The pond had in the meanwhile skimmed over in the shadiest and shallowest coves,some days or even weeks before the general freezing.The first ice is especially interesting and perfect,being hard,dark,and transparent,and affords the best opportunity that ever offers for examining the bottom where it is shallow;for you can lie at your length on ice only an inch thick,like a skater insect on the surface of the water,and study the bottom at your leisure,only two or three inches distant,like a picture behind a glass,and the water is necessarily always smooth then.There are many furrows in the sand where some creature has travelled about and doubled on its tracks;and,for wrecks,it is strewn with the cases of caddis-worms made of minute grains of white quartz.Perhaps these have creased it,for you find some of their cases in the furrows,though they are deep and broad for them to make.But the ice itself is the object of most interest,though you must improve the earliest opportunity to study it.If you examine it closely the morning after it freezes,you find that the greater part of the bubbles,which at first appeared to be within it,are against its under surface,and that more are continually rising from the bottom;while the ice is as yet comparatively solid and dark,that is,you see the water through it.These bubbles are from an eightieth to an eighth of an inch in diameter,very clear and beautiful,and you see your face reflected in them through the ice.There may be thirty or forty of them to a square inch.There are also already within the ice narrow oblong perpendicular bubbles about half an inch long,sharp cones with the apex upward;or oftener,if the ice is quite fresh,minute spherical bubbles one directly above another,like a string of beads.But these within the ice are not so numerous nor obvious as those beneath.I sometimes used to cast on stones to try the strength of the ice,and those which broke through carried in air with them,which formed very large and conspicuous white bubbles beneath.One day when I came to the same place forty-eight hours afterward,I found that those large bubbles were still perfect,though an inch more of ice had formed,as I could see distinctly by the seam in the edge of a cake.But as the last two days had been very warm,like an Indian summer,the ice was not now transparent,showing the dark green color of the water,and the bottom,but opaque and whitish or gray,and though twice as thick was hardly stronger than before,for the air bubbles had greatly expanded under this heat and run together,and lost their regularity;they were no longer one directly over another,but often like silvery coins poured from a bag,one overlapping another,or in thin flakes,as if occupying slight cleavages.The beauty of the ice was gone,and it was too late to study the bottom.Being curious to know what position my great bubbles occupied with regard to the new ice,I broke out a cake containing a middling sized one,and turned it bottom upward.The new ice had formed around and under the bubble,so that it was included between the two ices.It was wholly in the lower ice,but close against the upper,and was flattish,or perhaps slightly lenticular,with a rounded edge,a quarter of an inch deep by four inches in diameter;and I was surprised to find that directly under the bubble the ice was melted with great regularity in the form of a saucer reversed,to the height of five eighths of an inch in the middle,leaving a thin partition there between the water and the bubble,hardly an eighth of an inch thick;and in many places the small bubbles in this partition had burst out downward,and probably there was no ice at all under the largest bubbles,which were a foot in diameter.I inferred that the infinite number of minute bubbles which I had first seen against the under surface of the ice were now frozen in likewise,and that each,in its degree,had operated like a burning-glass on the ice beneath to melt and rot it.These are the little air-guns which contribute to make the ice crack and whoop.
