turning of the water at rapid shoals materially assists the growth of ice at points below where the current grows gentle. Streams of this character, whose beds are free from accumulations of vegetable mold, or other sources which generate gases, produce clear and sparkling ice of greater thickness than is found on still ponds or lakes in the same vicinity, and exposed to the same temperature. THE USEFULNESS OF SNOW.--Snow, as it is well known, is a great impediment to the inroads of frost into anything enveloped by it. A covering of snow on an ice field is a great impediment to the escape of heat from the water, as well as protecting the ice from the direct action of the cold air, and greatly retards the growth of the ice. It is essential to remove this snow as early as practicable, as the ice harvester has always in view a possible thaw or rain, and endeavors to secure his crop at the earliest practicable moment.