"Those are all six-sided crystals. Frost on the window pane is made in the same way. All those designs that look like lace work or trees or ferns are six-sided crystals produced by water-vapor, in the air, cooling and crystallizing on the cold glass. Ice crystals grow from each other quite readily. This is called twinning."
"But why are they always so regular?"
The Forecaster shook his head.
"You're always expecting everything to be regular, Ross," he said. "They're not regular at all. There are thousands of different forms. The United States is fortunate in having one man who's the world's expert on snow crystals, and he examines and photographs thousands every year and adds, perhaps, two or three new examples each season."
"Who's that, sir?" asked Fred.
"Wilson A. Bentley, of Jericho, Vermont," the Forecaster answered. "He's made thousands of photographs of snow crystals through a microscope. What's more, he's done it for the love of the work. Why don't you send him a copy of the Review, Fred? I'm sure he'd like to see it. Perhaps he might send you some prints of his snow crystals. He'd appreciate a plate of Cæsar's sunsets and Ralph's clouds, I'm sure."
"I'll send them to him right away," the editor answered.
"Why is it," queried Anton, "that when snow-flakes fall slowly and only a few of them at a time, they are big, but when there's a heavy snow-storm the flakes are small?"
"Because they are manufactured in different layers of the air," the Forecaster answered, "in the upper air, eight or ten miles up, where the faintest cirrus clouds are, they are not flakes at all, but tiny needle-like crystals, called spicules. In the depth of the Arctic winter, near the North Pole and especially on the Greenland ice-cap—one of the coldest regions of the world—the wind is full of these spicules, which one can't very well call snow.
"Snow-flakes that come from the cold regions of the air, three or four miles high, generally have a solid form. All, of course, show the six-sided form of the snow crystals. Being smaller and heavier in proportion to their surface they fall more quickly. In the layers of the atmosphere, one or two miles high, where the air is not as cold and where the content of water vapor is higher, the flakes have more opportunity to grow as they slowly sink through the air. Snow-flakes that have been formed only a short distance above the ground become large and feathery, the kind of which northern peoples say that 'the old woman of the sky is plucking her geese.'"